Improving the resolution bandwidth of the SA

While trying to do some analysis of narrow band signals it became obvious the current design of the spectrum analyzer has two limitations.
Below picture shows both of them.

The signal to analyze is at 575kHz. The resolution filter is clearly too wide, about 30kHz at -3dB, and the staircase patter shows scanning is done in 10kHz steps caused by the minimum frequency steps of the first mixer LO, a ADF4351. Making very narrow RBW filters is a considerable effort and an FFT could be an alternative.
So it was time to go test if a mixed mode SA can work. At high spans the SA works with the log detector but as soon as the minimum frequency step is below 10kHz the log detector is no longer used. Instead a third mixer is used  to convert to an IF of 50kHz. This is fed into the PC line-in of a good audio card at 192kHz sampling rate and analyzed using a 1024 point FFT. As the usable buckets of the FFT are limited due to the RBW, now acting as the 3 IF filter, multiple FFT's, spaced 10kHz apart, are stitched together. The FFT bucket width is about 100Hz, About 300 times better compared to the RBW filter used above,
The result is a nice sharp signal due to the flattop window function applied. The scan is 1000 points wide. Measurement speed is considerably faster as instead of 1000 steps the ADF4351 has to step only 10 times. It takes about twice the time for the audio samples to be collected compared to the stabilization of the log detector so in total still 50 times faster



There are still many thing to improve or test such as:
- The dynamic range of the audio input should be in the order of 110dB (24 bit audio card). This needs to be validated together with the behavior of the third mixer
- The frequency calibration and peak labeling needs to be improved for this much higher resolution.
- The noise floor shows a repetitive pattern so something is still wrong in the signal path
- Instead of averaging or duplication of FFT buckets to match the required resolution a better approach is probably to have an adaptive FFT length. A 10k FFT will result in 10Hz RBW resolution (and run 10 times slower) and a 128 bin FFT (for 1kHz resolution) will be much faster
So many things to try!