[time-nuts] WWVB SDR discussion

[paul swed]

Should add that on the East Coast the signal is about 10 uv. On the large
10' X 10' loop I get about 63 uv during the day. That loop at night can
easily hit 1000 uv.
Regards
Paul

On Tue, Aug 11, 2020 at 3:37 PM paul swed <paulswedb at gmail.com> wrote:

> 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
>>
>>
>> _______________________________________________
>> time-nuts mailing list -- time-nuts at lists.febo.com
>> To unsubscribe, go to
>> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
>> and follow the instructions there.
>>
>