[time-nuts] New WWVB modulation format receivers

[d0ct0r]

Its still interesting to read an article from Radio-Electronics Magazine 
with date stamp back to August 1973. In that article Don Lancaster 
explain few classical techniques how to handle WWVB band.

Regards,
V.P.


On 2014-02-20 20:42, Clint Turner wrote:
> Several years ago I spotted a clever PIC-based software (DSP-ish)
> approach to WWVB modulation - but it has thusfar defied my attempts to
> find it via Google.  It was from the late 90's, early 2000's - and I
> may have it in an archive somewhere.
> 
> The exact details escape me, but I believe that it sampled at 8 kHz
> and was fed a crystal-filtered WWVB signal at 60 kHz, putting this
> bandwidth-limited, AGC-leveled signal directly into the PIC's A/D.
> 
> If I've done my math correctly, that would yield a frequency-inverted
> alias at 4 kHz.  The A/D was then mixed and/or decimated significantly
> and a simple software-based carrier recovery scheme (a Costas loop,
> maybe?) was implemented in this rather low-end PIC.  Because the TRF
> bandwidth was on the order of just a few Hertz, it took a fairly
> trivial amount of horsepower to implement.
> 
> Presumably, at just one baud it should be practical to do this on more
> modern PICs and AVRs using the same scheme.  The trick to homebrewing
> this is to find a 60.003 kHz crystal - but one of these could be
> swiped from a WWVB receiver module, or, perhaps, a source-follower
> could be used to recover the phase component of the received carrier,
> tapping off the signal from the BPF itself and making it available to
> the processor.
> 
> * * *
> 
> Another scheme - one that I believe was poo-poohed a while back on
> this list - is to simply take a bandpass filtered sample of around 60
> kHz and throw it into a four-quadrant multiplier to yield a 120 kHz
> signal sans phase shift.  I believe that the initial critique of this
> was that this was not a particularly good way to recover a weak
> signal, but I found it to be quite useful on a project some (15) years
> ago.
> 
> On this project, I had a 100 kHz pilot carrier modulated with NRZ BPSK
> telemetry data and this same carrier was used to convey the reference
> frequency to multiple, simulcast transmitters via a 33cm microwave
> link.  At 100 kHz, I simply had an L/C bandpass filter that was
> roughly 3-5 kHz wide on the transmit (to control the occupied
> bandwidth when XOR-gate modulated) and a similar filter on the receive
> end.  "Listening" to this 100 kHz center frequency, 3-5 kHz bandwidth
> was a 1496 configured as a multiplier, the output of which was passed
> through a simple filter constructed using 200 kHz crystals. The 200
> kHz from the doubler output was then divided-by-two and used to
> synchronously demodulate the BPSK data (after being filtered with
> either a Bessel or Gaussian LPF) and this same recovered 100 kHz
> signal was then made available to the master 10 MHz frequency
> reference for locking.
> 
> What impressed me was the fact that my input signal S/N could go about
> 40 dB below the detection bandwidth of the BPSK signal and still
> maintain perfect lock on the 100 kHz carrier, despite the fact that
> the 1496 - which really doesn't make all that great of a doubler
> compared with other available (but more expensive!) devices was being
> pelted with 3-5 kHz of garbage when the S/N was purposely compromised.
>  IIRC, the detection bandwidth of the crystal-based carrier recover
> filter was on the order of a few 10ths of Hz.  Yes, the phase did vary
> with temperature, but the rate-of-change was fairly slow and this fact
> was inconsequential in our application.
> 
> * * *
> 
> The upshot of this is that it should be quite easy to do a simple
> doubler-based carrier recovery system at 120 kHz (or something else,
> if it's frequency-converted) and, since it may be a bit tricky to find
> a cheap 120.006 kHz crystal, use an SCF clocked from a VCXO (or a
> simple fractional divider/DDS implemented in software) to provide a
> very narrow detection bandwidth that would satisfy the dynamics
> associated with the usable signal range over which the WWVB carrier
> could be reconstructed and the phase data could likely be recovered.
> The AM output of a standard WWVB clock module could then be used to
> aid in the windowing of a synchronous demodulator integrate-and-dump
> filter to recover the phase information and make these two pieces
> available to something like a PIC or an AVR/Arduino for crunching.
> 
> In the (likely!) event of a signal that was too weak to recover the
> amplitude information from the broader-bandwidth WWVB receiver module
> it should be practical to oversample (say, by 8x) the output of the
> synchronous demodulator and then infer the timing of the phase change
> over a period of time since the minimum period of this is well known
> (1 second!) and such timing could be (initially) autonomously applied
> with very good stability until the timing of the phase change resolved
> itself - something that could be correlated with a statistical
> analysis of the output of the amplitude detector, as well.
> 
> To a large degree, this sounds like a candidate for a "front end"
> consisting of good old 4000 CMOS logic and a few op amps with the
> output handed off to a fairly low-end, cheap processor module!
> 
> 73,
> 
> Clint
> KA7OEI
> 
> 
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-- 
WBW,

V.P.