[time-nuts] New WWVB modulation format receivers

[Clint Turner]

Hi Paul,

Without digging through the archives, I'll rely on your memory of that 
past thread!

The scheme of using the doubler relied on the 100 kHz carrier recovery 
relied on the fact that the 200 kHz bandpass filters, being based on 
quartz crystals, was extremely narrow - on the order of fractions of 
Hz.  This effectively made them frequency-selective integrators (not the 
right word, but you get the idea...) and they were effectively immune to 
noise pulses as they simply could not react quickly.

IIRC - and I'll have to review my old notes - I used the first 200 kHz 
crystal as a series element and then passed it to a source-follower and 
then a bipolar amplifier with ridiculous gain (e.g. grounded emitter, 
high collector resistance) to form a limiter - and then ran it through 
another 200 kHz crystal and JFET/limiter. It took a couple of seconds 
for the outputs of the two limiters to saturate due to the narrow 
bandwidth and it was extremely tolerant of amplitude variations.  There 
was a phase shift with different amplitude levels, but since, on an FM 
microwave link the amplitude wasn't going to change much, that - and the 
phase shift related to temperature - was inconsequential.

On this simple recover scheme you could remove the input carrier for 
nearly a second (or blot it out with noise) and there would be almost no 
measurable effect on the output, aside from a phase shift of a few 10's 
of degrees which quickly rectified itself once the signal was returned.  
Had added some better tuning of the resonators I could have likely 
minimized this.  (I happened to have these 200 kHz HC-6 style units in 
my semi-large collection of 40-80's vintage crystals.)

The trick to replicating such a filter would be to find a suitable 
bandpass filter for the doubled frequency - in this case, a 120.005 kHz 
crystal (or thereabouts) - but it should be practical to convert the 
previously-filtered 60 kHz signal to a frequency for which a suitable 
crystal could be located.

The 60.003 kHz crystal to which I referred was a bandpass filter rather 
than an oscillator:  The TRF units found in WWVB clocks use these since 
most standard 60.000 kHz units end up being low in frequency when used 
in this sort of mode and they are a bit tricky to "pull" this far.

Rather than try to find such a crystal I would probably throw together a 
"Tayloe" commutating mixer with RC lowpass filtering with a time 
constant of a hundred milliseconds or so - this, filter/mixer being 
clocked at the nominal 60 kHz receive signal.

I would then follow it with another commutating mixer to translate the 
quadrature signal to any convenient frequency (say, audio - no doubt 
available from the 4060 or 4040 counter I'd be using!) where I would 
then do my frequency doubling and then follow it by yet another 
extremely narrow filter - this time, using an 8-capacitor SCF where I 
could set the detection bandwidth to a tiny fraction of 1 Hz just using 
a bunch of electrolytics!  It should be easy to set the carrier 
detection bandwidth to be a fraction of the information bandwidth so 
that reliable carrier recovery can be maintained under any conditions 
under which the BPSK data could be recovered.

(An example of an 8-capacitor "Roanoake" type SCF may be seen here: 
http://ka7oei.com/emm2a_scf.html  )

This recovered (and slightly filtered) signal, divided-by-two, could 
then be used to synchronously demodulate the original 
frequency-converted signal, at which point one should have a reasonable 
representation of the phase (and amplitude) of the transmitted signal - 
albeit, delayed by a fairly consistent amount.

Of course, all of this could be done by throwing a 16 bit A/D and DSP 
chip at it, but sometimes there's a simple pleasure in doing it with a 
bunch of 4000 CMOS and a few op-amps, handing the recovered baseband off 
to a PIC or Arduino only at the very end!

* * *

Many years ago I built a WWVB carrier recovery circuit using just a 
single-stage LC bandpass filter (to get rid of the VLF powerhouses) and 
an NE565 phase detector along with a 6 MHz VCXO divided down to 60 kHz 
as the comparison.  What amazed me was that even with the practically 
nonexistant filtering in front of the '565 (you really couldn't see the 
60 kHz carrier with the oscilloscope) that '565 would always find its 
way into lock over time - and then it would stay firmly there owing to 
that effect that occurs in which the effective loop bandwidth seems to 
decrease once lock has been achieved.  (WWVB's 45 degree phase shift 
"ID" would always throw it for a loop, though - pun intended!)

73,

Clint
KA7OEI


> Date: Thu, 20 Feb 2014 22:10:26 -0500
> From: paul swed<paulswedb at gmail.com>
> To: Discussion of precise time and frequency measurement
> 	<time-nuts at febo.com>
> Subject: Re: [time-nuts] New WWVB modulation format receivers
> Message-ID:
> 	<CAD2JfAhZvjSZ1vZiHBH05BwNc+DHd2gLQsTv1cAJc40UE1-gjw at mail.gmail.com>
> Content-Type: text/plain; charset=ISO-8859-1
>
> Clint
> I don't know if it was me or not the said the doubling scheme did not work.
> It does work but profoundly unreliably at least on the east coast. If you
> miss one cycle of carrier you loose phase making it useless. Jfor here on
> Time nuts and I tried a lot of things to get around the issues because
> simple is best. Now I do know that folks much closer to wwvb use the
> doubling method. Someone posted that here.
>
> You brought up a really interesting comment on the mix down method and I
> have been curious about that and thinking about it. Especially since we are
> looking for a 1Hz phase flip. You mention the 60.003 crystal as an
> oscillator or filter?
>
> Very hard to get those today, not so as little as 5 years ago. I found an
> ebay supplier that sold something like 25 for $5 so picked up a pack hoping
> that some crystals would actually work as a filter in the RF chain and they
> actually do, but you actually have to hand pick them. As an oscillator
> pretty poor behavior.
>
> I have released a RF frontend design to time nuts some 6 months ago and
> also a traditional costas loop using cd 4000 series chips. It does work and
> does hold phase over multiple days. It can get tripped up. But all in all
> for literally a few dollars does well. But I absolutely believe there is a
> better way as you are suggesting.
>
> Regards
> Paul.
> WB8TSL