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CMU200-maintenance-notes: new article
author Mychaela Falconia <falcon@freecalypso.org>
date Thu, 13 Jan 2022 08:18:03 +0000
parents
children d2fef140ed53
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83
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
1 Rohde & Schwarz CMU200 instrument is an absolutely essential piece of test
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
2 equipment for anyone in the business (or hobby) of designing and building his
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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3 or her own personal cellphones of 2G and/or 3G variety. I (Mother Mychaela)
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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4 currently only work with GSM, but depending on installed hw and sw options,
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
5 CMU200 instruments also support AMPS, IS-136, IS-95 (CDMA 2G) and both WCDMA
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
6 and CDMA2000 varieties of 3G.
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
7
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
8 Over the course of owning and maintaining a CMU200 instrument since 2017 and
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
9 having had to repair it twice now (as of 2022-01), and having conversed with
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
10 another CMU200 owner who had to repair his instrument in the same way, I
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
11 started observing a pattern in that many of these instruments are now failing
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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12 in the field in exactly the same ways. All of these failures happen in the
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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13 RXTX board, and the purpose of this article is to educate other CMU instrument
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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14 owners about these failures and most importantly, how to repair them.
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
15
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
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16 Credit attribution
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
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17 ==================
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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18
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
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19 I sincerely thank Michael Katzmann, NV3Z / VK2BEA / G4NYV, for his invaluable
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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20 help in reverse-engineering the insides of the culprit RXTX board, identifying
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
21 various critical components on that board, including the ones that habitally
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
22 fail, and identifying Eccosorb-caused galvanic corrosion as the root cause of
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
23 these failures. Without his help, I would not have made it this far!
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
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24
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
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25 What is this RXTX board
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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26 =======================
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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27
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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28 This board is common among CMU200, CMU300 and CRTU-RU instruments from R&S - or
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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29 at least these are the ones I know - maybe there are others I don't know about.
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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30 This board encapsulates the instrument's main RF Rx and Tx chains: on the Rx
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
31 side it takes RF input from the front end and performs triple IF downconversion
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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32 to 10.7 MHz IF3, and on the Tx side it takes 13.85 MHz IF3 input and upconverts
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
33 it to RF output, going through IF2 and IF1 in the process - triple IF in both
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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34 directions.
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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35
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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36 Every CMU200 instrument always has one RXTX board - it is an absolutely required
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
37 component irrespective of option configurations. The hardware architecture of
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
38 this instrument also has a place for an optional second RXTX board, providing a
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
39 complete second Rx and Tx channel - however, as far as I can tell, CMU200
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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40 software won't do anything with it, i.e., there are no test modes or
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
41 applications in CMU200 software repertoire that can make use of a second RXTX
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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42 board. Instead it seems that configurations with two RXTX boards are better
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
43 supported on the CRTU-RU platform - but I know next to nothing about that one.
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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44
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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45 Also note: if your CMU200 is equipped with Aux Tx model B96 (as opposed to B95),
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
46 there is an output from that B96 add-on that goes to the front end input that
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
47 was originally meant for second RXTX.
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
48
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
49 RXTX board failures
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
50 ===================
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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51
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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52 In terms of externally visible symptoms, almost all CMU200 units are now failing
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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53 in the same ways:
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
54
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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55 1) If Tx side fails, the visible symptom is completely absent or extremely weak
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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56 output, and the internal loopback test fails with no signal detected at any of
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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57 the frequencies in the test sequence. A key point is that this failure mode is
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
58 independent of the selected output frequency.
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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59
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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60 2) If Rx side fails, different frequency ranges are affected differently. As I
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
61 shall explain momentarily, there are two different IF1 Rx paths inside the RXTX
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
62 board: one handles the frequency range from > 1200 to <= 2200 MHz, and the other
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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63 handles lower (<= 1200 MHz) and higher (> 2200 MHz) input frequencies. When a
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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64 given RXTX board develops Rx path failure, this failure happens separately in
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
65 each of these two IF1 Rx paths. The resulting symptoms vary: if only one of
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
66 the two IF1 Rx paths fails, then only that frequency range will be affected,
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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67 or if both fail, the observed loss will typically be different between the two
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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68 frequency ranges. The failure symptom is unexpected large attenuation:
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
69 sometimes around 5 to 6 dB of loss, othertimes as much as 25 dB of loss.
