FreeCalypso > hg > freecalypso-docs

comparison CMU200-maintenance-notes @ 85:7fc66c5b54dd

Find changesets by keywords (author, files, the commit message), revision number or hash, or revset expression.
CMU200-maintenance-notes: updates from Michael VK2BEA
author Mychaela Falconia <falcon@freecalypso.org>
date Fri, 21 Jan 2022 05:26:17 +0000
parents d2fef140ed53
children 2ac06a49dfbc
comparison
equal deleted inserted replaced
84:d2fef140ed53 85:7fc66c5b54dd
26 ======================= 26 =======================
27 27
28 This board is common among CMU200, CMU300 and CRTU-RU instruments from R&S - or 28 This board is common among CMU200, CMU300 and CRTU-RU instruments from R&S - or
29 at least these are the ones I know - maybe there are others I don't know about. 29 at least these are the ones I know - maybe there are others I don't know about.
30 This board encapsulates the instrument's main RF Rx and Tx chains: on the Rx 30 This board encapsulates the instrument's main RF Rx and Tx chains: on the Rx
31 side it takes RF input from the front end and performs triple IF downconversion 31 side it takes RF input from the front end and performs triple (or quadruple,
32 to 10.7 MHz IF3, and on the Tx side it takes 13.85 MHz IF3 input and upconverts 32 explained later in this article) IF downconversion to 10.7 MHz IF3, and on the
33 it to RF output, going through IF2 and IF1 in the process - triple IF in both 33 Tx side it takes 13.85 MHz IF3 input and upconverts it to RF output, going
34 directions. 34 through IF2 and IF1 in the process - triple or quadruple IF in both directions,
35 as explained in more detail later in this article.
35 36
36 Every CMU200 instrument always has one RXTX board - it is an absolutely required 37 Every CMU200 instrument always has one RXTX board - it is an absolutely required
37 component irrespective of option configurations. The hardware architecture of 38 component irrespective of option configurations. The hardware architecture of
38 this instrument also has a place for an optional second RXTX board, providing a 39 this instrument also has a place for an optional second RXTX board, providing a
39 complete second Rx and Tx channel - however, as far as I can tell, CMU200 40 complete second Rx and Tx channel - however, as far as I can tell, CMU200
66 the two IF1 Rx paths fails, then only that frequency range will be affected, 67 the two IF1 Rx paths fails, then only that frequency range will be affected,
67 or if both fail, the observed loss will typically be different between the two 68 or if both fail, the observed loss will typically be different between the two
68 frequency ranges. The failure symptom is unexpected large attenuation: 69 frequency ranges. The failure symptom is unexpected large attenuation:
69 sometimes around 5 to 6 dB of loss, othertimes as much as 25 dB of loss. 70 sometimes around 5 to 6 dB of loss, othertimes as much as 25 dB of loss.
70 71
72 The internal loopback test invoked from the Maintenance menu is a good first
73 step in diagnosis. In this test the instrument software configures both Rx and
74 Tx chains to connect to RF1 (and then RF2 if you press Continue), and then it
75 tests a longish list of different frequencies in sequence, spanning the full
76 range from 10 to 2700 MHz. For each test frequency, the instrument software
77 configures both the signal generator and the Rx chain, and it reports what was
78 measured on Rx vs. what was put out on Tx. The test is considered a failure if
79 nothing was received or if the Rx signal level was too far from the expected
80 value, otherwise the test is declared as passed.
81
82 If the loopback test fails at every frequency with no signal detected, then you
83 don't really know what's going on, and you will need to manually test Rx and Tx
84 separately (using an external spectrum analyzer and an external signal
85 generator) in order to figure out what is broken. However, out of the commonly
86 observed failure modes, dead Tx will produce this symptom.
87
88 If the Tx side is OK but Rx IF1 filters (one or both paths) have gone bad, the
89 visible symptom in the loopback test will be Rx signal level that is lower than
90 it should be. The instrument software may declare the test as either passed or
91 failed depending on the magnitude of the error: in this Mother's experience,
92 when one of the two IF1 paths on my CMU200 developed a loss of some 5.8 dB, the
93 loopback test was reported as passing - but a closer look at the numbers in the
94 report window showed the unexpected attenuation.
