On 1/20/20 1:57 PM, Attila Kinali wrote:
And then there ia third way, which is IMHO even better:
Your application is an SDR system, i.e. you already need some
signal processing for the system to work. Why not extend this
to use it for the reference as well? Add another ADC and feed
the reference signal to that, then track the phase/frequency
relation between the sampling clock and the reference and
compensate any drift in the signal path. This way you get to
disable the reference if it is not needed and save a lot of power
and at the same time are able to use references with any frequency
and can change the "loop frequency" freely without the need to
worry about PLL stability or tempco of filters in the multiplier
version.
This ...
This is the way of the future. The problem is that there are enough
legacy systems out there where you need "control" vs "knowledge"
And, in the SDR world: while theoretically, you can do this in software,
a lot of times the software is either a black box, or incomprehensible
in finite time, or architected in a way that makes it hard, that it's
actually faster and easier to discipline the reference oscillator than
to fix the software.
I say this as someone who makes his living designing, building, and
using SDRs - a Curse on Matt Ettus and USRPs, gnuradio, pothos, etc. and
their ease of use, allowing positive legions of people to produce
software which is horrible, without realizing the implications and
defects within. They should all be consulting *me* before engaging in
these ill advised implementations based on textbook descriptions from
Oppenheim and Schaefer, etc.
But yes, the *best* way to do it is to *measure* the oscillator and use
that to correct the digitized data, rather than driving the oscillator.
It is challenging, though, to do this in a system where there is a need
for full duplex operation (i.e. the transmitted signal needs to be
adjusted to match the received signal).
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