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annotate Quadband-ideas @ 25:c01155dec65b

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MEMIF-wait-states: updates for the newly discovered CAL000/A v0.8 document
author Mychaela Falconia <falcon@freecalypso.org>
date Sun, 10 Nov 2019 01:26:11 +0000
parents 00216b7cfc4d
children 3799892b1a79
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1 Triband status quo
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2 ==================
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3
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4 Our current Openmoko-based Calypso+RF modem core is very very good, but it has
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5 one shortcoming compared to TI's Leonardo+ reference design: it is triband
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Mychaela Falconia <falcon@freecalypso.org>
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6 rather than quadband. This triband restriction stems from OM's use of discrete
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Mychaela Falconia <falcon@freecalypso.org>
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7 antenna switch and SAW filter components as opposed to an integrated FEM (front
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8 end module) like on Leonardo+. In addition to the band restriction, our current
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Mychaela Falconia <falcon@freecalypso.org>
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9 triband RF design suffers from one other very unpleasant problem: we have no
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Mychaela Falconia <falcon@freecalypso.org>
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10 datasheet for the antenna switch component which we have to use. We know from
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Mychaela Falconia <falcon@freecalypso.org>
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11 Openmoko's BOM data that the manufacturer is Darfon and that the part number for
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Mychaela Falconia <falcon@freecalypso.org>
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12 this antenna switch component is ASM4532T0P06-1, we are able to buy this part
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Mychaela Falconia <falcon@freecalypso.org>
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13 from our Chinese grey market suppliers, we build our boards with these parts and
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Mychaela Falconia <falcon@freecalypso.org>
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14 our boards do work perfectly fine when we get a good batch, but we have to do
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Mychaela Falconia <falcon@freecalypso.org>
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15 this entire process blindly, without any datasheet or other documentation for
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Mychaela Falconia <falcon@freecalypso.org>
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16 this mystery part.
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17
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18 While we lack any official documentation for our triband ASM, we know its basic
00216b7cfc4d Quadband-ideas: better explanation of DCS/PCS switching or diplexing mystery
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19 logical function: it has two Tx inputs (low band and high band Tx coming from
00216b7cfc4d Quadband-ideas: better explanation of DCS/PCS switching or diplexing mystery
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20 the PA), 3 Rx outputs going to 3 separate SAW filters for the 3 supported bands,
00216b7cfc4d Quadband-ideas: better explanation of DCS/PCS switching or diplexing mystery
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21 and 3 switch control inputs. Two of these switch control inputs are Tx controls
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22 (low band Tx enable and high band Tx enable) which appear to be the same across
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23 all common RFFEs, whether they are 2-band (single region), triband or quadband.
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24 The 3rd switch control input gets a logic high voltage applied to it during PCS
00216b7cfc4d Quadband-ideas: better explanation of DCS/PCS switching or diplexing mystery
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25 band Rx, thus it appears to be a switch that diverts the high band Rx path
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26 between DCS and PCS SAW filters. The same arrangement is found in most other
00216b7cfc4d Quadband-ideas: better explanation of DCS/PCS switching or diplexing mystery
Mychaela Falconia <falcon@freecalypso.org>
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27 triband phones and modems from that era, i.e., they also have two Tx switches
00216b7cfc4d Quadband-ideas: better explanation of DCS/PCS switching or diplexing mystery
Mychaela Falconia <falcon@freecalypso.org>
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28 and one Rx path switch selecting between DCS and PCS, plus 3 discrete SAW
00216b7cfc4d Quadband-ideas: better explanation of DCS/PCS switching or diplexing mystery
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29 filter components (outside the ASM) for the 3 bands.
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30
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31 This article outlines some ideas for how we may be able to move from this RFFE
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32 to a different one, replacing our current mystery ASM with something less
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33 mysterious and better documented, and improving our radio capability from
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Mychaela Falconia <falcon@freecalypso.org>
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34 triband to quadband at the same time.
