Topband: noise

[Rick Karlquist]

W7lr at aol.com wrote:

> This system may be one used by Motorola in some of their equipment.   It
> requires finding a "clear" noise frequency near the desired signal
> frequency.  It
> is a lot more effective than a noise blanker which tries to  blank out
> noise
> pulses.

I don't know about Motorola, but Collins had a system with a
40 MHz receiver.  However, it was just a noise blanker.  The
only thing unique was listening on 40 MHz instead of near the
receive frequency.

If your noise blanker works fine in the absence of nearby strong
signals, but doesn't work during a contest, then you need to listen
out of band, away from the big signals.  If the noise is power line
interference, you can synchronize a pulse generator to the AC line
that blanks the receiver at a 60 Hz rate.  You need to manually
adjust the phase and duration for optimum results.  This of course
is totally immune to big signals.  Somewhere on the web, there is
a write up of this that I saw.  It used a diode mixer as an RF switch.
You could also use PIN diodes, but you would have to get the long
lifetime ones (5 usec).  You could also patch into your receiver's
existing noise blanker circuitry and do the blanking in the receiver.

Rick N6RK