Let's not forget that most PAs are powered continuously, and it is the
exciter that is keyed on and off. Any signal that mixes in the PA will
likely be amplified at full power, regardless of whether the transmitter
is "on" or not. In order to prevent this from happening, it is
sometimes necessary to use the "belt-and-suspenders" approach: A dual
circulator, followed by a sharply-tuned bandpass cavity or two, between
the PA and the duplexer TX port. This combination limits most incoming
signals to a narrow band that can be diverted into the load by the
circulator. The common bandpass/bandreject (BpBr) duplexers have almost
no bandpass selectivity, so this must be provided by separate
bandpass-only cavities.
73, Eric Lemmon WB6FLY
Someone wrote:
...What bothers me here about using a circulator is that you say it only
occurs when your TX is active. This doesn't sound like IM to me, as that
would occur regardless of whether the tx was active or not (IM is
produced
> when external RF comes down the TX line and mixes in the PA stages. This does
> NOT require that the PA actually be making power)...
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