[time-nuts] Near-perfect chip for Loran-C frequency receiver
phk at phk.freebsd.dk
Fri Jul 4 03:32:44 EDT 2008
In message <1215126999.6104.9.camel at bigdog.icmp.com>, Carl Walker writes:
>On Fri, 2008-07-04 at 01:02 +0200, Magnus Danielson wrote:
>There's more than meets the eye initially when you attempt a receiver
>design of this type - at least as far as the analog section goes.
I hate to say so, but actual experience proces you wrong on all counts.
Remember we are not talking about fast signals like RTTY or voice,
if we were, you would be right.
We are talking about measuring one or more frequency/phase signals
and with an OCXO or Rubidium timebase we have minutes, hours or
even days of integration time at our disposal.
Currently, my AGC is a manual trimmer and I have not touched it
for years. I used to have a 246kHz LW station in visual range
which set the level for me, and that gave me no trouble at all.
Besides, a loop antenna can null out one strong signal very
A 12bit A/D gives about 72 dB dynamic range to work with, and and
as I said, I can detect Loran-C signals as far away as 8830 and
LORAN-C is both spread-spectrum and spread-time, those two signals
have amplitudes below one bit, but once they're despread, they
stand about 1 bit taller than the remaining noise.
The entire point about software defined radio is to do away with
the complicated analog side, and do it in the CPU, and VLF
time/frequency signals are about the easiest signals you can
work with for this.
See my earlier work here:
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
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