Tom:
"Antennas and systems we use have very poor (if effectively any)
response to F>20MHz signals. Any pulse is a series of sinewaves of
various amplitudes and frequencies."
Brad:
Some time ago, I wondered what kinds of noise pulses reach the input
ports of our preamps. For one measurement, I connected a beverage and
then my 40m yagi to a 200 MHz BW oscilloscope. Of course, I saw
components of local am and fm broadcast stations, but there was a
substantial amount of random noise with rise and 1/2-cycle times under
50 nS. This was done in December when Texas doesn't have too much
thunderstorm activity. If I were to do it in this time of year and
when there are T-storms in the area, I'm sure see much more
narrow-pulse activity. What would be the amplitude of the pulses?
Remember, my test pulses were in the few tens of microvolt range. The
random stuff I saw in December was in that range too. Noise from
electrical storms 25 miles distant would probably be much stronger.
"In this application higher frequency sinewaves can not pass through
any part of the system without rapidly increasing attenuation with
increased frequency."
You're right in saying the pulses wouldn't make it very far through
the receiver chain. But my question is What does the preamp do with
them? If it shuts down or rings, it doesn't matter how much
attenuation would be available downstream.
"This rounds and filters pulses, leaving only lower frequency energy.
A 500Hz filter limits reponse time to the order of milliseconds or
longer, not nanoseconds. That's where all the "pulse stretching" or
group delay errors are."
This is probably the most telling reason not to worry about what
preamps are doing out at the antenna input. Even if they shut down
for a few tens of microseconds...or even a millisecond or two...it
doesn't matter because the rest of the receiver won't miss the
interruption.
On the other hand, why not optimize each stage of the receiver for its
particular task? An example from another area of receiver design:
Recall how much we dislike the gain distribution in the MP and MK V
receivers. The gain is there, but it's at the wrong end of the IF.
Having it in the post amp creates noise problems that wouldn't be
there if more of it were available up front.
Regarding preamps, I'm not sure what a typical ham receiver would do
in my hypothetical case in which the preamp actually gates signals
while being shut down by noise pulses. Would it interfere with our
ability to "hear through" noise when we switch off the filters, set
the AGC to "Fast," and try to copy with ESP full-on?
"By the time the power level is high enough to gain-compress or
overload a reasonably good preamplifier, the rest of the system
(including the S/N ratio of the desired signal and our ears) are
toast."
Remember, we're not talking about gain-compression here. The levels I
used were well under the 1 dB compression points of all of the
preamps.
72,
Brad, KV5V
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