[time-nuts] WWVB SDR discussion

[Mark Haun]

Hi Detlef,

On 11-Aug-20 3:46 AM, Detlef Schuecker via time-nuts wrote:
> you do not necessarily need a variable ( physical ) oscillator. You mix 
> the signal down in the digital domain. A 'digital local oscilator' is a 
> mere complex value, which is rotated and the power is adjusted:
My proposed block diagram does actually have a digital LO, only mine is
1, 0, -1, 0... (in-phase) and 0, -1, 0, 1...  (quadrature).  You could
of course use an variable-frequency NCO, but I need a physical
oscillator in any case to clock the MCU.  I am also thinking in terms of
a WWVB-DO where I want a stable local reference to steer.  (Although in
fairness, for WWVB I think you probably want stability over the diurnal
propagation variation, and my crappy OCXO has no chance at that.)
>> part, unfortunately.  My tuned loop seems still too broadband, even
>> after a couple more poles of op-amp filter.  I have a bunch of 60-kHz
>> tuning-fork crystals and wanted to try a crystal filter like the "pros"
> Good point. 
> Firstly I tried a tuned resonant LC circuit with a BF245 preamplifier to 
> keep a high Q.
Besides having minimal analog design experience, I think what is
confusing me is these crystals have a really high impedance far away
from their parallel resonance.  Even at series resonance the motional
resistance is in the 10s of kohms, IIRC.  So I'm not exactly sure how to
deploy one in place of an LC tank circuit, e.g. in a collector or drain
circuit.
> Secondly an accurate ADC is an option. With 24Bit you get more than 100dB 
> dynamic range, so you dont care about a 60dB stronger nearby interferer.

Fancier ADC shouldn't make any difference.  Even though the STM32L4 ADC
has an SNR of (IIRC) ~ 69 dB, you can sample at several MHz.  After
downsampling to something like 100 Hz (post LO), the SNR is well over
100 dB, which should be plenty.

(Of course, this presupposes that there are not strong interferers or
SMPS noise spurs, etc. within a 100-Hz BW centered on 60 kHz.  But if
there are, a fancy ADC wouldn't help you anyway.  The main thing is to
make sure the interference isn't causing any clipping or nonlinearity
before you sample.)

I should note that my big tuned air loop and preamp, which is modeled
after Joe Magliacane's, may be adequate even though I can't see WWVB on
the oscilloscope.  I was hoping to build confidence there before hooking
it up to the MCU and doing SDR stuff.  I also want a small loopstick
version of this so I can embed it in my own nixie wall clock, hence the
interest in crystal filters, etc.

Mark