95
96 Because different input frequency ranges are handled via different Rx paths as
97 explained in the following section, when Rx IF1 filters fail, the loss behaviour
98 will be frequency-dependent. In the internal loopback test, you will see one
99 behaviour for frequencies from 10 to 1200 MHz, then a marked change for
100 frequencies from 1205 to 2200 MHz, and then another change (most likely a
101 reversion to low frequency behaviour) at the highest frequencies above 2200 MHz.
102
103 We do not currently know if there are any other failure modes elsewhere in the
104 CMU200 instrument that can also cause a stepwise change in behaviour at these
105 frequency cutover points. It is my (Mother Mychaela's) suspicion that the front
106 end may have some filters too, each covering a wide frequency swath, with
107 instrument software switching these filters depending on the configured
108 listening frequency - but we don't know for certain if any such additional
109 filters are there or not. If you find yourself wondering whether the problem
110 you are seeing is in the RXTX board or the front end, the best way to narrow it
111 down would be to remove the semi-rigid coax pieces that carry RF between the two
112 and use an external spectrum analyser to look at the Tx output from the RXTX
113 board and/or the Rx output from the front end.
114
71 RXTX board architecture explained 115 RXTX board architecture explained
72 ================================= 116 =================================
73 117
74 Unfortunately R&S' official service manual for CMU200 instruments is only a part 118 Unfortunately R&S' official service manual for CMU200 instruments is only a part
75 swapper guide: it tells you which boards do what in general terms and tells you 119 swapper guide: it tells you which boards do what in general terms and tells you
76 how to remove and replace each part, but no schematics, and no detailed 120 how to remove and replace each part, but no schematics, and no detailed
77 explanation of what happens inside each board. However, I draw the reader to 121 explanation of what happens inside each board. They do provide a little bit of
78 the block diagram on page 3.2 of this manual - this block diagram does provide 122 info: I draw the reader to the block diagram on page 3.2 of this manual - this
79 an important starting point for understanding what happens inside the RXTX 123 block diagram does provide an important starting point for understanding what
80 board. 124 happens inside the RXTX board - however, it is simplified and incomplete.
81 125
82 In the Tx direction, 13.85 MHz IF3 comes in from the digital board - or from 126 In the Tx direction, 13.85 MHz IF3 comes in from the digital board - or from
83 B68 board in WCDMA test modes. This Tx IF3 is mixed with Tx LO3 to produce 127 B68 board in WCDMA test modes. This Tx IF3 is mixed with Tx LO3 to produce
84 Tx IF2. This Tx IF2 is fixed at 487.52 MHz, thus one would think that Tx LO3 128 Tx IF2. This Tx IF2 is fixed at 487.52 MHz, thus one would think that Tx LO3
85 frequency ought to be fixed as well - but it seems to be a synthesized variable 129 frequency ought to be fixed as well - but it seems to be a synthesized variable
86 frequency. (Remember, all of this understanding is from reverse engineering, 130 frequency, and the manual describes it as "LO3TX with small tuning range".
87 hence we can only figure out so much.) 131 Calculations done by Michael VK2BEA put Tx LO3 at 473.67 MHz (needs
132 confirmation), but it is still not clear why it is a synthesized frequency
133 "with small tuning range", as opposed to simply fixed.
88 134
89 Tx IF2 of 487.52 MHz is then passed through a pair of identical SAW filters, 135 Tx IF2 of 487.52 MHz is then passed through a pair of identical SAW filters,
90 Sawtek 855272 - two cascaded identical filters, with an amplifier in between. 136 Sawtek 855272 - two cascaded identical filters, with an amplifier in between.
