[time-nuts] 60 KHz Receiver

[Bob Camp]

Hi

The other answer is that DSP was not really available when the original waveforms were developed. A modern system would not have a "must be able to work with manual delay lines and an oscilloscope" requirement on it.

Bob


On Oct 5, 2010, at 6:56 PM, Poul-Henning Kamp wrote:

> In message <4CABA343.8C5812B1 at cox.net>, WB6BNQ writes:
> 
>> Please explain to me how spread spectrum would enhance any process
>> of frequency or time recovery ?
> 
> Ok, it is late and I'm probably going to botch this, but I'll try:
> 
> The really short explanation is that your carrier transitions have
> random-ish looking signs, which, if properly designed, allows you to
> balance out pretty much any kind of CW or random noise.
> 
> This is, in essence, why you can separate the different GPS
> sattelites, even though they all send on the same frequency.
> 
> Technically speaking, Loran-C is spread spectrum, but they botched
> this aspect slightly, by not properly balancing the signs of (all)
> the codes.
> 
> The Austron 2000 has a switch that allows you to disregard certain
> bits in the codes to balance them, this increases the imunity to
> CW interference.
> 
> So given that you can trivially get a good OCXO today, I would design
> our "low-power-time-transmitter" to send one fix per hour.
> 
> For instance 127 bits of PRNG at 28 seconds per bit with a four
> second gap before the next timestamp (send ID ?)
> 
> On the receiver side, you know what time it is +/- one 28sec bit,
> so you digitize the signal and correlate the PRNG in a window around
> your local clock.
> 
> After an hour, you pick the correlation bucket that correlated best
> and have an instant estimate of the difference between your local
> clock and the average of that hours transmissions.
> 
> If xmitted as NFSK at around 100kHz and digitized at 1MSPS, you
> would get 1µsec resolution without resorting to interpolation.
> 
> By choosing all your magic numbers to be nonprime to normal
> CW signals inside their respective periods, you supress those
> by averaging.
> 
> This is why the NELS LORAN-C chains got new 4-digit GRI's: they
> are imune to pretty much traditional CW interference because
> they do not divide seconds or kHz on relevant timescales.
> 
> Poul-Henning
> 
> PS: DCF77 already does SS, but on a second to second basis.
> 
> -- 
> Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
> phk at FreeBSD.ORG         | TCP/IP since RFC 956
> FreeBSD committer       | BSD since 4.3-tahoe    
> Never attribute to malice what can adequately be explained by incompetence.
> 
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