[time-nuts] Dephasing WWVB

[John Reed]

First on the Schmidt trigger - The problem is that at the start of each 
"bit" that WWVB transmits the squared 60 KHz signal is essentially dead and 
the trigger must pick a new starting point.  This point seems to be random 
and can apparently end up as a positive or negative, so you end up with 
phase changes of 180 after the flip flop.  No trigger can fix this.  The 
system has to have some memory of the phase and this is why the Costas loop 
works.

I thought about getting rid of the 100 KHz front end filter in the Tracor 
and seeing if I could modify it by squaring the LO signal.  This isn't 
straightforward either.  The Tracor has a complex method of generating the 
IF signal.

I wasn't aware that 60 KHz crystals are available.  I would have used these 
instead of the LC filters.  I had some old telephone loading ferrite toroid 
coils, so most of the hardware was available.

Thanks for all the comments on this.  At least I understand the problem now, 
and why the solution will take some work.

John

-----Original Message----- 
From: paul swed
Sent: Monday, June 30, 2014 9:49 AM
To: Martin VE3OAT ; Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Dephasing WWVB

Many of the old receivers use them spectracoms come to mind. They are big
units and +- 40 Hz BW and I am totally unaware that they can be found
today. That also goes for nice transformers and inductors to build higher Q
circuits.
I built a opamp chain and it worked well but those crazy amps do draw
power. I like the ua consumption level. But thats a personnel preference.

I used the 60 KHz watch Xtals and its in the schematics of the WWVB rcvr I
released to time-nuts a year ago. These little crystals are interesting to
work with and available from China 25 xtals for a few $ at the pay site. I
purchased 2 packs so that I could sift through them. The trick is to very
very lightly load them. I could learn much more about them actually. They
seem useful overall.
The first re-modulator used them directly as the 60 KHz source. I stepped
up to the 15.360 MHz osc only because I believed they were not accurate
enough and that turned out not to be the case as I found.

The other comment to note is that these xtals cause an actual signal gap at
the phase transition. Because at that point the signal is actually 2 X 60
Khz. The crystal gaps for at least 8 cycles from what I have seen.
Regards
Paul
WB8TSL




On Sun, Jun 29, 2014 at 6:39 PM, Martin VE3OAT <ve3oat at storm.ca> wrote:

> John Reed wrote :
>
> >
> > By the way, my 5 section synchronous filter is an LC with
> > op-amps between each stage to bring the gain up for the
> > squaring chip.  It has a 2 KHz -6 dB bandwidth at 60 KHz.
> >
>
> John, have you thought of using a single 60.0 kHz crystal as a bandpass
> filter?
>
> I can't remember which receiver it was, but I think one of the old
> commercial WWVB receivers used a crystal as the tuning element.
>
> ... Martin   VE3OAT
>
>
>
>
>
>
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