[time-nuts] Near-perfect chip for Loran-C frequency receiver
carl at icmp.com
Thu Jul 3 20:23:25 EDT 2008
I concur with your observations, Bruce; you've made the points I was
alluding to in a far more succinct manner.
One broadband beast that solves all of the problems simultaneously and
works in any location is a tough nut to crack while maintaining really
good weak signal performance. Everything is a compromise.
When faced with a similar problem back in the late 70's - tracking
LORAN-C and WWVB simultaneously with as high a performance possible in a
single receiver - we did just as you suggest; two separate analog
receiver channels behind a moderate gain active antenna coupler, with
nothing more in terms of filtering than low pass roll off to avoid AM
broadcast overload of the first amplifier stage. Each analog channel had
its own bandpass filtering centered on the channel operating frequency
and specific bandwidth for the signals of concern. Worked like a charm
in the marine environment where it was intended to operate - so we never
had to worry about 1st stage overload from being too close to the
transmitter in Fort Collins at 60 KHz. One gain setting was sufficient.
On Fri, 2008-07-04 at 11:47 +1200, Bruce Griffiths wrote:
> Carl Walker wrote:
> > On Fri, 2008-07-04 at 01:02 +0200, Magnus Danielson wrote:
> >>> The analog side would need to allow for those signals also then.
> >> Naturally. The antenna-amplifier design will need to be more wideband
> >> oriented. Should not be too hard thought.
> >> Cheers,
> >> Magnus
> > I respectfully disagree; while making the wide-band receiver is an easy
> > task, you now have a family of unrelated signals - often of widely
> > varying signal strength. Once any of those signals becomes large enough
> > to drive the receiver non-linear, you rapidly run into issues.
> > AGC to maintain linearity isn't practical in this case - since reducing
> > overall receiver gain to compensate for one large signal - like WWVB if
> > you're close by that one transmitter - will potentially drive down the
> > gain for desired LORAN and other signals to the point where you can't
> > acquire and track many of the weaker but never the less desired signals.
> > 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. Once
> > it becomes 1's and 0's it's all straight forward - at least as far as
> > this old ex-analog guy is concerned ;-)
> > -Carl
> This is only true if one is attempting to simultaneously track all the
> signals with a single receiver.
> This isnt practical unless one uses separate analog receivers (each
> using AGC) dedicated to each signal to boost all signals to a similar
> level before sampling them with the ADC.
> In practice, it may be simpler to dedicate one digital receiver to each
> signal with the analog input filtering selected to suit a particular
> signal and location.
> There's little point in dedicating resources to receive signals that in
> particular locations are too weak to be useful.
> The 12 bit ADC's dynamic range will limit the disparity in signal levels
> that can be handled simultaneously long before the analog circuitry
> causes problems (at least in the case where the analog circuitry isnt
> driven into compression).
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