[Elecraft] K3: Noise Reduction (NR) in FM Mode

[Geoffrey Mackenzie-Kennedy]

If I remember correctly it was Collins who first came up with the idea of 
using a simple secondary 'noise receiver' to blank the main receiver in HF 
SSB mobile installations using the KWM -1 and / or KWM-2A transceivers. The 
secondary 'noise' receiver was tuned to a frequency above 30 MHz and had its 
own small whip antenna.

The FM receivers in our old RCA 'Carphone' series of VHF Land Mobile rigs 
used hard limiting before the discriminator to get rid of impulse noise and 
other AM gremlins.

A modified version of the Collins system provides very effective noise 
blanking when used in a receiver covering the ham bands. Usually the noise 
receiver which provides the blanking pulses is tuned to a frequency clear of 
signals just outside of the band in use so that the main receiver is not 
blanked by strong unwanted in-band signals, which can be a problem with 
blankers driven by in-band noise especially during contests. One name given 
to this arrangement many years ago is the 'Evasive Noise Blanker'.

Further improvement in overall receiver performance can be gained by 
removing the noise blanking gate, which is usually in the main receiver's 
signal path ahead of the IF filter(s), and use the blanking pulses from the 
noise receiver to turn the LO injection to the first mixer on and off - 
quite easy to do if any 'logic' is used in the LO system. The next 
improvement is aimed at dealing with storm static!

Sorry to have drifted off subject!

73,
Geoff
GM4ESD.


Matt Zilmer  <mzilmer at verizon.net> wrote on Friday, June 06, 2008 at 6:24 AM



> At least with the Motorola Syntor X, they used a separate and very
> simple receiver to pick out impulse noise and used this secondary RX
> detector output to poke holes in the main RX audio.  They called it an
> Extender and I believe it was only included with the low-band Syntor
> X.  It is described as a Noise Blanker.