91 This SAW filter has a center frequency of 479.75 MHz with 20 MHz bandwidth, 137 This SAW filter has a center frequency of 479.75 MHz with 20 MHz bandwidth,
92 thus the passband spans from 469.75 to 489.75 MHz. Notice how Tx IF2 of 138 thus the passband spans from 469.75 to 489.75 MHz. Notice how Tx IF2 of
93 487.52 MHz stands just 2.23 MHz away from the edge of the passband - is it 139 487.52 MHz stands just 2.23 MHz away from the edge of the passband - is it
94 intentional? What are they filtering? Without original design notes, we can 140 intentional? What are they filtering? Without original design notes, we can
95 only guess. As I shall explain later in this article, one of these two Tx IF2 141 only guess. As I shall explain later in this article, one of these two Tx IF2
96 SAW filters is a component prone to failure. 142 SAW filters is a component prone to failure.
143
144 [Note from Michael VK2BEA: "The LO frequency is only 13.85 MHz from the IF. It
145 makes sense to shift this to the edge of the passband to help the suppression
146 of LO feed through. Also explains the use of SAW filters (sharp skirts) and
147 that there are two."]
97 148
98 After these cascaded SAW filters, Tx IF2 is mixed with LO2. Unlike LO1 and LO3, 149 After these cascaded SAW filters, Tx IF2 is mixed with LO2. Unlike LO1 and LO3,
99 there is only one LO2 for both Rx and Tx, and it is fixed at 1329.6 MHz. When 150 there is only one LO2 for both Rx and Tx, and it is fixed at 1329.6 MHz. When
100 Tx IF2 at fixed 487.52 MHz is mixed with LO2 at fixed 1329.6 MHz, the output of 151 Tx IF2 at fixed 487.52 MHz is mixed with LO2 at fixed 1329.6 MHz, the output of
101 this mixer will always contain two frequencies: 842.08 MHz and 1817.12 MHz. 152 this mixer will always contain two frequencies: 842.08 MHz and 1817.12 MHz.
105 high Tx IF1, controlling switches before and/or after the filters. I have not 156 high Tx IF1, controlling switches before and/or after the filters. I have not
106 investigated to see if the frequency ranges for high vs. low Tx IF1 are the same 157 investigated to see if the frequency ranges for high vs. low Tx IF1 are the same
107 as on the Rx side or not - maybe they are the same, maybe they are different. 158 as on the Rx side or not - maybe they are the same, maybe they are different.
108 159
109 After Tx IF1 output is combined or switched from the two filters, it is mixed 160 After Tx IF1 output is combined or switched from the two filters, it is mixed
110 with Tx LO1 to produce the final RF output. The mixer that does this job is 161 with Tx LO1 to produce an output that may or may not be final RF. The mixer
111 MACOM SM4T, which is one of the larger, prominently visible components on the 162 that does this job is MACOM SM4T, which is one of the larger, prominently
112 board. There also seems to be a fourth mixer and LO stage that kicks in only 163 visible components on the board. Tx LO1 has "large tuning range and very fine
113 for frequencies above 2200 MHz, but I haven't really studied that one as my main 164 frequency resolution used for setting the desired transmitter frequency" - quote
114 interest is in the classic cellular frequency bands, 1900 MHz and below. 165 from the manual; by doing some frequency arithmetics, we can see that this Tx
166 LO1 tuning range needs to span from 1827.12 to 3042.08 MHz in order to produce
167 output frequencies from 10 to 2200 MHz starting from 842.08 MHz or 1817.12 MHz
168 IF1. (LO1 - IF1 is the desired output frequency, whereas the sum will be a
169 much higher frequency above 2.7 GHz - I presume that the latter must be
170 suppressed by some LPF somewhere.)
171
172 The "RF" output from Tx SM4T mixer (LO1-IF1 as explained above) is indeed the
173 final RF output going to the front end for output frequencies below 2200 MHz.
174 In the uppermost frequency range of 2200 to 2700 MHz, a fourth mixer and LO
175 stage come into play - NOT shown on the block diagram in the manual! In this
176 highest frequency range, the output from SM4T mixer should be considered a
177 fourth IF - but because it is not covered at all in the manual and not named,
178 we have to invent our own name for it. I (Mother Mychaela) propose that we
179 call it IF0, and refer to the corresponding LO as LO0 - this way we remain
180 consistent with official naming that puts IF1 closest to RF and IF3 closest to
181 digital.