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35
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Mychaela Falconia <falcon@freecalypso.org>
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36 Epcos M034F
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37 ===========
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38
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Mychaela Falconia <falcon@freecalypso.org>
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39 TI's Leonardo+ and E-Sample boards used a magic component made by Epcos (the
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Mychaela Falconia <falcon@freecalypso.org>
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40 canonical SAW filter manufacturer during that era) called M034 or M034F (the
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Mychaela Falconia <falcon@freecalypso.org>
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41 exact proper designation is unclear). It was an integrated quadband FEM,
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Mychaela Falconia <falcon@freecalypso.org>
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42 integrating the antenna switch and SAW filters in one component package, with a
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Mychaela Falconia <falcon@freecalypso.org>
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43 special twist. The special twist is that even though there are 4 separate Rx
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Mychaela Falconia <falcon@freecalypso.org>
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44 band SAW filters inside that M034 "chip" module, corresponding to its advertised
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Mychaela Falconia <falcon@freecalypso.org>
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45 quadband capability, only 3 Rx signal path differential pairs come out of it,
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Mychaela Falconia <falcon@freecalypso.org>
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46 neatly corresponding to the 3 LNA inputs on TI's Rita transceiver. This twist
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Mychaela Falconia <falcon@freecalypso.org>
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47 is important because even though the Rita transceiver itself is fully quadband
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Mychaela Falconia <falcon@freecalypso.org>
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48 internally, it has only 3 LNA inputs, with GSM850 and EGSM bands sharing the
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Mychaela Falconia <falcon@freecalypso.org>
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49 same LNA input while each of DCS and PCS get their own.
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50
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Mychaela Falconia <falcon@freecalypso.org>
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51 We do have an M034F.pdf datasheet for this magic component (came along with
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Mychaela Falconia <falcon@freecalypso.org>
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52 Calypso and Leonardo docs), and the block diagram on page 6 shows the magic
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Mychaela Falconia <falcon@freecalypso.org>
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53 quite clearly: there is a baseband-controlled switch selecting between EGSM Rx
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Mychaela Falconia <falcon@freecalypso.org>
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54 and GSM850 Rx (in addition to the two usual Tx switches), this switch directs
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Mychaela Falconia <falcon@freecalypso.org>
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55 the low band Rx path toward one of two different SAW filters, and the outputs
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Mychaela Falconia <falcon@freecalypso.org>
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56 of those two filters are then joined. The high band Rx path always goes to both
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57 DCS and PCS band SAW filters, and each of those high band Rx SAW filters gets
00216b7cfc4d Quadband-ideas: better explanation of DCS/PCS switching or diplexing mystery
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58 its own output going to its own dedicated Rita LNA input. Note the lack of a
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59 baseband-controlled switch between DCS and PCS in the high band Rx path: this
00216b7cfc4d Quadband-ideas: better explanation of DCS/PCS switching or diplexing mystery
Mychaela Falconia <falcon@freecalypso.org>
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60 switch is present in all triband RFFE designs I have seen, thus a big question
00216b7cfc4d Quadband-ideas: better explanation of DCS/PCS switching or diplexing mystery
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61 is raised as to how this magic M034 component functions without one. I can
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62 think of two possibilities:
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63
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64 Possibility 1: perhaps they do a 50/50 split of the total incoming energy
00216b7cfc4d Quadband-ideas: better explanation of DCS/PCS switching or diplexing mystery
Mychaela Falconia <falcon@freecalypso.org>
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65 between DCS and PCS Rx paths, with each path suffering by 3 dB as a result.
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66
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67 Possibility 2: perhaps by virtue of integrating the ASM and the SAW filters
00216b7cfc4d Quadband-ideas: better explanation of DCS/PCS switching or diplexing mystery
Mychaela Falconia <falcon@freecalypso.org>
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68 into a single monolithic FEM, Epcos found some way to have unswitched DCS and
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69 PCS Rx without incurring that 3 dB penalty. Perhaps they successfully
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70 implemented some form of frequency diplexer such that out of the total incoming
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71 energy picked up by the wideband antenna, DCS downlink frequencies go through
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72 the DCS Rx SAW filter, PCS downlink frequencies go through the PCS Rx SAW
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73 filter, and no needless losses are incurred. This hypothesis is supported by
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74 the observation that the available M034F.pdf document gives approximately the
00216b7cfc4d Quadband-ideas: better explanation of DCS/PCS switching or diplexing mystery
Mychaela Falconia <falcon@freecalypso.org>
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75 same insertion loss numbers for all 4 Rx bands, i.e., the same between the
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76 switched low bands and the unswitched high bands. Note that they could not
00216b7cfc4d Quadband-ideas: better explanation of DCS/PCS switching or diplexing mystery
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77 have similarly eliminated the GSM850 Rx switch: both EGSM Rx and GSM850 Rx need
00216b7cfc4d Quadband-ideas: better explanation of DCS/PCS switching or diplexing mystery
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78 to go to the same LNA on the transceiver, thus a switch is needed somewhere.
