How do you detect the broadband noise level?

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The traditional diode detector does not distinguish between broadband
noise and narrowband signals.

Is there a way to detect the overall broadband noise level? In this
coming semester, I will be working on an automatic noise cancelling
device for amateur radio. The input for the feedback network will be
provided by a circuit that responds to broadband noise but does not
respond to narrowband signals. I would prefer to monitor the noise at
the RF level (BEFORE the signals and noise reach the receiver), but it
would probably be easier and cheaper to monitor the audio noise.

The only idea I can think of is using an A/D converter to convert the
audio output of the receiver into discrete-time signals, using DSP to
convert these signals into a DFT, and then integrating over the DFT to
come up with the noise floor measurement. Of course, there needs to
be a way to take the absolute value of the discrete points.

Has anyone here done anything like this? I studied DSP last semester,
but we never touched a single DSP chip in the class. Using and
implementing DSP hardware is something I'll be picking up on my own.

Jason Hsu, AG4DG
[email protected]@@@jasonhsu.com

 

If you really do want to measure the RF signal level at the input of
your receiver, it might be worth your while looking at the Analog
Devices AD361 or AD362 broad-band true rms detectors.

 

I think this requires a DSP solution. Do a analog to digital conversion
and then analyse the frequency content and reject those components
that you think are narrow band signals. Take the rest as "noise"

Good luck.
Bill K7NOM

 

<reminiscence>This has been being done very successfully since about 1965 that I know
about. It takes one tube if you do it the 1965 way.</reminiscence>

It's even simpler than that--most of the broadband noise you want to reject in a
receiver is impulse noise, which consists of a train of large spikes. What you do is
to put a Schmitt trigger before the first IF filter, looking for large pulses, and
use a retriggerable monostable to turn off the IF gain until a few settling times
after the pulse has gone away again. The delay in the IF filter guarantees that the
gain will be off by the time the pulse arrives at the IF amplifier. Shorting out the
input to the IF filter is even better, because you don't have to wait for the filter
to settle afterwards.

There's no point in trying to do this at AF or even following the IF filter, because
a narrowband filter turns a big narrow spike into a long tone burst, at which point
it's a good deal harder to get rid of.

Using a DSP correlator at the audio output is more or less like trying to undo the
action of the IF filter, which seems a bit pointless when you can get the same effect
by moving the wire from one place to another. Another way of saying this is that the
autocorrelation of wideband noise becomes quite similar to the autocorrelation of the
signal, once you pass the IF filter, so there's less you can do about it.

Cheers,

Phil Hobbs