Products > Test Equipment
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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