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
70
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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71 RXTX board architecture explained
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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72 =================================
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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73
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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74 Unfortunately R&S' official service manual for CMU200 instruments is only a part
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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75 swapper guide: it tells you which boards do what in general terms and tells you
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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76 how to remove and replace each part, but no schematics, and no detailed
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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77 explanation of what happens inside each board. However, I draw the reader to
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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78 the block diagram on page 3.2 of this manual - this block diagram does provide
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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79 an important starting point for understanding what happens inside the RXTX
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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80 board.
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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81
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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82 In the Tx direction, 13.85 MHz IF3 comes in from the digital board - or from
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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83 B68 board in WCDMA test modes. This Tx IF3 is mixed with Tx LO3 to produce
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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84 Tx IF2. This Tx IF2 is fixed at 487.52 MHz, thus one would think that Tx LO3
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
85 frequency ought to be fixed as well - but it seems to be a synthesized variable
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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86 frequency. (Remember, all of this understanding is from reverse engineering,
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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87 hence we can only figure out so much.)
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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88
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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89 Tx IF2 of 487.52 MHz is then passed through a pair of identical SAW filters,
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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90 Sawtek 855272 - two cascaded identical filters, with an amplifier in between.
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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91 This SAW filter has a center frequency of 479.75 MHz with 20 MHz bandwidth,
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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92 thus the passband spans from 469.75 to 489.75 MHz. Notice how Tx IF2 of
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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93 487.52 MHz stands just 2.23 MHz away from the edge of the passband - is it
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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94 intentional? What are they filtering? Without original design notes, we can
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
95 only guess. As I shall explain later in this article, one of these two Tx IF2
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
96 SAW filters is a component prone to failure.
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
97
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
98 After these cascaded SAW filters, Tx IF2 is mixed with LO2. Unlike LO1 and LO3,
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
99 there is only one LO2 for both Rx and Tx, and it is fixed at 1329.6 MHz. When
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
100 Tx IF2 at fixed 487.52 MHz is mixed with LO2 at fixed 1329.6 MHz, the output of
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
101 this mixer will always contain two frequencies: 842.08 MHz and 1817.12 MHz.
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
102 These are the two possible Tx IF1 frequencies, and there is a frequency-
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
103 selective filter for each of these two Tx IF1 modes. Based on the final output
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
104 frequency to be generated, instrument control software selects either low or
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
105 high Tx IF1, controlling switches before and/or after the filters. I have not
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
106 investigated to see if the frequency ranges for high vs. low Tx IF1 are the same
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
107 as on the Rx side or not - maybe they are the same, maybe they are different.
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
108
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
109 After Tx IF1 output is combined or switched from the two filters, it is mixed
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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110 with Tx LO1 to produce the final RF output. The mixer that does this job is
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
111 MACOM SM4T, which is one of the larger, prominently visible components on the
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
112 board. There also seems to be a fourth mixer and LO stage that kicks in only
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
113 for frequencies above 2200 MHz, but I haven't really studied that one as my main
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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114 interest is in the classic cellular frequency bands, 1900 MHz and below.
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
115
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
116 On the Rx side the same process happens in reverse, but the specific frequencies
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
117 used for IF1, IF2 and IF3 are slightly different. At first there is a stage
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
118 that only kicks in for frequencies above 2200 MHz (bypassed otherwise), and
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
119 then there is an SM4T mixer (identical to the one on Tx side) that takes in RF
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
120 and Rx LO1 to produce Rx IF1. High-side injection is used, i.e., Rx LO1 is
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
121 programmed to generate frequency equal to the external RF of interest PLUS the
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
122 desired Rx IF1 output.
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
123
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
124 Rx LO1 is programmed as follows by the instrument control software:
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
125
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
126 * Rx IF1 will be at 1816.115 MHz (call it high) if the listening frequency is
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
127 <= 1200 MHz or > 2200 MHz;
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
128
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
129 * Rx IF1 will be at 843.085 MHz (call it low) if the listening frequency is in
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
130 the intermediate range, i.e., 1200 MHz < RF <= 2200 MHz.
