seeking radio hacking advice

plug.org at 2nerds.com plug.org at 2nerds.com
Fri Feb 9 11:24:37 MST 2007


On Sat, 3 Feb 2007, Hans Fugal wrote:
> Why is the static (noise) where there's no signal (e.g. between
> channels) so loud? It seems to be the same effect on TV, AM, FM, etc. as
> long as there's nothing squelching. Why, if the noise floor is so far
> below the signal floor, is the noise so loud?

These questions piqued my curiosity, so I asked a double-E buddy about
it.  He gave a very cogent explanation which I will paraphrase below.
I'm a programming geek, not an EE, so my summarization probably
wouldn't make my friend proud, but I hope it conveys the basic
principles in a way that is comprehensible to other programming geeks.
If any EEs are reading this, I'm sure they'll be tempted to light
their flamethrowers. :-)

Anyway, the answers regarding non-squelched static are quite different
for AM and FM receivers.

For AM, the short answer is that AM receivers have a built-in AGC
stage (automatic gain control) which internally amplifies the input
signal before the detector stage.  Because of the way AM detectors
work (the detector is the final stage of the receiver), AM detectors
require a constant-amplitude input. The preceding AGC stage therefore
raises or lowers the gain to bring the input signal to a constant
amplitude.  When a signal is being received, the AGC will turn the
internal gain down; similarly, when no input signal is present, the
AGC turns up the gain so that the noise is raised to (roughly) the
same level as the signal.  Hence, when no signal is present, you might
say the receiver really does just turn up the volume on the noise.

For FM, the principle is different.  FM detectors operate by measuring
the timing of zero-crossings of the received signal. When a signal is
being received, the signal is at a much higher amplitude than the
noise, so the zero-crossings of the signal dominate the detection
process such that the noise is essentially suppressed.  When no signal
is present, all the detector can do is measure the zero-crossings of
the noise itself, so the output of the receiver is basically just a
representation of that noise.  You might think of an FM signal as a
big sine wave with a much smaller superimposed noise waveform: when
the signal (big sine wave) has sufficient amplitude, the amplitude of
the big sine wave will have well-defined zero-crossings despite the
much smaller (in amplitude) superimposed noise.  When that big sine
wave goes away, the zero-crossings of the noise are all that is left,
and the detector happily produces an output which represents the
randomness of those zero-crossings.  There are other nonlinear effects
at work in FM (e.g. the capture effect), but the basic idea has to do
with the timing of zero crossings.

Chris



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