In a message dated 30/06/05 13:35:58 GMT Daylight Time,
[hidden email] writes:
I also see that Q5, the transistor that shorts out the output filter tank,
has no bias at all. There is no DC return for the base to ground. Is this
correct? Maybe the base should not be floated? Maybe Q5 is open?
Reply:-------------------------------------------------------
Rich,
Q5 on the KNB2 is either on or off, there is no in-between state so no bias
is applied to the transistor apart from when it is switched on by the one shot
Q3/4. Possibly looking at the base and collector of Q5 with a scope would
help to see the spikes of enabling voltage and suppression of the segment of
signal during a short duration noise burst. The circuit details with the
schematic on the last page of the KNB2 manual should give an insight into the
method of operation.
The problem with checking most noise blanker is generating the necessary
test conditions to see if the operation is correct. Most radio equipment
manufacturers are rather vague in this respect and I have yet to find a piece of
test equipment that will provide interference pulses on demand. Possibly an
automobile spark plug driven by an ignition coil and contact breaker pulse source
may be the answer. Have tried using electric drills and other brush motors
for generating noise pulses with only limited success. Otherwise it is
sometimes a case of awaiting a RF noisy auto to go past as the KNB2 only functions
on short duration spikes of noise.
Regards,
Bob, G3VVT
K2 #4168 with KNB2
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