--- a/Quadband-ideas	Sat Dec 14 06:24:24 2019 +0000
+++ b/Quadband-ideas	Sat Jan 25 22:41:11 2020 +0000
@@ -1,44 +1,48 @@
-Triband status quo
-==================
-
-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.
+I, Mother Mychaela, hold the belief that any newly designed FreeCalypso hardware
+made in 2020 or later needs to be fully quadband, supporting all 4 possible GSM
+frequency bands of 850, 900, 1800 and 1900 MHz, nothing less.  Furthermore, we
+do not need to invent or innovate anything in order to produce a quadband
+Calypso phone or modem: TI already had a quadband reference design back in the
+early 2000s, called Leonardo or Leonardo+ (the exact proper designation is
+unclear), we just need to resurrect it, adding some updates of our own like we
+always do.
 
-While we lack any official documentation for our triband ASM, we know its basic
-logical function: it has two Tx inputs (low band and high band Tx coming from
-the PA), 3 Rx outputs going to 3 separate SAW filters for the 3 supported bands,
-and 3 switch control inputs.  Two of these switch control inputs are Tx controls
-(low band Tx enable and high band Tx enable) which appear to be the same across
-all common RFFEs, whether they are 2-band (single region), triband or quadband.
-The 3rd switch control input gets a logic high voltage applied to it during PCS
-band Rx, thus it appears to be a switch that diverts the high band Rx path
-between DCS and PCS SAW filters.  The same arrangement is found in most other
-triband phones and modems from that era, i.e., they also have two Tx switches
-and one Rx path switch selecting between DCS and PCS, plus 3 discrete SAW
-filter components (outside the ASM) for the 3 bands.
+Our current FCDEV3B modem board is triband rather than quadband because of
+historical circumstances: while I had *always* desired a quadband solution
+since the very beginning of FreeCalypso (since I started gathering TI Calypso
+docs from Chinese sites in 2011), back in 2015 we did not have the necessary
+know-how to confidently resurrect TI's lost quadband reference design by doing
+a new PCB layout on the basis of the available surviving documentation, which
+consists of just schematics and a floorplan drawing - instead going with
+Openmoko's PCB layout and their triband RFFE was the only viable option at that
+time.  We have now successfully produced both 900 MHz and 850 MHz versions of
+our FCDEV3B (different SAW filter parts populated on the same PCB footprint)
+and both work flawlessly as verified with our CMU200 instrument, so we do
+already support all 4 GSM frequency bands in a way - but only 3 at a time with
+our current hardware.
 
-This article outlines some ideas for how we may be able to move from this RFFE
-to a different one, replacing our current mystery ASM with something less
-mysterious and better documented, and improving our radio capability from
-triband to quadband at the same time.
+A major breakthrough happened in December of 2019: we found a certain extremely
+obscure historical commercial Calypso modem module that is almost a verbatim
+clone of the core section of TI's Leonardo+ quadband reference design, and the
+knowledge gained from examination of this obscure historically-made modem module
+(both physical PCB reverse eng and evaluation of its RF performance with our
+CMU200 instrument) has given us the necessary confidence boost with this
+particular way of implementing a quadband GSM MS.
 
 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,
+canonical SAW filter manufacturer during that era) that was called M034 or M034F
+at the time of TI's designs in question.  Epcos later gave it a completely
+different name when they released it into volume production, thus the name that
+is needed in order to buy this part and that is physically marked on the
+component package bears to resemblance to M034 - but we will not be able to
+disclose this other name until we physically produce our first FreeCalypso
+hardware product with this quadband FEM in it, meaning until and unless someone
+pays for it.
+
+The component in question is an integrated quadband FEM (front end module),
 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
@@ -46,7 +50,12 @@
 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.
+same LNA input while each of DCS and PCS get their own.  Thus this M034 FEM is
+special in that it facilitates building a quadband GSM MS using transceivers
+like TI Rita or Silabs Si4200 with only 3 LNA inputs, and the name M034 itself
+reflects this specialness: the digit 3 in the name refers to the 3 differential
+Rx signal paths, while the digit 4 refers to the 4 SAW filters inside which
+provide full quadband capability.
 