182
183 The preliminary analysis by Michael VK2BEA is that Tx LO0 frequency is fixed at
184 3318.46 MHz (same as its counterpart on the Rx side), with IF0 (taking the place
185 of lower RF) ranging from 1118.46 to 618.46 MHz (reverse range) to produce final
186 output frequencies of 2200 to 2700 MHz. However, these numbers have NOT been
187 confirmed by actual measurements yet.
115 188
116 On the Rx side the same process happens in reverse, but the specific frequencies 189 On the Rx side the same process happens in reverse, but the specific frequencies
117 used for IF1, IF2 and IF3 are slightly different. At first there is a stage 190 used for IF1, IF2 and IF3 are slightly different. At first there is a stage
118 that only kicks in for frequencies above 2200 MHz (bypassed otherwise), and 191 that only kicks in for frequencies above 2200 MHz (bypassed otherwise), and
119 then there is an SM4T mixer (identical to the one on Tx side) that takes in RF 192 then there is an SM4T mixer (identical to the one on Tx side) that takes in RF
158 cascaded Sawtek 855272 filters, two identical filters with an amplifier in 231 cascaded Sawtek 855272 filters, two identical filters with an amplifier in
159 between, exactly the same as on the Tx side. Then there is Rx LO3 and the final 232 between, exactly the same as on the Tx side. Then there is Rx LO3 and the final
160 mixer, producing Rx IF3 at 10.7 MHz that goes to the digital board, to the rear 233 mixer, producing Rx IF3 at 10.7 MHz that goes to the digital board, to the rear
161 panel BNC output and to the WCDMA board (B68) if the latter is present. 234 panel BNC output and to the WCDMA board (B68) if the latter is present.
162 235
236 Frequency conversion tables
237 ===========================
238
239 Michael VK2BEA worked out a pair of frequency conversion tables, one for Rx and
240 one for Tx. Here are these tables, with further corrections by Mother Mychaela:
241
242 Rx frequency conversion, RF to IF1:
243
244 RF (MHz) LO0 (MHz) IF0 (MHz) LO1 (MHz) IF1 (MHz)
245 ----------------------------------------------------------------------
246 10-1200 1826.115-3016.115 1816.115
247 1200-2200 2043.085-3043.085 843.085
248 2200-2700 3318.46 1118.46-618.46 2934.575-2434.575 1816.115
249
250 Rx frequency conversion, IF1 to IF3:
251
252 IF1 (MHz) LO2 (MHz) IF2 (MHz) LO3 (MHz) IF3 (MHz)
253 --------------------------------------------------------
254 1816.115 1329.6 486.515 497.215 10.7
255 843.085 1329.6 486.515 497.215 10.7
256
257 Tx frequency conversion, IF3 to IF1:
258
259 IF3 (MHz) LO3 (MHz) IF2 (MHz) LO2 (MHz) IF1 (MHz)
260 ---------------------------------------------------------
261 13.85 473.67 487.52 1329.6 1817.12
262 13.85 473.67 487.52 1329.6 842.08
263
264 Tx frequency conversion, IF1 to RF:
265
266 IF1 (MHz) LO1 (MHz) IF0 (MHz) LO0 (MHz) RF(MHz)
267 ------------------------------------------------------------------
268 1817.12 1827.12-3017.12 10-1200
269 842.08 2042.08-3042.08 1200-2200
270 1817.12 2935.58-2435.58 1118.46-618.46 3318.46 2200-2700
271
272 Notes:
273
274 * In Michael's original version each table covered the full chain from RF on
275 one end to IF3 on the other end, but I (Mychaela) had to split each table
276 into two in order to fit within 80 columns.
277
278 * The numbers for LO3 (473.67 MHz for Tx, 497.215 MHz for Rx) are from Michael;
279 I (Mychaela) have not verified them.
280
281 * All details for the IF0/LO0 stage (upper frequency range) are from Michael;
282 his notes indicate that the numbers are confirmed for Rx, but not for Tx.
