At 5/5/2004 05:34 PM, you wrote:
>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.
If the (class C) PA isn't being driven by the exciter, the devices will be
off. In that condition, it would be rather difficult for a low-level
signal (milliwatt or less) to drive the collector of an RF power transistor
hard enough to cause mixing. Most all of your problems will be with your
TX on.
> 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.
This is all good, though a dual isolator is overkill unless you have
in-band transmitters close to your TX freq. at your site. The bandpass
cavity(ies) is(are) probably the most overlooked component in a clean,
neighborly system.
Bob NO6B
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