Frequency Hopper Synchronization

[Andre Kesteloot]

On 3/23/2014 16:45 PM, Frank Eliot wrote:
> Gents -
> 	On Thursday, Andre and Maitland presented a way to synchronize a frequency hopper, using the 1 pps output from a GPS board. This pulse is accurate to one millisecond, which is good enough for a hopper that hops 128 frequencies/second. This solution should work, but isn’t very elegant, and is sorta brute force.

Frank,

Thanks for you suggestions.
Finally, we are getting somewhere: members such as you and  --I hope-- 
many others, will participate in this debate.

To make things clear, and in order to get the ball rolling, I built a 
real transmitter that really hops at 127 hops/sec, with a 1pps sequence, 
and I built it in the hope that all sorts of ideas and solutions for the 
receiving end would be generated by other AMRAD members.

Maitland came up with the concept of a receiver based on the Fun Cube 
Pro dongle, (a rather expensive item)  but, by next week I hope, I shall 
be able to transmit on the 70 cm band.
Then all these dongles should be able to receive it, and this should 
open the field to any one who owns even a cheaper dongle.

Now, as the proud father of the 1pps pulse idea, I feel I must state 
that I personally think that my 1pps sync pulse is beautiful and:
a) a pure work of art (!),
b) elegant,
and that
c) "elegance" is in the eye of the Beholder  :-)

My 1pps reset scheme is  --to my mind--  no more "brute-force" that the 
V and H synchronizing pulses that we had in RETMA TV for 50 years.

To me, simplicity in electronics, and particularly in Ham Radio, can be 
a virtue.
>   After thinking about it, I suggest trying to synch without the requirement of having a GPS decoder and being able to hear a GPS signal.

OK

>
> 	Here’s a thought experiment. Sample the signal level from the receiver at all possible 128 frequencies, at a rate of 128 samples/second, and store the sampled levels in memory for several seconds. This data rate is 128 x 128 samples/second, so you don’t want to store much. The receiving site knows the sequence of the 128 frequencies - it just doesn’t know where it starts. In software, sum the signal levels of the 128 known frequencies in the known sequence starting arbitrarily at sample zero. Then do the same sum for the 128 frequencies in sequence starting at sample one. Do this sum for all 128 starting points. One of the sums should be noticeably larger than the others, and that defines the correct starting point. Feed this starting point to the decoder. During a QSO, you could continue running the correlator at plus or minus one count, and switch when the adjacent count rises. Except for the initial sampling, all the heavy lifting is performed in a computer, working on stored data. This is where the work should be done.
>
> 	This tradeoff wasn’t discussed on Thursday. Maybe it is out of the question. We have nifty spectrum analyzers. Can we sample 128 points on the spectrum at 128 times/second? That seems like the most difficult thing to do. The correlation calculation seems doable, as it is mainly summing, and doesn’t have to be done in quite real time.
>
> 	This synchronization scheme appears substantially more complex to implement than the one I suggested a few months ago for autonomously phase-locking a synchronous detector to a received signal. That technique would buy a few dB’s S/N on a PSK31 type of signal by means of signal processing.
>
> 	

OK Frank, dust off you fastest laptop, and start coding :-)

Here are the tentative specs I intend to implement, just to get the ball 
rolling:
Hops: 127 per second.
Sequence start is triggered every 1 pps, GPS derived.
Occupied bandwidth about 200 KHz (will be widened later)
Frequency: between 446 and 447MHz
(more precision next week; I am resurrecting and modifying my original 
70cm Direct Sequence Tx, even as we speak)

>
> Just trying to start a discussion. Is this doable?? Frank W3WAG

The more solutions, the better, as far as I am concerned.

73
André N4ICK





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