[time-nuts] Jan-Derk's DDMTD

[jimlux]

On 9/1/19 10:10 AM, Bill Slade wrote:
> Hello,
> This is a topic that interest me greatly as well.  Some of the work we have done uses a digital frequency tracking loop to generate frequency errors directly from sampled data using a sampling system and ethernet system of our own design.  We opted for using a look-up table-based DDS in the final frequency conversion to baseband (you mentioned considering CORDIC) and CIC filters/decimation..all in FPGA (Altera Cyclone III).  This was done for testing two-way coherent transponders.  See a description here<https://www.researchgate.net/publication/332241644_Coherent_tracking_error_characterization_in_microwave_transponders>.

This is quite interesting.. It's always a challenge to adequately 
characterize the turnaround performance of a transponder, particularly 
in a rapidly changing Doppler environment.Say your probe is orbiting 
Europa or Titan, and you want to do precision radiometric measurements 
for radio science.

back in the day, with an all analog transponder, you could test at 
constant frequencies, and do some analytical stuff to estimate 
incremental ADEV in the turnaround, as well as the imperfections from 
tracking a higher order variation in Doppler than the loop order. And 
mission design didn't do orbits around moons, but flybys (e.g. Cassini, 
Galileo, Voyager)  But these days, most transponders implement the 
carrier tracking loop in software, and there's always all sorts of 
arguments about what the implementation of that loop does to the turnaround.


> 
> Yes, it seems to me that a band limiting filter on the input would be absolutely necessary, since the sampling BW of the ADC is so high.

and, probably, on the sampling clock - the clock input on these kind of 
devices is also quite wideband