[time-nuts] WWVB SDR discussion

[Bob kb8tq]

Hi

> On Aug 11, 2020, at 10:27 PM, paul swed <paulswedb at gmail.com> wrote:
> 
> How do the small AM WWVB clocks work then. They use the 60 KHz crystal and
> they don't actually do anything special. In measuring those clocks they are
> about 2-6 hz wide.

They pop up once a night and grab time from WWVB. The rest of the day, they 
drift around based on their local crystal oscillator. If they are off by 1/10 second,
there are very few people who are going to notice ….


> On the spectracoms the crystal is huge. Looks like a HC6
> but 3" long. About 1-2 Hz wide.

Which is how 60 KHz crystals were done before the watch industry converted to 
crystals. The larger package *might* give you higher Q and better TC. That’s not
from the package, but from the cut of crystal you would be able to use in the 
bigger package. 

> Using the same small crystals in BPF filters does work and does not
> seriously change within reasonable temperature. The one thing they do
> is follow the crystal with a hi Z amplifier.

The high-z amp is needed to keep losses down. 

This is not a super duper hard circuit to model. Drop it into LTSpice and take a 
look. The motional parameters for the crystal can be derived from it’s resistance
and Q ….

Bob


> Just saying they work for all those atomic clocks for $10.
> But back to the discussion here. Need some gain and filtering. There are
> many good answers.
> John night time is cheating. I get seriously crazy levels in Boston many
> nights.
> Enjoying the thread.
> Regards
> Paul.
> 
> On Tue, Aug 11, 2020 at 7:45 PM John Magliacane via time-nuts <
> time-nuts at lists.febo.com> wrote:
> 
>> On Tuesday, August 11, 2020, 07:14:12 PM EDT, Bob kb8tq <kb8tq at n1k.org>
>> wrote:
>> 
>>> The problem with the crystal is that it has a temperature coefficient.
>> As a
>>> narrow band filter, it will have a *lot* of delay. Crystal resonance
>> moves
>>> (with temperature) and the delay changes.
>> 
>> I agree. The crystal needs to be ovenized. ;-)
>> 
>> That very concern led me in my design to derive nearly all my receiver
>> selectivity at baseband (DC) using op-amps, forgo any crystal filters, and
>> keep the Q of the loop antenna low.
>> 
>> 
>> 73.000 de John, KD2BD
>> 
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