DDC frequency resolution/programming accuracy?

[jmfriedt]

The DDC of the X310 is not programmed with the right frequency/DDC DDS resolution?

A X310 fitted with two BasicRX (no frequency conversion before sampling and FPGA DDC) is externally clocked by an Octoclock fed by the output of a microstepper generating a 10 MHz and 1-PPS output. This same 10 MHz reference feeds a Rohde & Schwarz SMA100A synthesizer and HP 53131A frequency counter. The synthesizer is programmed to output a 70 MHz continuous wave, validated with the frequency counter:

The X310 is set to a sampling rate of 200/40=5 MS/s and the LO frequency of the DDC set to 70 MHz. The spectrum of the signal recorded for 5 seconds (0.2 Hz resolution) is as follows:

An obvious offset of 5 Hz is seen. It is unclear whether this offset is within the DDC DDS resolution which seems to be 200 MHz/2^24=12 Hz but sounds a poor resolution (see discussion on chat.gnuradio.org): could this resolution be confirmed?

Repeating the measurement at 143.05 MHz and using the second Nyquist zone, so setting the DDC frequency to 56.95 MHz and feeding the X310 with a synthesized signal

leads to

The reason for the Issue is that this behaviour is observed with 4.5.0.0 but not with 4.1.0: with the latter, the 70 MHz signal is measured with sub-Hz error, so a behaviour changed in between.

Setup Details

The flowchart is as attached

with external time and frequency selected on the X310, only one of the two channels used in this experiment, stored in file and post-processed with GNU Octave running

fs=5E6;
x=read_cshort_binary('fichier.bin',50e6);
x1=x(1:2:end)/32767;
x2=x(2:2:end)/32767;
freq=linspace(-fs/2,fs/2-fs/length(x2),length(x2));
plot(freq,abs(fftshift(fft(x2))));