[Elecraft] Question from a newbie > noise blanker

[Geoffrey Mackenzie-Kennedy]

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