Re: SDR transceiver having independent mixer and ADC sampling clocks

[Brian Padalino]

On Wed, Feb 26, 2020 at 4:23 PM Heikki Laamanen <address@hidden> wrote:

> So you want to produce something like a stable 10MHz reference and a 1PPS which is synchronous to UTC using broadcast/terrestrial signals such as DVB-T, ATSC, LTE, etc?  And your edges can be within +/-50ns of actual UTC?

 

Yes. All those signals (not sure about ATSC) are synchronized to UTC directly (via GPS) or indirectly (via PTP over fiber). Thus, the signals carry inherent timing information. There are naturally lots of issues related with radio wave propagation in each case like multipath propagation caused by reflections, groundwave/skywave phenomena concerning LF radio wave propagation, e.g. DCF-77 and the ancient C-Loran. The most ideal cases are line-of-sight wave paths like GPS satellite – GPS receiver.

 

I have one more question. I have not found any GNU Radio object utilizing the possibility of controlling the ADC sampling frequency via controllable PLLs on the RF HW. All sync-related objects seem to use – e.g., polyphase filters, for adjusting sampling phase. Am I right?

Some radios use a DAC to control a VCTCXO with a pullability of maybe +/-10ppm.  Others use an analog PLL to lock to an externally supplied reference (like 10MHz or so).

Most would probably want to keep jitter to a minimum and track the sampling offset digitally.  You may want to look into what it might take to do that instead of expecting a clock to be perfect.

Brian