[time-nuts] New WWVB format...

[Dennis Ferguson]

On 26 Sep, 2012, at 11:19 , Majdi S. Abbas wrote:

> On Wed, Sep 26, 2012 at 10:13:22AM -0700, Tom Van Baak wrote:
>> My reading of the document(s) is that the new format will in fact allow 
>> WWVB to be used as a frequency standard with even greater precision then 
>> before, though not with unmodified legacy WWVB carrier receivers. My hope 
>> is that one of you will produce a clever reference design for such a T&F 
>> receiver make it available to the group. It sounds like a very fun DSP 
>> project; one that we can all learn from. Bonus points for making it an 
>> open-source Arduino shield. Making it work with both DCF77 and WWVB would 
>> also be a plus.
> 
> 	DSP would be good, although I also think an microcontroller
> implementation could be interesting.  Atmel's ARM MCUs look like they'd
> be good candidates for this sort of thing.  (Pretty fast, enough storage
> to do interesting things with it, and a fast enough ADC for 60 KHz.)

This is fine, though to make it maximally useful for time and
frequency purposes I believe the hardware might need to provide a
way to synchronize the ADC clock to an external reference, and likely
some way to time-mark the incoming data (e.g. a quick-and-dirty version
might feed a PPS signal to the second channel of a stereo ADC, if no
more elegant solution is available).  A control loop to discipline an
oscillator's output might use that oscillator to clock the ADC and adjust
the oscillator to zero the ADC's phase alignment with the input signal,
if that can be made to work.  A system to measure WWVB propagation delays and
signal levels might instead clock the ADC and the time marker with a
known-accurate frequency and PPS (e.g. a GPSDO).

RFSpace makes commercial LF/MF/HF SDR equipment with almost the right inputs
for this (an external frequency input and a timing trigger).  What I'd like
is a tiny-budget version of this just for LF stations.

> 	I've got a couple of these that I might use as a development 
> platform:
> 
> 	https://www.olimex.com/Products/ARM/Atmel/SAM7-P256/
> 
> 	Has anyone come up with a reasonable algorithm to implement in
> a microcontroller?  (DSP development kits are a bit more spendy than I'd
> like to invest in a prototype. :)

I guess the trouble with this is only that the availability of brute force
can sometimes make it unnecessary to deal with a lot of complexity.  If your
job is to do a convolution of a model of what you know was transmitted
against the incoming signal to measure the time alignment then using a
platform where you can store big blocks of data and do Fourier transforms
with wild abandon can provide really good results without having to spend
a lot of time thinking about it.  Even quite modest modern PC hardware comes
with a boatload of memory and is exceedingly speedy, and for some purposes
it can save a lot of time and effort just to make use of that compared to
trying to do without.

I have a quick-hack DCF77 PM detector which runs on PC hardware and makes use
of one of the above-mentioned RFSpace receivers for the data acquisition.  While
it is now in boxes being moved, when I get it back up I would love to lose the
RFSpace receiver in favor of something much less costly, but would hate trying
to make this work with something less capable than the PC.  Using a microcontroller
like that to do the A/D conversions and send the data collected out (say) an
ethernet port to a PC which does the heavy computational lifting (that's what the
RFSpace receiver does) would appeal to me, but trying to do without the PC would
not.

Dennis Ferguson