 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
@@ -56,104 +65,40 @@
 of those two filters are then joined.  The high band Rx path always goes to both
 DCS and PCS band SAW filters, and each of those high band Rx SAW filters gets
 its own output going to its own dedicated Rita LNA input.  Note the lack of a
-baseband-controlled switch between DCS and PCS in the high band Rx path: this
-switch is present in all triband RFFE designs I have seen, thus a big question
-is raised as to how this magic M034 component functions without one.  I can
-think of two possibilities:
-
-Possibility 1: perhaps they do a 50/50 split of the total incoming energy
-between DCS and PCS Rx paths, with each path suffering by 3 dB as a result.
-
-Possibility 2: perhaps by virtue of integrating the ASM and the SAW filters
-into a single monolithic FEM, Epcos found some way to have unswitched DCS and
-PCS Rx without incurring that 3 dB penalty.  Perhaps they successfully
-implemented some form of frequency diplexer such that out of the total incoming
-energy picked up by the wideband antenna, DCS downlink frequencies go through
-the DCS Rx SAW filter, PCS downlink frequencies go through the PCS Rx SAW
-filter, and no needless losses are incurred.  This hypothesis is supported by
-the observation that the available M034F.pdf document gives approximately the
-same insertion loss numbers for all 4 Rx bands, i.e., the same between the
-switched low bands and the unswitched high bands.  Note that they could not
-have similarly eliminated the GSM850 Rx switch: both EGSM Rx and GSM850 Rx need
-to go to the same LNA on the transceiver, thus a switch is needed somewhere.
+baseband-controlled switch between DCS and PCS in the high band Rx path!
 
-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
-=================================
+For a long time prior to the 2019-12 breakthrough I was concerned about this
+lack of a baseband-controlled switch between DCS and PCS in the high band Rx
+path: this switch is present in every triband RFFE design I am familiar with,
+including the one we got from Openmoko, and I was concerned that feeding the
+high band Rx path to both DCS and PCS SAW filters without a switch would
+introduce a 3 dB penalty into these high band Rx paths.  But when I got my
+hands on the newly discovered Tango modem modules, looked at the GMagic numbers
+written into their FFS as part of the manufacturer's production calibration and
+then independently verified them with our CMU200, all fears were dispelled: the
+Rx performance of this M034 FEM in all 4 bands is exactly the same as our
+current Openmoko-based triband RFFE, with GMagic around 200 half-dB units.
 
-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.
+Now that we have physical proof that our desired quadband RFFE based on this
+Epcos M034 FEM and other aspects of TI's Leonardo design that go with it really
+works exactly as we would like and has been used successfully by a historical
+commercial modem module manufacturer (albeit an obscure non-mainstream one),
+the way forward for FreeCalypso is clear: instead of continuing with Openmoko's
+PCB layout and triband RFFE for our future hardware products, we can confidently
+go with a new PCB layout based on Leonardo/Tango, using Epcos M034 as our FEM
+and achieving the full quadband capability we have always wanted.
 
-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.
+The matching networks ("RF black magic") that would need to be placed between
+M034 Rx outputs and Rita LNA inputs were also a big area of concern for a long
+time - I did not feel confident with blindly going with the matching networks
+depicted on Leonardo schematics (or the slightly different E-Sample version)
+with no ability to actually understand them - but the new Tango find has once
+again saved the day.  Tango uses exactly the same matching networks as depicted
+on Leonardo+ schematics (not the slightly different E-Sample version), or more
+precisely, it uses exactly the same topology (PCB layout), but with slightly
+different component values populated.  The safest approach for FreeCalypso is
+once again clear: copy Leonardo+ matching network topology and Tango component
+values.
 
