I'm new to the GPSDO topic.
I bought a GPSDO during an Aliexpress sale out of curiosity, and it later turned out to be (per reviews) one of decent enough models, known as TM4313.
When the antenna is placed outside (it's enough for it to be
just outside), it seems to work as expected: it starts receiving the GNSS signal(s) rather quickly, then switches to the "tracking" state in a few minutes, and then in some more minutes (~20-ish) it switches to what seems to be "fine" tracking, as described in more details at
https://tomverbeure.github.io/2023/07/09/TM4313-GPSDO-Teardown.htmlOnce it is in this tracking state, it agrees to within a fraction of Hz with my TinySA Ultra's calibration output, and once both are warmed up, the frequency difference remains stable at 0.3-0.4 Hz or ~30-40 ppb. Since they both agree so well, and looking at the status messages received over the serial (to USB) interface, I assume that it is this state of the GPSDO that is correct.
Now, I decided to let it run overnight and moved the antenna inside the room, leaving it stuck to the window with some scotch tape (later in the morning I found that the tape failed and the antenna was lying on the window sill). I don't know how many satellites the GPSDO can see in either of those locations, but my smartphone barely sees one or two when placed right on the window glass inside, and a lot of them once just outside.
After a night spent with the antenna being inside, the messages from the controller (I changed the jumpers to receive the microcontroller's messages instead of the GPS module's messages over the serial-to-USB interface) looked way off, like this:
Ph = 4760, D = 1039687, GL = 0, LOS = 3, ST = 78213, AF = 37, RT = 78519
Ph = 4900, D = 1058441, GL = 0, LOS = 3, ST = 78214, AF = 37, RT = 78520
Ph = 4901, D = 1077805, GL = 0, LOS = 3, ST = 78215, AF = 37, RT = 78521
Ph = 4759, D = 1097213, GL = 0, LOS = 3, ST = 78216, AF = 37, RT = 78522
Ph = 4604, D = 1116081, GL = 0, LOS = 3, ST = 78217, AF = 37, RT = 78523
...
Ph = 281, D = 1162668, GL = 0, LOS = 3, ST = 78255, AF = 37, RT = 78561
Ph = 144, D = 1153634, GL = 0, LOS = 3, ST = 78256, AF = 37, RT = 78562
Ph = 6, D = 1144387, GL = 0, LOS = 3, ST = 78257, AF = 37, RT = 78563
Ph = -136, D = 1134960, GL = 0, LOS = 3, ST = 78258, AF = 37, RT = 78564
Ph = -280, D = 1125348, GL = 0, LOS = 3, ST = 78259, AF = 37, RT = 78565
...
Ph = -5755, D = 1051665, GL = 0, LOS = 3, ST = 78293, AF = 37, RT = 78599
Ph = -5918, D = 1027427, GL = 0, LOS = 3, ST = 78294, AF = 37, RT = 78600
Ph = -5943, D = 1002474, GL = 0, LOS = 3, ST = 78295, AF = 37, RT = 78601
Ph = -5826, D = 977371, GL = 0, LOS = 3, ST = 78296, AF = 37, RT = 78602
Ph = -5700, D = 952698, GL = 0, LOS = 3, ST = 78297, AF = 37, RT = 78603
...
Ph = -254, D = 937942, GL = 0, LOS = 3, ST = 78377, AF = 37, RT = 78683
Ph = -139, D = 945679, GL = 0, LOS = 3, ST = 78378, AF = 37, RT = 78684
Ph = -23, D = 953570, GL = 0, LOS = 3, ST = 78379, AF = 37, RT = 78685
Ph = 95, D = 961609, GL = 0, LOS = 3, ST = 78380, AF = 37, RT = 78686
Ph = 216, D = 969799, GL = 0, LOS = 3, ST = 78381, AF = 37, RT = 78687
...
The "Ph" (phase difference?) parameter oscillated up and down, never converging to zero, the "D" parameter was always way above normal (see below), and the AF parameter remained at the value indicating what seems to be "coarse" tracking. Weirdly enough, it never thought it lost tracking, which is indicated by the ST parameter, which means seconds since tracking started or stopped (and RT means seconds since power-on).
Most importantly, the frequency it produced in this state was way off, at least tens of Hz difference against the TinySA. I also suspect that the OCXO used in my unit is not the best, as the frequency quickly wanders off once GPS tracking is lost, but that's a different story.
