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OK, here's the deal: Unlike FM, SSB has no threshold where the signal
"drops out"; you can add more and more gain to the receiver (or more
and more antenna capture area) to bring up any signal that is above
the noise floor. However, there is a finite signal strength that any
given receiver is able to deal with. Signal strength is voltage, and
there is a maximum voltage that the receive amplifier(s) can accept.
In a SSB/CW receiver, the receive amplification is generally greater
than is needed most of the time, in order to have the ability to
receive weak signals. A strong signal is detected by the AGC, which
"turns down the power" momentarily in the receive amplifier.
Therefore, receive signal strength is limited to a preset maximum
voltage level. If a station you are receiving is at maximum signal
strength for your receiver at 100 watts and that station increases
power to 1000 watts, his signal will be no louder at your receiver;
but other signals will be momentarily weaker because the AGC is
reducing the gain. That is not an attenuator, but is one of the
reasons for the attenuator.
An attenuator is simply a resistive voltage divider in the receive
path from the antenna, upstream of the T/R switch so it doesn't
affect transmit. It simply reduces the overall voltage level and
hence signal strength by a given percentage, expressed as a certain
db; typically or 20 db. The way it does this is to shunt a certain
percentage of the signal to ground.
This protects the receive circuitry from overload when operating in
an environment of excessively high level signals. This overload makes
it difficult to hear weaker signals both directly as a result of the
overload, and indirectly as a result of the AGC action. The overload
also reduces the lifespan of the active devices in the receive
amplifier, in the form of a gradually reduced ability to amplify
(causing the receiver to gradually become "deaf") or, in extreme
cases, in the form of sudden blowout.
The attenuator is also useful, as pointed out in another post, in the
case where the strong signal is the one you are listening to. It can
be used to reduce the level of all signals so that the strong signal
is weaker but still perfectly copyable, and weaker signals fall below
the audible range. Then the desired signal is brought back up in
volume by increasing the audio gain.
There is also, in some receivers, an RF gain control. This is simply
an adjustable attenuator. IMO, the best combination for most uses is
a sensitive receiver with a fast AGC, combined with an adjustable RF
gain control AND a fixed, switchable 20db attenuator.
Tracy, KU4FL