283
163 How these RXTX boards fail 284 How these RXTX boards fail
164 ========================== 285 ==========================
165 286
166 There are 3 specific components on this RXTX board that have been seen to fail 287 There are 3 specific components on this RXTX board that have been seen to fail
167 over and over in the field: 288 over and over in the field:
187 The complete RXTX board assembly consists of the traditional PCBA plus heavy 308 The complete RXTX board assembly consists of the traditional PCBA plus heavy
188 metal shields on both sides; the front and back metal shield pieces are custom- 309 metal shields on both sides; the front and back metal shield pieces are custom-
189 made for this board, with individually shielded cavities matching different 310 made for this board, with individually shielded cavities matching different
190 sections of the board. Some (not all) of these cavities are filled with a 311 sections of the board. Some (not all) of these cavities are filled with a
191 special black foam called Eccosorb - it is an RF absorber, presumably added to 312 special black foam called Eccosorb - it is an RF absorber, presumably added to
192 prevent these cavities from acting as parasitic oscillators. Trouble occurs 313 lower the Q of these cavities to prevent parasitic oscillations. Trouble occurs
193 when this Eccosorb foam comes into direct contact with metal surfaces of 314 when this Eccosorb foam comes into direct contact with metal surfaces of
194 components on the board: the result is galvanic corrosion, a process that takes 315 components on the board: the result is galvanic corrosion, a process that takes
195 many years before it results in component failure. The reason why only 3 316 many years before it results in component failure. The reason why only 3
196 particular filter components fail is because they got the bad luck of residing 317 particular filter components fail is because they got the bad luck of residing
197 in cavities with Eccosorb - the other identical components that don't fail 318 in cavities with Eccosorb - the other identical components that don't fail
198 reside in cavities without Eccosorb. 319 reside in cavities without Eccosorb.
199 320
200 We don't know how R&S allowed this design flaw to escape and remain in their 321 [Note from Michael VK2BEA: "The copper surface of these filters form an integral
201 sold and field-deployed products: there is the "innocent" explanation that they 322 part of the component. It is this copper that forms the cavity of the combline
202 simply didn't notice, and there is the conspiratorial view that this slow 323 filter. When this is compromised by corrosion, the filter is detuned and there
203 failure mechanism is intentional as in planned obsolescense - pick your choice 324 is leakage causing excessive loss."]
204 of hypothesis. 325
326 It appears that R&S only noticed this design flaw toward the end of "product
327 life" of these instruments, probably because failures occur only after many
328 years. Some of the newer boards have had modifications to prevent contact
329 between Eccosorb and the two troubled Rx filters, either by way of thinner
330 Eccosorb fill or by way of an added plastic barrier. It is not clear if these
331 modifications were applied to newer produced RXTX boards from the start, or if
332 they are a result of field service repairs.
205 333
206 How to repair failed boards 334 How to repair failed boards
207 =========================== 335 ===========================
208 336
209 All 3 of the failing filter components (one SAW filter part and two ceramic 337 All 3 of the failing filter components (one SAW filter part and two ceramic
337 and then covering the component with Kapton tape before putting the board back 465 and then covering the component with Kapton tape before putting the board back
338 into its metal casing. 466 into its metal casing.
339 467
340 Unlike the failing filters, this MACOM SM4T mixer is still available new - but 468 Unlike the failing filters, this MACOM SM4T mixer is still available new - but
341 it's an expensive component, so let's protect these mixers from corrosion. 469 it's an expensive component, so let's protect these mixers from corrosion.
470
471 [Michael VK2BEA notes: "The case of the mixer is purely for shielding and is
472 much thicker than the thin copper of the filter that is essential for
473 operation." Mother Mychaela's response: it may be so, but if you are going to
474 take the RXTX board out of your CMU, take it out of its metal casing and either
475 replace filters or at least cover them with Kapton tape for protection, it
476 won't hurt to put the same Kaptop tape on the mixers too - and the signs of
477 corrosion are very real.]