[time-nuts] New WWVB modulation format receivers

paul swed paulswedb at gmail.com
Fri Feb 21 22:25:34 EST 2014


Clint
Nice writeup but will say its to late to effectively reply perhaps tomorrow.
So early on I did all sorts of oscillators and such looking for the magical
carry through. They just didn't work.
The other thing is that I can do a lot of things just for myself with parts
no one else can get and thats not been the goal. I want to be able to say
get a digikey this or a mouser that. I never plan to build kits. Bless
those time-nuts that do make that effort. It really is a job. So I like to
build what I hope are reproducible things.

You mention using things like CD4000 series. I am not proud and if that
works fire up the iron I have plenty of the 4000 series. But as I mention I
have been introduced to the STM eval board and it can run Forth and it
moves along well with 12 bit A>Ds and D>As. Multiples of them. Its just
crazy the technology we have for $15 today.
Regards
Paul.


On Fri, Feb 21, 2014 at 9:55 PM, Clint Turner <turner at ussc.com> wrote:

> 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
>>
>
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