Re: Noise figure paradox

Hi Richard,

"Richard Clark" <kb7qhc@xxxxxxxxxxx> wrote in message
news:701js4lgrngrhinqdean8bobae4hvb4li1@xxxxxxxxxx
That was a curious objection to a solution answering a problem as it
was specifically stated. Are there angles to showing noise being
overcome by several means when you offered none?

My means were "reduce the noise figure of the amplifiers in your front-end"
and "reduce the phase noise of your oscillators/PLLs/etc." "Averaging the
input" is a clear winner here too.

What "noise" were you speaking about when through the course of this
thread it has most often been confined to kTB than, say, cross-talk,
splatter, spurs, whistlers, howlers, jamming, and a host of others?

For the sake of this thread, it's been just thermal and oscillator nose since
these are -- AIUI -- what limit traditional analog (AM/FM/PM) communication
systems. Most of the rest of what you've listed are certainly real-world
problems, but they're (hopefully) somewhat transient in nature and -- as
AIUI -- often all lumped into a single "fade margin" when designing the an
end-to-end system. E.g., the transmission medium is often modeled with
something no more complex than, say, the Friis equation and Rayleigh fading.
I do realize that in the real world things like spurs or splatter can end up
being very expensive (frequency changes, high-order/high-power filters, etc.)
if you're co-locating your radio with many others on a hilltop -- I've been
told that if you take a run-of-the-mill radio to a place like Mt. Diablo in
California, many of them just fall over from front end overload and cease to
function at all.

What constitutes "successfully?" Is this a personal sense of well
being, or is it supported by a metric?

Usually something like a 12dB SINAD standard is used for analog modulation
schemes or a 1e-3 bit-error rate for digital modulation techniques (before any
error correction coding is applied).

Spread Spectrum is so ubiquitous that waiting on anticipated exotic
failures of phase noise, on the face of an overwhelming absence of
problems, is wasted time indeed.

It's not ubiquitous on amateur radio, though.

But yeah, commercially it certainly is, and my understanding is that phase
noise in oscillators in a Big Deal for cell sites, requiring much more
strigent standards than what a 2m/440 HT's oscillator is likely to provide.
The network timing of cell sites is sync'd to atomic clocks via
GPS-disciplined oscillators system as well.

As to sampling error via the net. Time was when 16x over-sampling for
RS-232 was the norm.

I've meet many RS-232 routines that don't do any over-sampling at all -- I've
even written a few. :-) For most applications the SNR of an RS-232 signal is
typically well in excess of 20dB if you don't exceed the original specs for
cable length of bit rate. (Granted, as least historically before RS-232
starting falling out of use, it was probably one of the most "abused"
electrical interconnect standards in the world, and 16x oversampling certainly
would let you go further than simple-minded receivers with no oversampling.)

---Joel


.

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