Scott,
A ferrite circulator is simply a three-terminal device that is designed
to "steer" RF currents so that they move in one direction around a
circular path between ports that are spaced 120 degrees apart. What
comes in at one port will leave at the next port if the terminating
impedance is 50 ohms. Your assumption of its operation is exactly
correct.
An isolator is a circulator that includes a 50 ohm load on the third
port. Some manufacturers use the terms interchangeably, but generally
the term circulator describes the ferrite, garnet, and magnetic guts
that are inside the box.
A single isolator generally provides about 35 dB of isolation to protect
a PA from incoming RF that can cause intermodulation. A dual isolator
is just two single circulators in series, with two external dummy loads,
that can provide about 70 dB of isolation.
Since a circulator is a non-linear device, it will always generate a
strong second harmonic. For this reason, any ferrite
circulator/isolator must always be followed by a notch filter, a
low-pass filter, or a bandpass cavity to eliminate or greatly reduce the
second harmonic.
73, Eric Lemmon WB6FLY
wn1b8 wrote:
>
> List members,
>
> Is there any reason that a circulator could not be used as an isolator?
> Assuming appropriate power handling capabilities, I would think the former
> would work just fine. Amp output into port 1, Bp/antenna on port 2, dummy
> load on port 3, and any reflected power from the Bp/antenna should be
> absorbed by the dummy load. Am I missing anything here?
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