Things did not change when I moved the GPS antenna outside. It still could not catch up after ~30 min or so.
Then I disconnected and reconnected the antenna.
It promptly detected the loss of signal (GL=1):
Ph = -3064, D = 738791, GL = 0, LOS = 3, ST = 79728, AF = 37, RT = 80034
Ph = -3005, D = 742375, GL = 0, LOS = 3, ST = 79729, AF = 37, RT = 80035
Ph = -3005, D = 745982, GL = 1, LOS = 3, ST = 79730, AF = 37, RT = 80036
Ph = -3005, D = 745982, GL = 1, LOS = 4, ST = 0, AF = 37, sK = 2.06052, RT = 80037
Ph = -3005, D = 942403, GL = 1, LOS = 4, ST = 1, AF = 37, RT = 80038
Ph = -3005, D = 942401, GL = 1, LOS = 4, ST = 2, AF = 37, RT = 80039
Then it, as it would normally do on power-on, started to receive the GPS signal (GL=1 transitioning to GL=0):
Ph = -1343, D = 942376, GL = 1, LOS = 4, ST = 14, AF = 37, RT = 80051
Ph = -1228, D = 942374, GL = 1, LOS = 4, ST = 15, AF = 37, RT = 80052
Ph = -1110, D = 942372, GL = 0, LOS = 4, ST = 16, AF = 37, RT = 80053
Ph = -996, D = 425986, GL = 0, LOS = 2, ST = 0, AF = 37, RT = 80054
Ph = -951, D = 405986, GL = 0, LOS = 2, ST = 0, AF = 37, RT = 80055
-- at this point the 10 MHz frequency output becomes very close to the TinySA.
Then it starts tracking (LOS=2 -> LOS=3, ST resets, Ph fluctuates close to zero):
Ph = -3774, D = 565986, GL = 0, LOS = 2, ST = 40, AF = 37, RT = 80182
Ph = -3770, D = 565986, GL = 0, LOS = 2, ST = 41, AF = 37, cK = 0.003312E-9/LSB, RT = 80183
Ph = -3764, D = 550892, GL = 0, LOS = 2, ST = 42, AF = 37, RT = 80184
Ph = -3761, D = 550892, GL = 0, LOS = 2, ST = 43, AF = 37, RT = 80185
Ph = -3760, D = 550892, GL = 0, LOS = 2, ST = 44, AF = 37, RT = 80186
Ph = -3761, D = 550892, GL = 0, LOS = 2, ST = 45, AF = 37, RT = 80187
Ph = -3760, D = 550892, GL = 0, LOS = 2, ST = 46, AF = 37, RT = 80188
Ph = 0, D = 550892, GL = 0, LOS = 3, ST = 0, AF = 37, RT = 80189
Ph = -1, D = 550892, GL = 0, LOS = 3, ST = 1, AF = 37, RT = 80190
Ph = 0, D = 550892, GL = 0, LOS = 3, ST = 2, AF = 37, RT = 80191
-- at this point the 10 MHz frequency has the same difference measured earlier against the TinySA and the difference remains stable.
And then, finally, it switches to what seems to be "fine" tracking state (AF=37 -> AF=73, and the steps of the changes in D become very small):
Ph = 1, D = 550704, GL = 0, LOS = 3, ST = 997, AF = 37, RT = 81186
Ph = 0, D = 550704, GL = 0, LOS = 3, ST = 998, AF = 37, RT = 81187
Ph = -1, D = 550704, GL = 0, LOS = 3, ST = 999, AF = 73, RT = 81188
Ph = 0, D = 550704, GL = 0, LOS = 3, ST = 1000, AF = 73, RT = 81189
No observable output frequency change at this point.
Now, just by looking at the output, I think it's safe to assume that this (the latest messages) is its normal state, and what I saw after a night of potentially poor signal reception and/or low number of satellites, was not. It seems that it got locked up in some bad state and needed an antenna reset to come out of it back to normal. Would it be able to resolve that bad state on its own with the antenna placed outside? I don't know, I didn't have the patience to wait.
What was it that I observed? Was it expected? There's not much information on the internet about this device, but I believe there are a few people here on this forum that own it, so maybe they have some thoughts, or maybe some general GPSDO considerations may be applicable.