Handbook

Denver Channelised CE
User/Maintenance Handbook
Handbook Nō.-50-063708HBKM Issue No:-2
Date:-03/11/2003
Page:-
32 of 51
5.1.3 Ferrite Isolator (08-930003)
5.1.3.1 Description
The purpose of fitting an isolator to the output of a transmitter in a multi-transmitter
environment is such that each output is afforded a degree of isolation from every other.
Were this not to be the case, two simultaneous transmissions could interfere to create
intermodulation products, especially in the non-linear power amplifier output stages of
the transmitters. Whilst this effect would not affect the intelligibility of the two original
transmissions, a further two new transmissions would be created which could themselves
cause interference to third party users. In this case it is to isolate the duplex and simplex
uplink power amplifiers from inter-modulating each other.
A ferrite-isolator, (or junction circulator, as it is sometimes known) generally consists of
several major components; for example a ferrite region, a magnetic circuit and matching
circuitry which can form a three port isolator.
The ferrite isolator is a ferro-magnetic device, which has directional properties. In the
forward direction, RF arriving at the input is passed to the output with minimal
attenuation. In the reverse direction, RF arriving at the output due to reflected power
from a badly matched load, or due to coupling with another transmitter, is routed into an
RF load where it is absorbed. The isolator therefore functions to prevent reflected RF
energy reaching the power amplifier where it could cause intermodulation products or
premature device failure.
Ferrite materials form the active part of the junction in which the actual circulation signal
flow occurs. When a signal encounters a ferrite disk (e.g. port l) biased with a magnetic
field, it divides and part of the signal flows in a clockwise direction, while a portion of
the signal travels in the counter-clockwise direction, each with a different velocity. The
combined effects of the two rotating signals, create a standing wave around the perimeter
of the ferrite disk. By choosing the proper value of the ferrite’s magnetisation and the
magnetic field, the standing wave can be made to create a voltage null at port 3, such that
no power is transferred to it and a peak voltage at port 2, transferring maximum power to
it from port l. If a termination is placed on port 3 to absorb the signal flow in the reverse
direction the device functions as an isolator.