Clearly I am not EE staff but since theyhave not responded; I do have an external GPS sync-ed reference that has a very pure sine wave output. It is excellent.
What I have noticed are spurs ... those extra signals (sometimes called birdies) in the pass band of the receiver that were not there without the reference. Actually many are present with the internal reference and just shifted slightly clearly due to the interenal & external frequency difference.
Back in the day that reference oscillators were designed to make the extra birdies that appeared near the upper ham bands for convenience in calibrating the rcvr inlieu of something similar a WWV. However, because GPS references actually are used to directly set the xcvr's frequency (it is *the* reference), therefore those birdies are no longer needed and are now just noise.
I once made a simple experiment by adding a 10dB attenuator pad between my reference & the SDR to investigate if the birdies would lessen. The pad made no change on the level of those birdies at all. I do have a couple of old HP step attenuators & coupled together gives 1dB steps up to ~100dB, but given the zero effect the 10dB pad made, I lost interest and decided the SDR squares the reference signal internally and unnecessarily for ham radio use and there is nothing I can do about it ... opinion. I might revisit this sometime in the next couple of monts.
