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+++ b/Quadband-ideas	Tue Oct 22 00:04:25 2019 +0000
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+Our current Openmoko-based Calypso+RF modem core is very very good, but it has
+one shortcoming compared to TI's Leonardo+ reference design: it is triband
+rather than quadband.  This triband restriction stems from OM's use of discrete
+antenna switch and SAW filter components as opposed to an integrated FEM (front
+end module) like on Leonardo+.  In addition to the band restriction, our current
+triband RF design suffers from one other very unpleasant problem: we have no
+datasheet for the antenna switch component which we have to use.  We know from
+Openmoko's BOM data that the manufacturer is Darfon and that the part number for
+this antenna switch component is ASM4532T0P06-1, we are able to buy this part
+from our Chinese grey market suppliers, we build our boards with these parts and
+our boards do work perfectly fine when we get a good batch, but we have to do
+this entire process blindly, without any datasheet or other documentation for
+this mystery part.
+
+This article outlines some ideas for how we may be able to move from this RFFE
+to a different one, replacing our current mystery antenna switch with something
+less mysterious and better documented, and improving our radio capability from
+triband to quadband at the same time.
+
+Epcos M034F
+===========
+
+TI's Leonardo+ and E-Sample boards used a magic component made by Epcos (the
+canonical SAW filter manufacturer during that era) called M034 or M034F (the
+exact proper designation is unclear).  It was an integrated quadband FEM,
+integrating the antenna switch and SAW filters in one component package, with a
+special twist.  The special twist is that even though there are 4 separate Rx
+band SAW filters inside that M034 "chip" module, corresponding to its advertised
+quadband capability, only 3 Rx signal path differential pairs come out of it,
+neatly corresponding to the 3 LNA inputs on TI's Rita transceiver.  This twist
+is important because even though the Rita transceiver itself is fully quadband
+internally, it has only 3 LNA inputs, with GSM850 and EGSM bands sharing the
+same LNA input while each of DCS and PCS get their own.
+
+We do have an M034F.pdf datasheet for this magic component (came along with
+Calypso and Leonardo docs), and the block diagram on page 6 shows the magic
+quite clearly: there is a baseband-controlled switch selecting between EGSM Rx
+and GSM850 Rx (in addition to the two usual Tx switches), this switch directs
+the low band Rx path toward one of two different SAW filters, and the outputs
+of those two filters are then joined.  The high band Rx path always goes to both
+DCS and PCS band SAW filters (I assume it is a 50/50 split of the total incoming
+energy, with each path suffering by 3 dB as a result), and each of those high
+band Rx SAW filters gets its own output going to its own dedicated Rita LNA
+input.
+
+I (Mother Mychaela) would absolutely love to play with an M034-based quadband
+Calypso+Iota+Rita board in my lab with the trusty CMU200 instrument, and to see
+how well it actually performs, especially in comparison with our current
+OM-based triband version.  However, in all of my years of searching I have never
+found a physical Leonardo board (any version), nor have we ever found any
+Leonardo PCB layout files which would allow us to build a modern recreation -
+thus the magic of M034 remains elusive.
+
+Unless a miracle happens and we are able to obtain either a physical Leonardo+
+board or a PADS PCB file for one, there is no quick or low-effort way to "just
+try" this M034 RFFE.  Instead if we wish to build a FreeCalypso board with this
+RFFE, it would have to be "the full 9 yards": a full-blown PCB design and layout
+effort.  There is no way to just "drop" the M034 into our existing PCB design
+in the place of our current triband RFFE, it would have to be either a very
+disruptive RF section layout change or an entirely new PCB layout, making this
+idea very open-ended - an open-ended venture with quite uncertain outcome, but
+with a high dollar cost attached to it.  Given the massive effort required and
+PCB layout labor costs, I currently have no active plans to pursue this idea
+beyond hypothetical.
