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Is 1.25E-10 (125 picoseconds) is a correct result, or did I forget to divide it by 2?
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At least, I have not found any significant difference in the output signal of the GPSDO, with and without the qErr correction (ADEV from the GPSDO itself was showing the difference, of course). I made a lot of experiments, receiving QO-100 and telemetry beacons from Es'hail-1 and -2 sats, using my GPSDO for clocking the receiver. I had to pause two months ago, but I hope to continue soon.
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The reason for the preferrably long time constant is the uncertainty (noise) of the timing reference signal. If you can remove uncertainty from the measurement you can use a shorter time constant.
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To compare OCXO-based (Lars' with M6N GPS, IMG_5455) and TCXO-based (Leo Bodnar's with M8T, IMG_5454) GPSDOs, a beacon from one of Es'hail sats was received. (The mirror image is from the SDR software, please ignore). In both cases, an SI5338 converter was uses to get clocks for the LNB and for the SDR. Same GPS antenna. Same scale on the SDR software!
In both cases, the frequency drift was of the same order (theoretically acceptable to receive FT8). However, there is a noticeable difference in the noise (almost nil - or - 100% copy of any FT8 signal).
This is why I highly recommend taking a look at QO-100 and other sats, which in our case are inexpensive "magnifying glasses" for exploring GPSDOs. Important: you even do not need a dish to be able to receive beacons; just point your LNB in the sky.
Hi Andrew!
I used to experiment with different types of LNBs, but later I bought a dozen of Eurosky UTP-5 Twin, and tried different methods to inject 25 MHz into the LNB, comparing the results.
Please do not inject 25 MHz into the same wire which is used for RF, as many people recommend. Instead, use one of the LNB's sockets for RF and power, and the other to injecting 25 MHz.
First, disassemble the LNB and power it up from 12V. By using an oscilloscope with 1:1 input (more capacitance than 1:10), find a pin of the XTAL, which has higher amplitude of 25 MHz signal. This is the output of the generator. Cut the trace, and then feed 25 MHz from external source through the XTAL. A 75 Ohm resistor to ground is recommended. Of course, the second socket should be disconnected from the existing circuit in the LNB.
Now you will need two cables to connect your LNB, but this will pay back. If you wish to be able inject 24 MHz, as well, just add another 24 MHz XTAL in parallel to 25 MHz.
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The reason for the preferrably long time constant is the uncertainty (noise) of the timing reference signal. If you can remove uncertainty from the measurement you can use a shorter time constant.
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I have a different opinion about this. Clearly to me the reason why the time constant in Lars' DIY GPSDO is set in the range 100~1000s is because as in any GPSDO we are trying to combine the stability of the inexpensive OCXO with the stability of the PPS from the inexpensive GPS receiver, and we only want to start "disciplining" the OCXO at around the point where the two ADEV curves from the OCXO and PPS intersect.
In other words, in an inexpensive DIY GPSDO (using inexpensive OCXOs and inexpensive GPS receivers with inexpensive antennas), there is nothing to be gained by using a shorter time constant, on the contrary: if we "discipline" the OCXO at shorter intervals we are essentially introducing instability in the system and worsening the ADEV of the GPSDO in the region tau < 1000s.
A zero-cost improvement to the TIC with an order of magnitude better resolution is indeed a significant achievement but it won't change the ADEV curve for a typical Lars' DIY GPSDO build using an inexpensive OCXO and an inexpensive u-blox GPS receiver.
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Let's have a look at this claim. I've made some simulations in octave using GPS 1PPS data from Leapsecond.com. Find attached three ADEV plots.
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The simulated GPSDO is spec'd similar to Lars design, 1ns TIC resolution, 16 bit DAC, PI regulator.
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At short time constants (10-100secs) sawtooth correction probably is very useful for a GPSDO. At long time constants (1000-10000secs) a pre-filter probably is nearly as efficient I think.
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My own rule of thumb is also that AT cut OCXO´s (and good VCTCXO as DOT050V) need time constants in the GPSDO of about 10-100secs, SC cut OCXO 100-1000secs and Rubidium 5000-20000secs to match good GPS receivers.
Hi!
To study the qErr, I plotted it (in picoseconds) in TimeLab, and the ADEV looks quite smooth.
Idea: what if I feed qErr (instead of the TIC output) to the prefilter and so on, and then look at the DAC values in TimeLab? Will I see the difference with different time constants? I will do this experiment soon.
Two remarks:
1- ADEV for all data sets would be nice
2- 600 seconds is not really a lot to make a meaningful statement.
What causes the periodic sudden swings?
3 or 4 hours should be enough, otherwise the OCXO is not good.
It's been running for about a week. I wasn't expecting much for such a cheap device that's 15 years old, but I thought the MDEV looked OK?
I just want to understand what causes those jumps in diff_ns.
Mike