Hello Csaba, I am not sure if you know this, wanted to mention it: you could easily measure your SDR’s oscillator accuracy using the kalibrate tool. It uses nearby GSM BTSs if I am correct to give you a good estimation about how much off your oscillator is. I used to use it with rtl-sdr a lot, because those USB dongles are crazy off in terms off accuracy. Once you have an offset figured out for the current temperature I guess you could try to offset the frequency set by osmocom by that amount. Maybe even try to expose it as a variable if needed. Other solution would be using some kind of a GPSDO or similar 10 MHz input if the LineSDR supports it.
Sorry again if this is known info, but wanted to chime in with some hopefully helpful bits. Cheers, Domi > 2020. okt. 26. dátummal, 16:08 időpontban Sipos Csaba <[email protected]> > írta: > > > Hi Ralph, > > I attached the patch for the country specific settings for Hungary, you can > apply it against "master" . TAs are not yet verified, but the phone goes > "green" with all TAs from 1 to 15. The RF part and country code are verified. > > > One thing, maybe your MCR has drifted, due to ageing?! > > Well, that was my first impression too, as if you look at my picture ( > http://www.imagebam.com/image/d17e881357285965 ) you will see that the center > of the downlink and uplink transmissions are not exactly 10MHz apart, they > have a couple kHz drift. On the picture the two markers are indicating the > nominal frequency of the channel. As you can see the uplink burst is actually > closer to the nominal frequency, compared to the downlink. It is not in the > standard (or I was not able to find it, in general the RF spec is not very > detailed...), so I am not sure if the SDRs downlink signal is off, and the > phone is correct or the other way around. Although it sure looks like a low > layer fault, as the NMT network is not even detecting is as bad or incorrect > frames. > > I did a loopback test with a jumper between the TX and RX of the SDR and that > seemed to work (frame level and frame quality were 90+%). On the other hand > if the local oscillator would be off, the phone would not be able to lock on > the DL signal as well. I have no high precision counter at home, but with a > relatively primitive multimeter I was able to measure 12.7999MHz on the > 12.8MHz local oscillator. And both identical phones behave the same. > > The question is if I can somehow offset the uplink frequency on the SDR side > or via config/source modification to remove this couple kHz drift? > > If someone has seen a lot of these signals and can take a look at the image > above, would be lovely to hear if any of the signals are off. > > I will also get another more "modern" phone hopefully soon, so I can try with > that. > > Regards, > Csaba > > Ralph A. Schmid, dk5ras <[email protected]> ezt írta (időpont: 2020. okt. 26., > H, 15:33): >> Hi Chaba, >> >> >> >> I have a Nokia handheld phone from Hungary, I can try with your settings and >> let you know. >> >> >> >> Andreas seems to be difficult to reach these days, I found the same :/ >> >> >> With best regards >> >> >> Ralph. >> >> >> >> >> >> >> >> From: OpenBSC [mailto:[email protected]] On Behalf Of Sipos >> Csaba >> Sent: Monday, October 26, 2020 3:16 PM >> To: [email protected] <[email protected]> >> Subject: Fwd: NMT-450 - partial success >> >> >> >> Hi, >> >> >> >> I know it is a bit off topic, but as osmocom-analog has no dedicated mail >> list and my every attempt to contact Andreas lead to silence, I thought this >> is the closest one to discuss it. >> >> >> >> I try to create an NMT-450 network with Motorola MCR4800XL phones, and a >> LimeSDR-mini. As the phones are locked to "Hungary" using a specific raster >> and a large gap in the middle, first I needed to dig out the details, find >> out the country code and create a patch so at least the phone is willing to >> lock onto the DS signal. I managed to do all that, so now the phone is >> actually able to decode the network and lock onto the signal. >> >> >> >> My issue is with the uplink: when the phone tries Traffic Area update (the >> phone's uplink transmission burts is clearly seen with a spectrum analyzer), >> the network side is not able to detect the uplink burst at all. Not even as >> bad, or incorrectly formatted frame. Andreas has a site which describes how >> to set up the uplink side and do some tests: >> >> >> >> http://osmocom-analog.eversberg.eu/docs/sdr.html >> >> >> >> I followed that guide and when the uplink burst from the phone arrives, the >> RX IQ constellation monitor indicates a correct burst with proper power (the >> burst is nicely round and in the green area). If I try to set up a call to >> the phone using the correct country code and phone number, the phone clearly >> responds to the paging request, as the 3 paging attempt generates 3 uplink >> bursts. Again, with no reception/decoding on the network side. Tried with >> two phones of the same type, the effect is the same. >> >> >> >> I have two questions: >> >> >> >> 1. Where to send patches for the osmocom-analog project? >> >> 2. Does anyone have an idea what can be wrong with my setup? >> >> >> >> One more thing I noticed: compared to the channel frequency used to set the >> NMT network up, the uplink is a couple kHz shifted: >> >> >> >> http://www.imagebam.com/image/d17e881357285965 >> >> >> >> As it can be seen, the uplink burst appears 3-4kHz left relative to the >> downlink signal. >> >> >> >> The network is started with the following command: >> >> nmt -k 239 -k 235 -Y HU,1 --limesdr-mini --sdr-rx-gain 20 >> >> >> >> Any and all help is appreciated. >> >> >> >> Regards, >> >> Csaba >> > > <0001-Add-country-specific-settings-for-Hungary.patch>
