Jim Lux writes: > So now the challenge is to "line em up". An obvious approach is to > transmit an inband pilot tone with some sync pattern, received by all, > and I'm working on that too.
A maybe not-so obvious approach would be to use RTL-SDR that have been modified for direct sampling (usually via the Q branch) and inject your timing pulse there. That would limit the disturbance of the actual signal while still relatively easy to extract from the data stream. > But right now, I have the idea of capacitively coupling the 1pps pulse > from the GPS to the antenna input - the fast rising and falling edge > are broad band and show up in the sampled data. The trouble is that you are going to impair the already low dynamic range. The ENOB on the I/Q ADC is around 7bit only. > And you can see, no surprise, that the sample clock in the RTL isn't > dead on - over the 10 seconds, it looks like it drifts about 30- 50 > microseconds - that is, the RTL clock is slow by 3-5 ppm. Not all of these are created equal. Several manufacturers claim to factory calibrate their TCXO to better than 0.5ppm. I have currently two RTL-SDR that certainly are within 1ppm. These things get quite hot, so it definitely takes some time before they stabilize even if they do have a TCXO in them. > SO here's the question for the time-nuts hive-mind... > What's a good (or not so good) way to develop an estimator of the > timing/frequency error. Post processing minutes of data is just fine.. There is a program called rtl_test that just checks how many samples it gets in a certain amount of time. Let it run for a few hours on a PC with a GPS-disciplined PC clock and it'll give you a pretty accurate estimate of the mean sampling clock deviation. The other method is to tune to a signal of known frequency and check what it reads as. There is a program floating around that uses a GSM station for that purpose. Regards, Achim. -- +<[Q+ Matrix-12 WAVE#46+305 Neuron microQkb Andromeda XTk Blofeld]>+ SD adaptations for Waldorf Q V3.00R3 and Q+ V3.54R2: http://Synth.Stromeko.net/Downloads.html#WaldorfSDada _______________________________________________ time-nuts mailing list -- email@example.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.