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| TM4313 GPSDO: strange behavior after a night of poor reception |
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| tverbeure:
--- Quote from: shapirus on April 09, 2024, 07:41:20 pm ---I don't like the algorithm of what it does on signal loss. The output frequency changes quite abruptly when that happens. I don't have the numbers, but I see it on the scope: I'd estimate it at tens of Hz difference at least. Once it starts tracking again, it gets to sub-Hz difference against the TinySA cal output almost instantly. Apparently it simply stops applying the tuning voltage to the OCXO when the GNSS signal is lost, whereas I think the proper thing to do would be to retain the last known good voltage instead to at least let it run at a good enough frequency until the GNSS signal can be received again. I think that's a software flaw. --- End quote --- I recorded the frequency every second on my frequency counter, unplugged the antenna and then plugged it back in: StanfordResearchSystems,SR620,00182,1.48 0 9.99999999761E6 1 9.99999999769E6 2 9.99999999785E6 3 9.99999999766E6 4 9.99999999766E6 5 9.99999999815E6 6 9.99999999796E6 7 9.99999999774E6 8 9.99999999796E6 9 9.99999999764E6 10 9.99999999802E6 11 9.99999999788E6 12 9.99999999745E6 13 9.99999999755E6 14 9.99999999669E6 15 9.99999999747E6 16 9.99999999774E6 17 9.99999999758E6 18 9.99999999709E6 19 9.99999999802E6 20 9.99999999807E6 21 9.99999999780E6 22 9.99999999793E6 23 9.99999999802E6 24 9.99999999829E6 25 9.99999999907E6 26 9.99999999880E6 27 9.99999999918E6 <<<< Antenna unplugged 28 9.99999999883E6 29 9.99999999932E6 30 9.99999999926E6 31 9.99999999910E6 32 9.99999999924E6 33 9.99999999956E6 34 9.99999999940E6 35 9.99999999986E6 36 1.00000000001E7 37 1.00000000006E7 38 9.99999999934E6 39 9.99999999850E6 40 9.99999999856E6 41 9.99999999829E6 42 9.99999999858E6 43 9.99999999821E6 44 9.99999999861E6 45 9.99999999856E6 ... 138 9.99999999812E6 139 9.99999999802E6 140 9.99999999804E6 141 9.99999999802E6 142 9.99999999791E6 143 9.99999999845E6 144 9.99999999848E6 145 9.99999999829E6 146 9.99999999791E6 147 9.99999999788E6 <<< Antenna plugged in, immediate GPS relock 148 9.99999999815E6 149 9.99999999815E6 150 9.99999999793E6 151 9.99999999821E6 152 9.99999999875E6 153 9.99999999837E6 154 9.99999999856E6 155 9.99999999837E6 156 9.99999999823E6 157 9.99999999831E6 158 1.00000000010E7 159 1.00000000000E7 160 9.99999999921E6 161 9.99999999818E6 162 9.99999999717E6 163 9.99999999658E6 164 9.99999999620E6 165 9.99999999566E6 166 9.99999999536E6 167 9.99999999509E6 168 9.99999999419E6 169 9.99999999351E6 170 9.99999999397E6 171 9.99999999359E6 172 9.99999999340E6 173 9.99999999240E6 174 9.99999999232E6 175 9.99999999188E6 176 9.99999999202E6 177 9.99999999167E6 178 9.99999999183E6 179 9.99999999164E6 180 9.99999999245E6 I'd have to repeat this test a couple of times and create a plot, but the effect is minimal. Instead of have measurements that different by ~1 or 2 mHz, there's maybe a little jump but it's definitely the a couple of Hz that you're seeing. The interesting part is if there's a jump, it settle back down after that, so the change could just be due to the vibrations caused by unscrewing the antenna connector. Edit: I redid the exercise with the counter set to a 5s update rate. Unplugging has zero effect, plugging the cable back in is more disruptive (~10mHz) with the output frequency swinging around a bit, which is what you'd expect from a control loop. |
| shapirus:
I'll repeat the same test as yours with the antenna disconnection a bit later. Well, somewhat the same -- I don't have a freq counter, so will have to use the oscilloscope to watch two signals, GPSDO and TinySA, along each other. Assuming that the disconnection of the antenna from the GPSDO is not supposed to disturb the output of the TinySA, then if a significant difference occurs in what's seen on the scope (particularly in the XY mode), then it can only mean that it's the GPSDO that loses its frequency stabilization. As far as the 1 PPS output, there is a respective jack, and I will try to trace where it's connected to (not sure if it'll be possible with that black solder mask). Is it supposed to output nothing when there's no GPS reception? Right now the GPSDO is locked up again (has been since the evening -- see the ST value) and isn't recovering: --- Code: ---Ph = 49838453, D = -296976385, GL = 0, LOS = 3, ST = 40169, AF = 37, RT = 163986 Ph = 49838537, D = -288784385, GL = 0, LOS = 3, ST = 40170, AF = 37, RT = 163987 Ph = 49838566, D = -280559617, GL = 0, LOS = 3, ST = 40171, AF = 37, RT = 163988 Ph = 49838540, D = -272400385, GL = 0, LOS = 3, ST = 40172, AF = 37, RT = 163989 Ph = 49838443, D = -264208385, GL = 0, LOS = 3, ST = 40173, AF = 37, RT = 163990 Ph = 49838325, D = -256049153, GL = 0, LOS = 3, ST = 40174, AF = 37, RT = 163991 Ph = 49838364, D = -247857153, GL = 0, LOS = 3, ST = 40175, AF = 37, RT = 163992 Ph = 49838395, D = -239697921, GL = 0, LOS = 3, ST = 40176, AF = 37, RT = 163993 --- End code --- I have checked the 1 PPS output: it is outputting a pulsed signal, 1 Hz (I can't check how accurate), unipolar 0..4.75V square (into 1 MOhm), 100 ms pulse width. Does it mean that the GPS module is definitely not locked up? |
| nctnico:
I'd say it is a software issue. But it is quite possible that the type of jamming used near you, is designed to confuse GPS receivers to a point where they don't come up after the jamming ends. Makes it harder to pinpoint the transmitter. It would be very interesting if you could test other GPSDOs using different GPS receiver modules and look what is coming out of the module itself in the form of NMEA data. |
| shapirus:
--- Quote from: shapirus on April 10, 2024, 07:44:26 am ---Right now the GPSDO is locked up again (has been since the evening -- see the ST value) and isn't recovering: --- End quote --- Here's how it looks visually: Looks more like a software issue to me. Purple trace is a stable enough reference (TinySA Ultra), blue trace is the TM4313. I was going to record how it behaves when I disconnect and reconnect the antenna, but alas, have just had a power outage, and the GPSDO got powered off. Next time I'll power it from the UPS. Anyway, I will at least be able to record its reaction to a disconnection of the antenna when it's in a "good" tracking state to compare the results with those posted by tverbeure above. |
| shapirus:
--- Quote from: shapirus on April 10, 2024, 08:45:39 am ---Anyway, I will at least be able to record its reaction to a disconnection of the antenna when it's in a "good" tracking state to compare the results with those posted by tverbeure above. --- End quote --- ...and we have a match here. When the antenna is disconnected, there is no visually observable change in the output frequency, at least within a few minutes. When the antenna is connected back, it goes into "fine" tracking mode again almost instantly. This means that the OCXO tuning voltage level is maintained in the last known good state in this scenario. The next thing to test is to bring the antenna, without disconnecting it, further into inside of the room to simulate a signal level degradation. |
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