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79
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Mychaela Falconia <falcon@freecalypso.org>
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80 I (Mother Mychaela) would absolutely love to play with an M034-based quadband
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Mychaela Falconia <falcon@freecalypso.org>
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81 Calypso+Iota+Rita board in my lab with the trusty CMU200 instrument, and to see
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Mychaela Falconia <falcon@freecalypso.org>
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82 how well it actually performs, especially in comparison with our current
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Mychaela Falconia <falcon@freecalypso.org>
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83 OM-based triband version. However, in all of my years of searching I have never
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Mychaela Falconia <falcon@freecalypso.org>
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84 found a physical Leonardo board (any version), nor have we ever found any
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Mychaela Falconia <falcon@freecalypso.org>
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85 Leonardo PCB layout files which would allow us to build a modern recreation -
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Mychaela Falconia <falcon@freecalypso.org>
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86 thus the magic of M034 remains elusive.
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87
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Mychaela Falconia <falcon@freecalypso.org>
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88 Unless a miracle happens and we are able to obtain either a physical Leonardo+
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Mychaela Falconia <falcon@freecalypso.org>
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89 board or a PADS PCB file for one, there is no quick or low-effort way to "just
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Mychaela Falconia <falcon@freecalypso.org>
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90 try" this M034 RFFE. Instead if we wish to build a FreeCalypso board with this
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Mychaela Falconia <falcon@freecalypso.org>
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91 RFFE, it would have to be "the full 9 yards": a full-blown PCB design and layout
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Mychaela Falconia <falcon@freecalypso.org>
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92 effort. There is no way to just "drop" the M034 into our existing PCB design
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Mychaela Falconia <falcon@freecalypso.org>
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93 in the place of our current triband RFFE, it would have to be either a very
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Mychaela Falconia <falcon@freecalypso.org>
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94 disruptive RF section layout change or an entirely new PCB layout, making this
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Mychaela Falconia <falcon@freecalypso.org>
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95 idea very open-ended - an open-ended venture with quite uncertain outcome, but
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Mychaela Falconia <falcon@freecalypso.org>
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96 with a high dollar cost attached to it. Given the massive effort required and
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Mychaela Falconia <falcon@freecalypso.org>
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97 PCB layout labor costs, I currently have no active plans to pursue this idea
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Mychaela Falconia <falcon@freecalypso.org>
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98 beyond hypothetical.
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99
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Mychaela Falconia <falcon@freecalypso.org>
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100 Commissioning a new custom RF FEM
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101 =================================
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102
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Mychaela Falconia <falcon@freecalypso.org>
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103 Here is a wild thought: what if instead of twisting over backwards trying to
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Mychaela Falconia <falcon@freecalypso.org>
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104 hammer an existing RF FEM like M034F into our not-quite-fitting PCB design, we
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Mychaela Falconia <falcon@freecalypso.org>
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105 were to get an entirely new FEM made specially for us, made exactly the way we
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Mychaela Falconia <falcon@freecalypso.org>
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106 need it? If we were to venture that way, I would ask for a FEM very similar
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Mychaela Falconia <falcon@freecalypso.org>
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107 conceptually to M034F, but with a few changes:
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Mychaela Falconia <falcon@freecalypso.org>
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108
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Mychaela Falconia <falcon@freecalypso.org>
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109 1) Instead of diplexing between DCS and PCS SAW filter inputs with a 50/50
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Mychaela Falconia <falcon@freecalypso.org>
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110 energy split, implement another switch (just like the GSM850 Rx switch) for
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Mychaela Falconia <falcon@freecalypso.org>
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111 PCS Rx, exactly the same way how it is done in classic triband designs like
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Mychaela Falconia <falcon@freecalypso.org>
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112 our current OM-based one. This change should eliminate the extra 3 dB
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Mychaela Falconia <falcon@freecalypso.org>
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113 penalty which I assume (for lack of experimental data) must happen with the
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Mychaela Falconia <falcon@freecalypso.org>
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114 existing M034 FEM. Or as an alternative to making this change, if someone
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Mychaela Falconia <falcon@freecalypso.org>
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115 who is more knowledgeable than me in this area can explain to me why it isn't
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Mychaela Falconia <falcon@freecalypso.org>
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116 necessary, I would accept that option as well.
