Understood. I had been pondering a trigger derived from the noise receiver's
output pulses but with memory and variable delay added to freeze the
display, and allow one or several output noise pulses to be displayed.
Jack Smith wrote:
Actually, a swept spectrum analyzer is a very difficult machine to use for
tracking noise, unless the sweep is triggered with the power line zero
crossing.
<snip>
If one were to go about an effective noise blanker, I believe you could
do much worse than building a new version of the old Collins approach --
a broadband receiver tuned to 30-35 MHz to detect noise and then a fast
gate (with suitable delay for synchronization) to clip the received
signal.
I agree and use a version. The old Collins system was first used in mobile
installations I believe with the KWM-1, but the approach has undergone many
changes over the years. Possibly the most significant changes that have
evolved for HF use is to reduce the bandwidth of the noise receiver, and
tune the noise receiver to a clear frequency close to the system's working
frequency to get a more accurate sample of incoming noise. This mutation of
the system has become known as the Evasive Noise Blanker, and in Amateur use
the noise receiver is tuned to a clear frequency close to but outside of the
band in use. From using this type of blanker system for many years I find
that a noise receiver tuning range of 25 kHz above each operating band is
satisfactory. I opted to avoid using a noise gate in the main receiver's
signal chain in the interests of dynamic range, but use the noise pulses to
inhibit a fast flip-flop which provides the H-Mode signal mixer with it 50%
duty cycle LO squarewave injection, thus having the noise pulses switch
injection. This type of blanker will not respond to legitimate in-band
signals, a problem sometimes found during contests when using a blanker
which samples in-band noise.
73,
Geoff
GM4ESD
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