dnadlinger
commented
11 months ago Related: https://github.com/sinara-hw/Pounder/issues/110 (though this is apparently using downconversion to ~250 kHz IF, not DC, so the close-in properties will presumably be different).
Here is a screen capture for posterity (according to Robert, the axis scaling is off by a factor ~2000×):
It would be good to measure the noise floor with output connected to input and phase adjusted to zero mixer output. I'd do this both with an external instrument (via Stabilizer's analog in SMA) and via the Stabilizer's ADC. I'm afraid I don't have boards or people to do this myself in the near term.
https://doi.org/10.1109/FCS.2011.5977868 gives an idea of how you might do this and where you'd expect to get to if the mixer was limiting. The DDS is probably the limit here - see http://rubiola.org/pdf-slides/2012C-IFCS-DDS.pdf. Hopefully the shared clock circuitry, power supplies, and thermal environment make some of the noise sources highlighted by Rubiola common-mode out, but AFAIK nobody in the literature has tested this.
Originally posted by @dtcallcock in https://github.com/sinara-hw/Pounder/issues/84#issuecomment-741345552