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.

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.

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 aka D1016
=====================

TI's Leonardo+ and E-Sample boards used a magic component made by Epcos (the
canonical SAW filter manufacturer during that era) that was called M034 or M034F
at the time of TI's designs in question.  However, the same component is also
known by a completely different name of Epcos D1016; this new D1016 name is the
only one under which this component can be bought for the purpose of building
new quadband FreeCalypso hw, and the physical markings on the component package
only say "EPCOS D1016", no mention of M034.  We (FreeCalypso) have no idea as
to how and why this component got two completely different names.  Here is a
picture of the part:

https://www.freecalypso.org/members/falcon/pictures/Epcos_D1016.jpeg

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
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.  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
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, 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!

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.

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.

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
======================

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.  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.

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.

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.

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.