Topband: Receivers, Noise Blankers and Key Clicks

[Jim Jarvis]

I won't repeat the previous posts on the topic, except 
for this...

"DSP receivers with wide roofing filters can be a problem..."

For anyone with a 756PRO, if you haven't modified the receiver,
I can guarantee you that you're suffering from degraded dynamic
range on 160.  It's caused by the am detected NB drive signal
leaking into the main audio channel.  

K1KP concepted a fix, which I implemented, and which was published in 
QST a couple of years ago.  You can find it on the european mods
page (mods.dk?)....or in the QST archives. 

Depending on what signals are present within the 15KHz wide
roofing filter, what you hear can be anything from keyclicks
to 'thumps and bumps' from lowpass filtered keyclicks, to
a 'growling' sound on ssb...to the am detected envelope of
your signal of interest, injected after the product detector
audio.  Essentially, you're interfering with youself.  

Addition of a small 1000uF electrolytic stabilizes the 8v rail,
which is being pulled down proportional to the NB drive signal.  

In the PROII, ICOM changed the decoupling on the 8v rail to
eliminate the problem. 

The proII noise blanker works, and is adjustable, too.  The 
PRO design only works for limited waveform/amplitude situations.

N2EA
jimjarvis at ieee.org