--- a/Quadband-ideas	Wed Oct 23 00:43:21 2019 +0000
+++ b/Quadband-ideas	Sat Nov 09 23:45:15 2019 +0000
@@ -1,3 +1,6 @@
+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
@@ -12,9 +15,22 @@
 this entire process blindly, without any datasheet or other documentation for
 this mystery part.
 
+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.
+
 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
+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.
 
 Epcos M034F
@@ -38,10 +54,28 @@
 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.
+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.
 
 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