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
131
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
132 In addition to programming Rx LO1 to produce the desired IF1 per the logic
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
133 above, the software also controls switches that select one or the other IF1
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
134 filter: either the filter that passes low IF1 or the one that passes high IF1.
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
135
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
136 The filters used for low and high IF1 modes are the same on both Rx and Tx
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
137 sides. (The actual frequencies are slightly different, but in each case they
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
138 fit within the passband of the common filter parts.) The filter for low IF1 is
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
139 Murata DFC3R836P025HHD, package marking 836 CD, and the one for high IF1 is
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
140 DFC31R84P075HHA, package marking CR. The two filter packages are NOT the same
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
141 mechanically: the low IF1 filter is physically larger. Both parts are ceramic
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
142 monoblock filters from the same family, and it seems that these filter parts
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
143 were originally made for mobile phones, not for RF metrology instruments: the
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
144 "836 CD" filter is for AMPS uplink band, and the "CR" filter is for DCS downlink
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
145 band.
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
146
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
147 On the Tx side of the board there are only two IF1 filters: one for low Tx IF1
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
148 and one for high Tx IF1. However, on the Rx side there are 3 of these ceramic
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
149 filters in total: two for high IF1 (two cascaded identical filters with an
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
150 amplifier in between) and just one for low IF1. Why am I covering these filters
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
151 in so much detail? You probably guessed it: they are components that fail, as
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
152 will be covered shortly.
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
153
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
154 After the selection of either low or high IF1 filter, Rx IF1 coming out of the
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
155 selected filter (either 843.085 MHz or 1816.115 MHz) is mixed with LO2, which is
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
156 shared between Rx and Tx sides and fixed at 1329.6 MHz. The output of this
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
157 mixer is Rx IF2 at 486.515 MHz. This Rx IF2 then passes through a pair of
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
158 cascaded Sawtek 855272 filters, two identical filters with an amplifier in
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
159 between, exactly the same as on the Tx side. Then there is Rx LO3 and the final
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
160 mixer, producing Rx IF3 at 10.7 MHz that goes to the digital board, to the rear
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
161 panel BNC output and to the WCDMA board (B68) if the latter is present.
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
162
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
163 How these RXTX boards fail
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
164 ==========================
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
165
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
166 There are 3 specific components on this RXTX board that have been seen to fail
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
167 over and over in the field:
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
168
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
169 * The second of the two cascaded IF2 SAW filters (Sawtek 855272) on the Tx side
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
170 often fails, breaking the Tx chain (output totally gone or extremely weak)
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
171 for all frequencies. Note that there are a total of 4 identical Sawtek 855272
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
172 filters on this board (2 on Rx side, 2 on Tx side), and only one of the four
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
173 fails: Tx side, second filter in the cascade.
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
174
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
175 * The "836 CD" filter on the Rx side is prone to failure. When it fails, the
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
176 visible symptom is severe attenuation in measured Rx signal levels for input
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
177 frequencies in the 1200 MHz < RF <= 2200 MHz range. Only the Rx side filter
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
178 fails, not the identical one on the Tx side!
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
179
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
180 * One of the two cascaded "CR" filters on the Rx side likewise fails - this time
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
181 it is the first one in the cascade. The other two identical "CR" filters on
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
182 the same board (the second in cascade for Rx and the one for Tx) are likewise
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
183 NOT seen to fail.
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
184
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
185 The root cause of all 3 component failures has been traced to galvanic corrosion
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
186 caused by direct contact between these components and Eccosorb RF absorber foam.
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
187 The complete RXTX board assembly consists of the traditional PCBA plus heavy
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
188 metal shields on both sides; the front and back metal shield pieces are custom-
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
189 made for this board, with individually shielded cavities matching different
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
190 sections of the board. Some (not all) of these cavities are filled with a
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
191 special black foam called Eccosorb - it is an RF absorber, presumably added to
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
192 prevent these cavities from acting as parasitic oscillators. Trouble occurs
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
193 when this Eccosorb foam comes into direct contact with metal surfaces of
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
194 components on the board: the result is galvanic corrosion, a process that takes
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
195 many years before it results in component failure. The reason why only 3
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
196 particular filter components fail is because they got the bad luck of residing
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
197 in cavities with Eccosorb - the other identical components that don't fail
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
198 reside in cavities without Eccosorb.
