As I have posted before, I am trying to use an SDR to make peak frequency measurements at intervals using Spectrum Lab software to control the SDR and record the data.I don't have the full info about what algorithms are used to capture the peak frequency and was trying to look into various methods to estimate the peak frequency between FFT bins. The software also has a Phase measurement function that is described as:
Multiply the incoming signal with a local oscillator, here: a numerical controlled oscillator with two outputs (90°) Low-pass filter and decimate the mixed signal until the required (low) bandwidth is reached If the signal is MSK (minimum shift keying, as most VLF transmitters), square the decimated signal, and examine the 'peaks' in the spectrum at f_center +/- bitrate. The same principle is also used in the continuous sampling rate calibrator . Calculate the amplitude and the phase angle of the decimated signal (for MSK: Angle of the recovered carrier signal). I am not using MSK. I then use this function they provide to get the frequency at each sample time: pamN.freq returns the precisely measured frequency in Hz . Ideally the same as the configured center frequency (can be used as an additional "health check"). I don't know any more than that about the algorithms, but ideally it provides a measured frequency of the peak of the signal of interest. So, I should have a file with the measured frequency, sampled at the sample interval. I have checked this statically with specific input frequencies generated by an HP 8642A signal generator and it appears to provide a good estimate of the frequency. The algorithm wraps every Hz, so I use the poorer estimation of the peak to decide which Hz offset the frequency actually is. This seems to work well, at least at 0.1 Hz intervals a few Hz around the center frequency of the SDR. The measured frequency always is within 0.2Hz of what was set, including all variations. As a check, I connected both the reference clock of the SDR and the measured signal to the same reference source, a Trueposition GPSDO. Ideally this would provide a straight line on the ADEV plot that represents the inherent noise of the measurement system. So I have taken the inputs which represent the measured frequency at 0.6816 second intervals and imported them into Timelab as follows: File | Import ASCII Phase or Frequency Data Sampling Interval 0.6816 Sec Input Frequency 10e6 Hz Bin Density 29 Bin Threshold 4 Trace History 1 Numeric Field <chosen column> X 1.0 Data Format Decimal Comment Prefix USF( # if Channels 1 Frequency (Hz) Selected I have then taken readings at a slower rate, every 5.453 seconds and again checked that they match every 0.1 Hz around the SDR center frequency. I then connected both inputs to the same source again to reproduce the measurement of the internal noise, this time with frequency measurements every 5.453 seconds and imported as above with the following change: Sampling Interval 5.453 Sec Otherwise the connections and measurements are the same, just using a different sampling interval. I had Timelab calculate the ADEV of both. I would have expected them to be similar with no data below each sampling interval. The result is two curves of basically the same shape, but the curve of the higher sampling interval is offset vertically by a factor of about 4.5. Since these are two different runs a day apart, some variation is expected, but not a constant offset. This is repeatable for other measurements, the results from the larger sample rate are always offset vertically above the lower sample rate. I have looked at http://www.leapsecond.com/pages/adev-avg/ and I can see that the data has a small downward turn at the low end of the fast sample rate, which is likely due to some averaging, but the two graphs are very similarly shaped otherwise. I do not see an example there of such an offset. The Screen capture from Timelab and the raw data are at: https://drive.google.com/drive/folders/1jkORB_wujXOw-rknBt2pFCqLSvp91In6?usp=sharing There are three columns in each file. Column 1 is from the peak frequency algorithm and shows an offset and other anomalies. Columns 2 and 3 are from the phase measurement algorithm frequency measurement function. All data is in Hz at the respective sampling rates. Can anyone tell me what I may be doing wrong? Unfortunately, I can't get a TIC at this time, so I am trying to work with what I have. Regards, Mark _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.