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Mychaela Falconia <falcon@freecalypso.org>
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117
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Mychaela Falconia <falcon@freecalypso.org>
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118 2) I would ask for a rearranged pinout: the existing M034F pinout does not fit
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Mychaela Falconia <falcon@freecalypso.org>
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119 at all into our OM-based PCB layout, but it would fit much better with some
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Mychaela Falconia <falcon@freecalypso.org>
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120 rearrangement.
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121
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122 3) The hypothetical M034-like FEM would fit into our OM-based PCB layout a lot
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123 better if it were made a little smaller than the 8.2x5.5 mm size of M034F.
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124 Considering that the original M034F was created some 15-16 y ago, I assume
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125 that it should be possible to make a smaller version in 2020 or 2021 or
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126 whenever.
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127
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128 Timeline sequentiality
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129 ======================
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130
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131 All of the above ideas will be considered on a less hypothetical level after we
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132 get our already-committed FCM40 product built. FCM40 will be a modem module in
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133 the same 56.5x36 mm form factor as Huawei GTM900 (with a mostly-compatible FPC
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134 interface with only a few changes), featuring the same OM-based triband modem
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135 core as FCDEV3B V2. The reason for this sequencing is that our current FCDEV3B
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136 suffers from a couple of issues which FCM40 is expected to solve:
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137
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138 1) FCDEV3B has a very tight PCB layout: not only do we have the tightly laid out
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139 core from GTA02, but also the whole board is quite small for the implemented
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Mychaela Falconia <falcon@freecalypso.org>
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140 peripheral complexity, imposing further constraints from all sides. This
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141 tight and complex layout makes a poor choice of starting point for bold
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Mychaela Falconia <falcon@freecalypso.org>
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142 experiments like RFFE changes.
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143
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144 2) FCDEV3B is locked into Altium. Layout data migration from Altium to FOSS
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Mychaela Falconia <falcon@freecalypso.org>
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145 appears to be much less feasible than migration from PADS to FOSS, thus
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Mychaela Falconia <falcon@freecalypso.org>
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146 freeing our PCB layout from the clutches of proprietary software will most
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Mychaela Falconia <falcon@freecalypso.org>
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147 likely require giving up (or rather setting aside) all of FCDEV3B new layout
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148 and going back to the GTA02 starting point, which is in PADS format rather
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149 than Altium. Redoing all of FCDEV3B anew does not sound appealing at all,
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150 but the much simpler FCM40 board offers a perfect opportunity for a fresh
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151 start.
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152
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153 FCM40 will have exactly the same OM-based triband RFFE as our current FCDEV3B,
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154 but it will be a much simpler board, and if we can get it done in FOSS instead
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Mychaela Falconia <falcon@freecalypso.org>
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155 of continuing the Altium track, then we would have a very solid reference and a
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Mychaela Falconia <falcon@freecalypso.org>
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156 good starting point for potential RFFE change experiments.
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157
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158 Firmware compatibility
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159 ======================
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160
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161 Our current FreeCalypso firmwares drive TSPACT RFFE control signals as follows
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162 on FC hw family targets (CONFIG_TARGET_FCFAM):
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163
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164 TSPACT1 = Rx PCS band
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165 TSPACT2 = Tx high bands
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166 TSPACT4 = Tx low bands
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167 TSPACT5 = Rx GSM850 band
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168
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169 The driving of TSPACT1, TSPACT2 and TSPACT4 matches the way these signals have
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170 been assigned by Openmoko and thus the way they function on our current OM-based
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171 triband RFFE, whereas TSPACT5 is a new signal which is not wired anywhere on
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172 our current FCDEV3B. This signal driving arrangement is expected to be
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173 compatible with all 3 RFFE hw possibilities under consideration:
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174
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175 * On our current OM-based triband RFFE it works as is.
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176
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177 * If we use Epcos M034 or a semi-clone thereof that has the two Tx switches and
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178 a GSM850 Rx switch but no PCS Rx switch, then we will need to connect TSPACT2,
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179 TSPACT4 and TSPACT5 per the table above, and leave TSPACT1 unconnected.
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180
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181 * If we get a new M034-like FEM custom-made with a full set of all 4 switches,
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182 then all 4 TSPACT signals will need to be connected per the table above.