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
199
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
200 We don't know how R&S allowed this design flaw to escape and remain in their
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
201 sold and field-deployed products: there is the "innocent" explanation that they
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
202 simply didn't notice, and there is the conspiratorial view that this slow
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
203 failure mechanism is intentional as in planned obsolescense - pick your choice
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
204 of hypothesis.
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
205
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
206 How to repair failed boards
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
207 ===========================
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
208
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
209 All 3 of the failing filter components (one SAW filter part and two ceramic
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
210 monoblock filter parts) are now unobtainium. However, because so many of these
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
211 RXTX boards fail in exactly the same ways, our community at large is now
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
212 accumulating a very substantial "graveyard" of failed boards, and here is the
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
213 good news: we can make one good board out of every two failed ones. Suppose
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
214 that every RXTX board in our community's collective inventory has fully failed,
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
215 leaving no failure-free boards - what now? Here is the recipe for making one
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
216 good RXTX board out of two fully failed ones:
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
217
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
218 1) Out of the two failed boards, choose one to be the part donor and the other
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
219 to be the part recipient.
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
220
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
221 2) Take the part donor board and harvest 3 parts from it: one of the 3 Sawtek
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
222 855272 filters that aren't subject to corrosion, and the two IF1 filters
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
223 (one 836 CD and one CR) from the Tx side. Tx side IF1 filters aren't in
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
224 contact with Eccosorb and thus don't corrode, and 3 out of the 4 SAW filters
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
225 are likewise safe - hence we expect that every "dead" RXTX board can still
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
226 serve as a donor of good parts in this manner.
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
227
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
228 3) Take the part recipient board and transplant the donor parts onto it,
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
229 replacing all 3 corroded filters.
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
230
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
231 4) Before putting the repaired board back into its metal casing, cover all
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
232 corrosion-prone components with Kapton tape, preventing direct galvanic
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
233 contact with Eccosorb - this way the newly transplanted uncorroded components
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
234 won't suffer the same fate.
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
235
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
236 RXTX disassembly instructions
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
237 =============================
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
238
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
239 Before you can start working on an individual RXTX board, you first need to pull
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
240 it out of your CMU. Disassembly instructions are provided in the official part
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
241 swapper guide from R&S (which they call "service manual"), but here is the gist:
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
242
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
243 * Using a Torx T20 screwdriver, remove the 4 rear feet and lift the sleeve part
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
244 of the instrument case.
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
245
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
246 * Remove two small Phillips screws that secure the cover over the main board
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
247 cage, and lift that cover off.
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
248
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
249 * Unhook all MMCX little coax connections from the RXTX board: 3 on the top side
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
250 (IF3 interface) and one on the bottom (netclock input).
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
251
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
252 * Loosen and remove the two semi-rigid coax pieces that connect RF between the
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
253 RXTX board and the front end. In this Mother's opinion, this step is the
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
254 least pleasant of all, but it is unavoidable.
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
255
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
256 * After ensuring that nothing remains connected to the RXTX board on the bottom
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
257 side, pull the board out from the top.
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
258
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
259 Once you got the complete RXTX board assembly out, how do you extract the actual
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
260 board out of the metal casing? The not-immediately-obvious answer is that you
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
261 don't need to remove all of the screws, instead there are shortcuts that will
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
262 save you a lot of pain:
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
263
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
264 * There are two smooth thin metal plates, one on the front side of the board
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
265 (facing toward the front of the CMU when installed) and one on the back side.
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
266 Each is secured with a small Phillips screw. You only need to remove the one
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
267 on the front side. You don't need to remove the thin metal plate from the
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
268 back side of RXTX assembly - doing so will only add more clutter and loose
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
269 parts to your lab bench while the board is disassembled.
