[Elecraft] Another Better Mouse Trap (noise blanker)

[William Carver]

I have a TRF 40 MHz "noise receiver" about 500 KHz wide that generates
blanking pulses. The idea was stolen from the Collins which had an
accessory blanking receiver for the KWM2. In my case it blanked by
momentarily interrupting the LO to the first mixer of the receiver,
providing essentially infinite attenuation of the noise pulse.

No separate noise gate. It is very effective, essentially removing any
impulse thatis big enough to produce a blanking pulse. However it
requires a separate noise-pickup antenna (I used a cut down CB whip) and
is a bit more hardware.

You never get something for nothing and blanking isn't free. For those
who like to think in engineering terms, think of this act of momentarily
blanking noise as "modulation": we're digitally modulating the incoming
spectrum of signals, multiplying by 1 to pass signals and 0 to blank
signals them. Multiplying = modulation. A noise blanker modulates  ALL
the incoming signals and some of the sidebands produced by the blanking
(modulation) process fall into the i.f. filter. So although the noise is
blanked, there is a bit of crud introduced.

I have a nearby "cogeneration" plant that generates electricity from a
little river that's sold to the power company. At one point there was a
transformer failure impending ("dielecric breakdown") that produced s
narrow 1 microsecond pulse of very high amplitude at a 120 Hz rate. The
pulses, radiated by their power line, were so large I could see them on
an oscilloscope connected to the dipole. I had steady S9+20 dB noise on
40m.

The noise blanker took them down to S4. It was close to magic although
not quite as good as my normal S3 noise level. But I tuned around the
band and realized then that the rise in noise level was from
off-frequency stations being smeared, or spread around, by the 120 Hz
blanking pulses.

Bill - W7AAZ