Ok, I see why I connected the two technologies. Does the 'exact' timing needed
for CCW help with the 'very narrow bandwidth detector' methodology?
Is the CCW detector only allowing or gating for the signal at an 'exact' moment
in time?
hmmm, more questions. :-)
----- Original Message ----
From: Jim Lux
<[EMAIL PROTECTED]>
To: Ted <[EMAIL PROTECTED]>
Cc: flexradio@flex-radio.biz
Sent: Sunday, December 23, 2007 10:24:24 AM
Subject: Re: [Flexradio] CCW
Quoting Ted <[EMAIL PROTECTED]>, on Sat 22 Dec 2007 07:23:58 PM PST:
>
>
>
> I always thought that the ability of GPS receivers to detect signals
> that ride at or deep inside the noise levels was a direct
> descendant of the concepts of CCW.
Not exactly. GPS relies on spreading the signal, and then despreading
it in in the receiver with an internally generated perfect copy of the
original spreading signal. It's the phase of the spreading signal
that gives you the unambiguous range measurement needed.
They are similar in that they both rely on using a very narrow
bandwidth detector.
The math experts on this list know more
> about this topic than I do. But CCW was a pretty exciting concept
> that could dramatically improve reception of signals that couldn't
> be 'seen' or heard with the ear.
"was" is the operative term. It was very effective using the
technology available at the time. Nowadays when it's straightforward
getting ppb frequency references (GPS disciplined) and having a lot of
DSP available on the coding and decoding sides, the balance shifts...
I think that there are better comm methods for an HF channel. The
ionosphere puts a basic limit of a couple hertz on how narrow you can
get (doppler spreading). Furthermore, CW is an on/off modulation, so
the average power is lower than a constant envelope method.
Techniques such as PSK or MFSK are probably better, especially those
that do some amount of Forward Error Correction and that have
modulations "tuned" to the ionospheric channel.
>
>
> If the SDR can finally bring CCW into the ham 'bag of tricks' for
> long distance RF communication than it will be terrific event.
I suspect that a low speed form of PSK16 or MFSK can blow it away.
OTOH, CCW has some historical interest, and the beauty of a software
radio is that experimenting with it isn't particularly
more difficult
than any other modulation scheme.
Jim,W6RMK
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