[time-nuts] WWVB SDR discussion

[paul swed]

Mark
His antenna hit a preamp as I recall about 20 db of gain. To see something
on a scope add 40 more db approx. Unfortunately a purely broadband solution
will show 40 db of pure garbage these days. Using the 60 KHz watch crystals
$2.00 for 20 out of China you can most likely find a reasonable match.
Thats what I did. It is hi Z so it feeds one side of an opamp. Look at the
spectracom schematics to get a sense of what to do. I made a small socket
to plug them in and found the one that worked. As an alternative you can
build a bandpass filter with opamps lots of variations. Anything to get the
received bandwidth reduced. Look at Johns front end also.
Regards
Paul

On Tue, Aug 11, 2020 at 3:20 PM Mark Haun <mark at hau.nz> wrote:

> 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
>
>
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