In my previous post I set out to measure the phase difference between the QO-100 8APSK and BPSK beacons. One of the things I mentioned is that the frequency separation between these two beacons was approximately 1.6 Hz larger than the nominal 245 kHz.
A frequency error of a couple of Hz is typical when working with SDRs unless special care is taken. Many SDRs allow choosing the sample rate and centre frequency with great flexibility, but the drawback is that the frequencies that are achieved are often not exactly the ones we indicated. Fractional-N synthesis PLLs are used to generate the sampling clock and local oscillator, so there are small rounding errors in the generated frequencies.
With enough knowledge of how the SDR hardware works and how it is configured, it is possible to determine these frequency errors exactly, as a rational number \(p/q\) that we can calculate explicitly, multiplied by the reference frequency of the SDR. Then we can use this exact value to correct our measurements.
I have asked Mario Lorenz DL5MLO and Kurt Moraw DJ0ABR the details of how the beacons are generated in the Bochum groundstation. Two ADALM Pluto‘s are used: one generates the CW and BPSK beacons, and the other generates the 8APSK multimedia beacon. With the data they have given me, I have been able to compute the frequency separation of the 8APSK and BPSK beacons exactly, and the result matches well my experimental observations.
In this post we will look at how the fractional-N synthesis calculations for the Pluto can be done. Since the Pluto uses an AD9363 RFIC, these calculations are applicable to any product using one of the chips from the AD936x family, and to the FMCOMMS3 evaluation board.