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Quadband-ideas article written
author Mychaela Falconia <falcon@freecalypso.org>
date Tue, 22 Oct 2019 00:04:25 +0000
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19:f68ca40fa5c1 20:3fa4006696d0
1 Our current Openmoko-based Calypso+RF modem core is very very good, but it has
2 one shortcoming compared to TI's Leonardo+ reference design: it is triband
3 rather than quadband. This triband restriction stems from OM's use of discrete
4 antenna switch and SAW filter components as opposed to an integrated FEM (front
5 end module) like on Leonardo+. In addition to the band restriction, our current
6 triband RF design suffers from one other very unpleasant problem: we have no
7 datasheet for the antenna switch component which we have to use. We know from
8 Openmoko's BOM data that the manufacturer is Darfon and that the part number for
9 this antenna switch component is ASM4532T0P06-1, we are able to buy this part
10 from our Chinese grey market suppliers, we build our boards with these parts and
11 our boards do work perfectly fine when we get a good batch, but we have to do
12 this entire process blindly, without any datasheet or other documentation for
13 this mystery part.
14
15 This article outlines some ideas for how we may be able to move from this RFFE
16 to a different one, replacing our current mystery antenna switch with something
17 less mysterious and better documented, and improving our radio capability from
18 triband to quadband at the same time.
19
20 Epcos M034F
21 ===========
22
23 TI's Leonardo+ and E-Sample boards used a magic component made by Epcos (the
24 canonical SAW filter manufacturer during that era) called M034 or M034F (the
25 exact proper designation is unclear). It was an integrated quadband FEM,
26 integrating the antenna switch and SAW filters in one component package, with a
27 special twist. The special twist is that even though there are 4 separate Rx
28 band SAW filters inside that M034 "chip" module, corresponding to its advertised
29 quadband capability, only 3 Rx signal path differential pairs come out of it,
30 neatly corresponding to the 3 LNA inputs on TI's Rita transceiver. This twist
31 is important because even though the Rita transceiver itself is fully quadband
32 internally, it has only 3 LNA inputs, with GSM850 and EGSM bands sharing the
33 same LNA input while each of DCS and PCS get their own.
34
35 We do have an M034F.pdf datasheet for this magic component (came along with
36 Calypso and Leonardo docs), and the block diagram on page 6 shows the magic
37 quite clearly: there is a baseband-controlled switch selecting between EGSM Rx
38 and GSM850 Rx (in addition to the two usual Tx switches), this switch directs
39 the low band Rx path toward one of two different SAW filters, and the outputs
40 of those two filters are then joined. The high band Rx path always goes to both
41 DCS and PCS band SAW filters (I assume it is a 50/50 split of the total incoming
42 energy, with each path suffering by 3 dB as a result), and each of those high
43 band Rx SAW filters gets its own output going to its own dedicated Rita LNA
44 input.
45
46 I (Mother Mychaela) would absolutely love to play with an M034-based quadband
47 Calypso+Iota+Rita board in my lab with the trusty CMU200 instrument, and to see
48 how well it actually performs, especially in comparison with our current
49 OM-based triband version. However, in all of my years of searching I have never
50 found a physical Leonardo board (any version), nor have we ever found any
51 Leonardo PCB layout files which would allow us to build a modern recreation -
52 thus the magic of M034 remains elusive.
53
54 Unless a miracle happens and we are able to obtain either a physical Leonardo+
55 board or a PADS PCB file for one, there is no quick or low-effort way to "just
56 try" this M034 RFFE. Instead if we wish to build a FreeCalypso board with this
57 RFFE, it would have to be "the full 9 yards": a full-blown PCB design and layout
58 effort. There is no way to just "drop" the M034 into our existing PCB design
59 in the place of our current triband RFFE, it would have to be either a very
60 disruptive RF section layout change or an entirely new PCB layout, making this
61 idea very open-ended - an open-ended venture with quite uncertain outcome, but
62 with a high dollar cost attached to it. Given the massive effort required and
63 PCB layout labor costs, I currently have no active plans to pursue this idea
64 beyond hypothetical.
