# HG changeset patch # User Mychaela Falconia # Date 1571702665 0 # Node ID 3fa4006696d0bf7670a918a68211cc44be7b1ccd # Parent f68ca40fa5c1d6abef1e7453c573544aa3f95efc Quadband-ideas article written diff -r f68ca40fa5c1 -r 3fa4006696d0 Quadband-ideas --- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/Quadband-ideas Tue Oct 22 00:04:25 2019 +0000 @@ -0,0 +1,148 @@ +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.