Hello FreeCalypso community, It's time for me to give you all an update on what I've been doing during the last few weeks of silence. I still desire to produce more FCDEV3B boards, but I am currently fixated on the need to have a proper setup for RF calibration.
Background info for those who have joined the mailing list recently: every newly produced GSM MS (mobile station) device needs to have its RF tract tested and calibrated in both Rx and Tx directions for each band, and the standard test equipment for these calibration procedures is Rohde&Schwarz CMU200. I bought a CMU200 on ebay back in March (these instruments are in fairly abundant supply on ebay), and I have used it successfully to perform some calibration procedures, first manual, then automated. However, there are two issues which I need to resolve before further progress can be made, and the road toward the resolution of these two issues is taking quite a while. The two issues are: 1: The CMU200 can act as an RF signal analyzer or as a signal generator, or both at the same time. Some units, including mine, are also equipped with a secondary signal generator in addition to the main one. On my CMU200 the RF analyzer function works fine as does the auxiliary Tx generator, but the main signal generator is broken: no signal or an extremely weak signal comes out. There are two further problems with this broken signal generator issue: 1a: I discovered the fault quite late: I bought this CMU200 back in March, and because the RF analyzer function works, I thought that the whole unit was totally fine until some time in late May when I reached the point of needing the signal generator function. So that's two months right there from the time of purchase, ticking away against the seller's 90 day warranty. 1b: When I discovered the Tx problem in late May, I started the dialogue with the seller about it, and they seemed willing to help me, but then I needed to prepare for my presentation at REcon Montreal in mid-June (not only the presentation itself, but also logistical preparations for the big trip), and I had no time to deal with the CMU200 issue. I was only able to get back to this CMU200 about a week ago (early July), and by now the 90 day warranty is over. What I've been doing is I've been trying to troubleshoot this broken CMU200 signal gen problem on my own, i.e., trying to determine which internal module is at fault and get a replacement for that module. The CMU200 service manual explains what the major internal modules are and what they do; the two modules of interest for this troubleshooting chase are the RxTx board and the RF front end (FE). The 4 N-type RF connectors on the front panel of the CMU200 (where DUTs are connected) are part of the FE, and the FE has internal connections (made with pieces of rigid coax) to the RxTx board. In the Tx path the RxTx board generates the RF signal from an internal IF input that comes from another board, i.e., upconverts to the desired output frequency, and the RF output from the RxTx board goes to the FE via a piece of rigid coax. The FE switches this Tx output to one of 3 possible external connectors (of the 4 external RF connectors in total, two are bidirectional, RF3 is output only and RF4 is input only), and presumably does some amplification or attenuation as well depending on which output is selected, as the specified output levels for RF1, RF2 and RF3 are quite different. In my most recent experiment done only yesterday, I opened the CMU200 box and disconnected the rigid coax piece that carries the Tx output from the RxTx board to the FE. I then took the same N-to-SMA cable which I've been using to connect the CMU200 to FreeCalypso DUTs, connected the SMA end of this cable to the now-exposed output from the RxTx board, and connected the N end to front panel RF ports of the same CMU200 - I tried both the usual RF2 port as well as the more sensitive input-only RF4. I powered the unit on in this state, commanded it to transmit, and used the CMU200's own RF analyzer to measure the power level being put out - this is the CMU200 function which worked OK for measuring power levels put out by FreeCalypso DUTs, so why not. What I saw were really low signal levels. Whether I commanded the unit to transmit on RF1, RF2 or RF3 at the respective maximum power levels of -27 dBm, -10 dBm and +13 dBm, the signal coming out of the RxTx board maxed out somewhere around -70 dBm. Now we are still crippled by not knowing what the signal level *should* be at this point, i.e., how much amplification or attenuation happens in the FE in each mode, but around -70 dBm max seems too low to me. Hence my finger of suspicion points at the RxTx board, rather than the FE. Earlier today I went to ebay, searched for a spare/replacement CMU200 RxTx module, and found someone in Israel who had one of these modules for sale. It was $165 USD including shipping, so I grabbed it. Now I need to wait for it to arrive, and the uncertainties galore: we still have no solid proof, only a suspicion, that the RxTx module is the one at fault, and there is no certainty that the replacement module is good either. But this is all I am currently able to do in attempts to repair this faulty CMU200. Now onto the other major CMU200 issue: calibration of the unit itself. Out of the 3 calibration parts required for Calypso GSM devices (VCXO, Rx gain, Tx power levels) the last one (Tx power levels) is the trickiest, and the only one for which I have not yet developed the necessary automated calibration software. The biggest difficulty with Tx power level calibration is that the calibration process absolutely needs to know what the produced power levels are with high accuracy, and any not-accounted-for losses in the interconnecting RF cables or any slight miscalibration of the measuring instrument itself will throw the process off badly. Thus the CMU200 itself needs to be in good calibration standing, and the loss in the interconnecting cables needs to be measured and accounted for. The CMU200 that I have came from ebay, and its calibration history is unknown. Given that the seller shipped it to me without having caught the fault in the signal generator function, I feel that it would be foolish to put blind trust in its calibration status: while the Rx path (the RF analyzer function) works, we have no way of knowing if the power level measurements it reports are accurate within 0.5 dBm like they are supposed to be per the datasheet, or if that calibration is off. A related issue is that if I replace the RxTx board inside the unit in order to find the Tx path problem, the unit will almost certainly need to be recalibrated before its power level measurements can be trusted to the required degree of accuracy. And the problem with getting a CMU200 unit (re)calibrated, whether it be the unit I already have after RxTx board replacement or another cheap CMU200 off ebay with unknown calibration history, is that I still have not found any shop where I can get this (re)calibration done; I found some places that might be able to do it, but I haven't got a confirmation for sure, and I am afraid that their asking price for the calibration service may be astronomical. So far the only will-work-for-sure option I've been able to find would be to buy one of these expensive units: http://www.ebay.com/itm/222146419296 Their warranty is only 30 days, but now that I have already climbed the CMU200 learning curve, 30 days should be plenty enough to fully test the unit immediately upon arrival. And these expensive listings are the only ones I could find where the seller explicitly warrants that the unit is properly calibrated. The problem, of course, is the price. Midwest ATE (the company selling the unit linked above) has several CMU200 units with different option configs (most of these options aren't needed for FreeCalypso RF testing and calibration), and the one linked above with the asking price of $2995 USD is their lowest. Other companies with similar offerings that I found ask even more. Now they do have the "make offer" option, but I doubt that they would budge below $2000 USD, and the latter figure is still more than I can afford right now. So if any of the new people on the list (from REcon etc) would like to help the project move forward faster, and can donate $2000 USD toward the purchase of a known-good CMU200, here is your chance. :-) Otherwise I shall continue with first trying to fix the CMU200 which I already have, and then trying to get it calibrated, and the whole process may take a few months, during which time there will be no further progress on FreeCalypso. Hasta la Victoria, Siempre, Mychaela aka The Mother _______________________________________________ Community mailing list [email protected] https://www.freecalypso.org/mailman/listinfo/community
