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Quadband-ideas: better explanation of DCS/PCS switching or diplexing mystery
author Mychaela Falconia <falcon@freecalypso.org>
date Sat, 09 Nov 2019 23:45:15 +0000
parents 3fa4006696d0
children 3799892b1a79
comparison
equal deleted inserted replaced
23:14391ad53281 24:00216b7cfc4d
1 Triband status quo
2 ==================
3
1 Our current Openmoko-based Calypso+RF modem core is very very good, but it has 4 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 5 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 6 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 7 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 8 end module) like on Leonardo+. In addition to the band restriction, our current
10 from our Chinese grey market suppliers, we build our boards with these parts and 13 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 14 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 15 this entire process blindly, without any datasheet or other documentation for
13 this mystery part. 16 this mystery part.
14 17
18 While we lack any official documentation for our triband ASM, we know its basic
19 logical function: it has two Tx inputs (low band and high band Tx coming from
20 the PA), 3 Rx outputs going to 3 separate SAW filters for the 3 supported bands,
21 and 3 switch control inputs. Two of these switch control inputs are Tx controls
22 (low band Tx enable and high band Tx enable) which appear to be the same across
23 all common RFFEs, whether they are 2-band (single region), triband or quadband.
24 The 3rd switch control input gets a logic high voltage applied to it during PCS
25 band Rx, thus it appears to be a switch that diverts the high band Rx path
26 between DCS and PCS SAW filters. The same arrangement is found in most other
27 triband phones and modems from that era, i.e., they also have two Tx switches
28 and one Rx path switch selecting between DCS and PCS, plus 3 discrete SAW
29 filter components (outside the ASM) for the 3 bands.
30
15 This article outlines some ideas for how we may be able to move from this RFFE 31 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 32 to a different one, replacing our current mystery ASM with something less
17 less mysterious and better documented, and improving our radio capability from 33 mysterious and better documented, and improving our radio capability from
18 triband to quadband at the same time. 34 triband to quadband at the same time.
19 35
20 Epcos M034F 36 Epcos M034F
21 =========== 37 ===========
22 38
36 Calypso and Leonardo docs), and the block diagram on page 6 shows the magic 52 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 53 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 54 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 55 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 56 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 57 DCS and PCS band SAW filters, and each of those high band Rx SAW filters gets
42 energy, with each path suffering by 3 dB as a result), and each of those high 58 its own output going to its own dedicated Rita LNA input. Note the lack of a
43 band Rx SAW filters gets its own output going to its own dedicated Rita LNA 59 baseband-controlled switch between DCS and PCS in the high band Rx path: this
44 input. 60 switch is present in all triband RFFE designs I have seen, thus a big question
61 is raised as to how this magic M034 component functions without one. I can
62 think of two possibilities:
63
64 Possibility 1: perhaps they do a 50/50 split of the total incoming energy
65 between DCS and PCS Rx paths, with each path suffering by 3 dB as a result.
66
67 Possibility 2: perhaps by virtue of integrating the ASM and the SAW filters
68 into a single monolithic FEM, Epcos found some way to have unswitched DCS and
69 PCS Rx without incurring that 3 dB penalty. Perhaps they successfully
70 implemented some form of frequency diplexer such that out of the total incoming
71 energy picked up by the wideband antenna, DCS downlink frequencies go through
72 the DCS Rx SAW filter, PCS downlink frequencies go through the PCS Rx SAW
73 filter, and no needless losses are incurred. This hypothesis is supported by
74 the observation that the available M034F.pdf document gives approximately the
75 same insertion loss numbers for all 4 Rx bands, i.e., the same between the
76 switched low bands and the unswitched high bands. Note that they could not
77 have similarly eliminated the GSM850 Rx switch: both EGSM Rx and GSM850 Rx need
78 to go to the same LNA on the transceiver, thus a switch is needed somewhere.
45 79
46 I (Mother Mychaela) would absolutely love to play with an M034-based quadband 80 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 81 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 82 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 83 OM-based triband version. However, in all of my years of searching I have never