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
270
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
271 * Once you remove the thin metal plate from the *front* side of your RXTX
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
272 assembly, you will see all of the many screws that hold together the sandwich
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
273 of two heavy metal pieces with the board in the middle. These screws are
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
274 Torx T8.
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
275
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
276 * Put the board down on your bench so that the side that faces the front of the
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
277 CMU when installed (the side with the T8 screw heads) will become the top,
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
278 with the rear side becoming bottom.
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
279
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
280 * Each of the T8 screws passes through thread in the top metal piece, a hole in
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
281 the PCB, and then thread in the bottom metal piece. As you loosen these
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
282 screws, you don't need to remove them all the way - instead loosen each screw
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
283 so that its far end comes out of the thread in the bottom metal piece, but
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
284 let it remain captive in the top metal piece. Letting the screws remain
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
285 captive in the top metal piece will reduce bench clutter while the board is
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
286 disassembled, and there is a lot less screwing and unscrewing work to be done,
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
287 as there is no need to work through the thread in the top metal piece.
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
288
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
289 Once you loosen all of the T8 screws, the top metal piece should lift off,
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
290 leaving just the bottom metal piece and the PCBA. The bottom metal piece has
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
291 two thin metal pins sticking out of it; both the PCBA and the top metal piece
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
292 align on these two pins.
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
293
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
294 When you lift the top metal piece (the one with the screws), the side of the
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
295 board that will be immediately exposed to you is the side that faces the front
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
296 of the CMU when the board is installed. It is the Rx side, and you can confirm
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
297 that you are looking at the Rx side by noting that there are two "CR" filters
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
298 for high IF1, as opposed to just one. And chances are, right here at this step
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
299 in the disassembly process you will see the galvanic corrosion or the lead-up
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
300 to it.
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
301
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
302 As you lift the top metal piece from the board, look at its inside and note the
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
303 many individual cavities. Also note how some of these cavities are filled with
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
304 some black foam - that's the Eccosorb. And note how only some of the cavities
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
305 have Eccosorb in them, not all.
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
306
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
307 Now look at the ceramic IF1 filters on the Rx side of the board. The one "CR"
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
308 filter that is NOT in contact with Eccosorb will be bright copper-colored (it
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
309 actually is copper), whereas the two filters that are in contact with Eccosorb
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
310 (one 836 CD, one CR) will often be green instead of copper-colored on their top
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
311 surface - that's patinated copper! Furthermore, there will typically be some
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
312 black Eccosorb material directly adhered to the corroding top surfaces of those
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
313 two unlucky filters.
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
314
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
315 Now lift the PCBA off the two metal pins, separating it from the bottom metal
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
316 piece. Like you did with the top metal piece, observe the inside of the bottom
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
317 metal piece: note which cavities have Eccosorb in them and which don't. Then
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
318 flip the board over and look at its Tx side. You will see that there are only
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
319 two ceramic IF1 filters on this side (one 836 CD and one CR), and both should
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
320 be in pristine shape, bright copper-colored, no corrosion - these two are not
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
321 in contact with Eccosorb!
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
322
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
323 Now look at the two Sawtek 855272 filters on the Tx side. The one closer to
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
324 the middle of the board will often appear in worse physical condition that the
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
325 other 3 - and the culprit is once again in contact with Eccosorb.
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
326
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
327 MACOM SM4T mixer corrosion
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
328 ==========================
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
329
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
330 Neither I nor my collaborator on this project have seen an RXTX board on which
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
331 either the Rx SM4T mixer or the Tx one went bad - i.e., we haven't seen a
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
332 failure in this part *yet*. However, this mixer *is* in contact with Eccosorb,
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
333 and looking visually at the collection of RXTX boards in my possession, I
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
334 (Mother Mychaela) see definite signs of corrosion - the metal surface of this
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
335 SM4T mixer component is beginning to corrode. Therefore, as a preventative
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
336 measure, I recommend cleaning off any Eccosorb that is adhered to this component
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
337 and then covering the component with Kapton tape before putting the board back
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
338 into its metal casing.
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
339
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
340 Unlike the failing filters, this MACOM SM4T mixer is still available new - but
a87d9ee278fb CMU200-maintenance-notes: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
341 it's an expensive component, so let's protect these mixers from corrosion.