65
66 Commissioning a new custom RF FEM
67 =================================
68
69 Here is a wild thought: what if instead of twisting over backwards trying to
70 hammer an existing RF FEM like M034F into our not-quite-fitting PCB design, we
71 were to get an entirely new FEM made specially for us, made exactly the way we
72 need it? If we were to venture that way, I would ask for a FEM very similar
73 conceptually to M034F, but with a few changes:
74
75 1) Instead of diplexing between DCS and PCS SAW filter inputs with a 50/50
76 energy split, implement another switch (just like the GSM850 Rx switch) for
77 PCS Rx, exactly the same way how it is done in classic triband designs like
78 our current OM-based one. This change should eliminate the extra 3 dB
79 penalty which I assume (for lack of experimental data) must happen with the
80 existing M034 FEM. Or as an alternative to making this change, if someone
81 who is more knowledgeable than me in this area can explain to me why it isn't
82 necessary, I would accept that option as well.
83
84 2) I would ask for a rearranged pinout: the existing M034F pinout does not fit
85 at all into our OM-based PCB layout, but it would fit much better with some
86 rearrangement.
87
88 3) The hypothetical M034-like FEM would fit into our OM-based PCB layout a lot
89 better if it were made a little smaller than the 8.2x5.5 mm size of M034F.
90 Considering that the original M034F was created some 15-16 y ago, I assume
91 that it should be possible to make a smaller version in 2020 or 2021 or
92 whenever.
93
94 Timeline sequentiality
95 ======================
96
97 All of the above ideas will be considered on a less hypothetical level after we
98 get our already-committed FCM40 product built. FCM40 will be a modem module in
99 the same 56.5x36 mm form factor as Huawei GTM900 (with a mostly-compatible FPC
100 interface with only a few changes), featuring the same OM-based triband modem
101 core as FCDEV3B V2. The reason for this sequencing is that our current FCDEV3B
102 suffers from a couple of issues which FCM40 is expected to solve:
103
104 1) FCDEV3B has a very tight PCB layout: not only do we have the tightly laid out
105 core from GTA02, but also the whole board is quite small for the implemented
106 peripheral complexity, imposing further constraints from all sides. This
107 tight and complex layout makes a poor choice of starting point for bold
108 experiments like RFFE changes.
109
110 2) FCDEV3B is locked into Altium. Layout data migration from Altium to FOSS
111 appears to be much less feasible than migration from PADS to FOSS, thus
112 freeing our PCB layout from the clutches of proprietary software will most
113 likely require giving up (or rather setting aside) all of FCDEV3B new layout
114 and going back to the GTA02 starting point, which is in PADS format rather
115 than Altium. Redoing all of FCDEV3B anew does not sound appealing at all,
116 but the much simpler FCM40 board offers a perfect opportunity for a fresh
117 start.
118
119 FCM40 will have exactly the same OM-based triband RFFE as our current FCDEV3B,
120 but it will be a much simpler board, and if we can get it done in FOSS instead
121 of continuing the Altium track, then we would have a very solid reference and a
122 good starting point for potential RFFE change experiments.
123
124 Firmware compatibility
125 ======================
126
127 Our current FreeCalypso firmwares drive TSPACT RFFE control signals as follows
128 on FC hw family targets (CONFIG_TARGET_FCFAM):
129
130 TSPACT1 = Rx PCS band
131 TSPACT2 = Tx high bands
132 TSPACT4 = Tx low bands
133 TSPACT5 = Rx GSM850 band
134
135 The driving of TSPACT1, TSPACT2 and TSPACT4 matches the way these signals have
136 been assigned by Openmoko and thus the way they function on our current OM-based
137 triband RFFE, whereas TSPACT5 is a new signal which is not wired anywhere on
138 our current FCDEV3B. This signal driving arrangement is expected to be
139 compatible with all 3 RFFE hw possibilities under consideration:
140
141 * On our current OM-based triband RFFE it works as is.
142
143 * If we use Epcos M034 or a semi-clone thereof that has the two Tx switches and
144 a GSM850 Rx switch but no PCS Rx switch, then we will need to connect TSPACT2,
145 TSPACT4 and TSPACT5 per the table above, and leave TSPACT1 unconnected.
146
147 * If we get a new M034-like FEM custom-made with a full set of all 4 switches,
148 then all 4 TSPACT signals will need to be connected per the table above.