+
+Commissioning a new custom RF FEM
+=================================
+
+Here is a wild thought: what if instead of twisting over backwards trying to
+hammer an existing RF FEM like M034F into our not-quite-fitting PCB design, we
+were to get an entirely new FEM made specially for us, made exactly the way we
+need it?  If we were to venture that way, I would ask for a FEM very similar
+conceptually to M034F, but with a few changes:
+
+1) Instead of diplexing between DCS and PCS SAW filter inputs with a 50/50
+   energy split, implement another switch (just like the GSM850 Rx switch) for
+   PCS Rx, exactly the same way how it is done in classic triband designs like
+   our current OM-based one.  This change should eliminate the extra 3 dB
+   penalty which I assume (for lack of experimental data) must happen with the
+   existing M034 FEM.  Or as an alternative to making this change, if someone
+   who is more knowledgeable than me in this area can explain to me why it isn't
+   necessary, I would accept that option as well.
+
+2) I would ask for a rearranged pinout: the existing M034F pinout does not fit
+   at all into our OM-based PCB layout, but it would fit much better with some
+   rearrangement.
+
+3) The hypothetical M034-like FEM would fit into our OM-based PCB layout a lot
+   better if it were made a little smaller than the 8.2x5.5 mm size of M034F.
+   Considering that the original M034F was created some 15-16 y ago, I assume
+   that it should be possible to make a smaller version in 2020 or 2021 or
+   whenever.
+
+Timeline sequentiality
+======================
+
+All of the above ideas will be considered on a less hypothetical level after we
+get our already-committed FCM40 product built.  FCM40 will be a modem module in
+the same 56.5x36 mm form factor as Huawei GTM900 (with a mostly-compatible FPC
+interface with only a few changes), featuring the same OM-based triband modem
+core as FCDEV3B V2.  The reason for this sequencing is that our current FCDEV3B
+suffers from a couple of issues which FCM40 is expected to solve:
+
+1) FCDEV3B has a very tight PCB layout: not only do we have the tightly laid out
+   core from GTA02, but also the whole board is quite small for the implemented
+   peripheral complexity, imposing further constraints from all sides.  This
+   tight and complex layout makes a poor choice of starting point for bold
+   experiments like RFFE changes.
+
+2) FCDEV3B is locked into Altium.  Layout data migration from Altium to FOSS
+   appears to be much less feasible than migration from PADS to FOSS, thus
+   freeing our PCB layout from the clutches of proprietary software will most
+   likely require giving up (or rather setting aside) all of FCDEV3B new layout
+   and going back to the GTA02 starting point, which is in PADS format rather
+   than Altium.  Redoing all of FCDEV3B anew does not sound appealing at all,
+   but the much simpler FCM40 board offers a perfect opportunity for a fresh
+   start.
+
+FCM40 will have exactly the same OM-based triband RFFE as our current FCDEV3B,
+but it will be a much simpler board, and if we can get it done in FOSS instead
+of continuing the Altium track, then we would have a very solid reference and a
+good starting point for potential RFFE change experiments.
+
+Firmware compatibility
+======================
+
+Our current FreeCalypso firmwares drive TSPACT RFFE control signals as follows
+on FC hw family targets (CONFIG_TARGET_FCFAM):
+
+TSPACT1 = Rx PCS band
+TSPACT2 = Tx high bands
+TSPACT4 = Tx low bands
+TSPACT5 = Rx GSM850 band
+
+The driving of TSPACT1, TSPACT2 and TSPACT4 matches the way these signals have
+been assigned by Openmoko and thus the way they function on our current OM-based
+triband RFFE, whereas TSPACT5 is a new signal which is not wired anywhere on
+our current FCDEV3B.  This signal driving arrangement is expected to be
+compatible with all 3 RFFE hw possibilities under consideration:
+
+* On our current OM-based triband RFFE it works as is.
+
+* If we use Epcos M034 or a semi-clone thereof that has the two Tx switches and
+  a GSM850 Rx switch but no PCS Rx switch, then we will need to connect TSPACT2,
+  TSPACT4 and TSPACT5 per the table above, and leave TSPACT1 unconnected.
+
+* If we get a new M034-like FEM custom-made with a full set of all 4 switches,
+  then all 4 TSPACT signals will need to be connected per the table above.