 Firmware compatibility
 ======================
@@ -169,14 +114,68 @@
 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:
+our current FCDEV3B.  If and when we actually produce a new FreeCalypso hw
+product with the newly-confident M034 FEM, we will need to wire its control
+signals (going through logic-inverting PNP transistors as usual) to TSPACT2,
+TSPACT4 and TSPACT5 as mapped above, leaving TSPACT1 unconnected as there is no
+switch between DCS and PCS in the high band Rx path.  This new control signal
+wiring is an original FreeCalypso invention, different from TI's original
+Leonardo and E-Sample TSPACT signal assignments, but in my maternal opinion
+having the same fcmodem firmware build run on both legacy FCDEV3B and future FC
+quadband modems would be more valuable than paying tribute to the historical
+Leonardo.
 
-* On our current OM-based triband RFFE it works as is.
+Other ideas
+===========
+
+In a previous version of this article I entertained the idea of getting an
+entirely new M034-like FEM custom-designed and made specifically for us, asking
+someone with the necessary capabilities to produce a new FEM for us that would
+be very much like the historical M034F, but with just two key differences:
+
+1) A rearranged pinout so that the putative new FEM could be just "plopped"
+   into our current OM-based PCB layout in the place of OM's triband RFFE;
+
+2) Better-understood Rx output impedance specifications so that simpler and
+   more intuitive matching networks like OM's could be retained as well.
 
-* 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.
+That desire was driven by the mystery of Epcos M034 and the lack of empirical
+test data for it, stemming from the lack of an already-existing historically-
+made physical specimen that could be examined and tested: I simply could not
+stomach the idea of expending a monumental effort on a new PCB layout followed
+by thousands of dollars of cost to physically produce the new experimental board
+batch while doing it as a completely blind stab-in-the-dark with the unknown
+Epcos FEM.  But the discovery of Tango modems in 2019-12 has drastically changed
+the situation, and all mysteries and unknowns surrounding that M034 FEM have now
+been cleared.
+
+In the new reality that exists from 2020 onward that idea of commissioning the
+design of a new FEM to replace Epcos M034 no longer makes any sense and can be
+taken off the table: while it is true that using the existing M034 FEM will
+require doing an entirely new PCB layout (no more reuse of OM's version), it is
+now just a matter of doing the layout labor, no more risks or uncertainties,
+and the cost of this PCB layout job will certainly be much less than having a
+new FEM custom-made for us.
 
-* 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.
+Of course there are also myriad other, completely different ways of implementing
+a quadband GSM MS without using an M034-style FEM.  The approach used by TI in
+the days of Calypso, using FEMs that combine the antenna switch and SAW filters
+in one component while the PA is separate, fell out of fashion shortly
+afterward, replaced with a competing approach where the PA and the antenna
+switch are combined into one component while Rx SAW filters are external - the
+latter approach is featured on TI's I-Sample (LoCosto) board, and it is
+quadband - LoCosto has 4 separate LNA inputs, not 3 like Rita.  One could even
+implement the same approach while keeping the Calypso+Iota baseband, perhaps
+replacing TI's Rita transceiver with Silabs Si4210 (Aero II) that has 4 LNA
+inputs like LoCosto.  But such ideas are far outside the scope of FreeCalypso,
+so if anyone feels like pursuing them, feel free to do so on your own, not
+involving FC - my mission in life is NOT to invent or innovate anything new,
+instead I am all about resurrecting and bringing back to availability those
+perfectly good and perfectly working technical solutions which already existed
+once before, but which have been wrongfully discontinued and thus taken away
+from us.
+
+In the present situation, the goal of producing a quadband GSM MS while reusing
+as much of the already existing FreeCalypso IP and know-how as possible would
+be accomplished most efficiently by using the newly-confident M034 FEM and doing
+a new PCB layout with it, hence that is the approach I am currently pursuing.