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Ferretec's FERRETRAC filters contain a unique reference loop circuit that allows
a tunable filter to be locked to an RF signal (a receiver local oscillator, for
example). All the normal tuning errors are corrected by the loop gain. The designer
can now return to specifying the primary system goals of optimum RF performance.
The key to FERRETRAC filter operation is  an additional YIG sphere located in the
same magnetic structure as the filter. Thus,
any variations in magnetic field, which
would tend to tune the filter to other than the desired frequency, is also sensed
by this additional reference sphere. The reference sphere is surrounded by a small
air coil which can be biased to establish any desired fixed offset, up to +/-300
MHz, between the reference sphere and the filter. Together with elements of the
loop circuits, the reference sphere and air coil form a unique microwave discriminator.
The output of this discriminator is a voltage proportional to the error between
the quiescent tuned frequency of the YIG filter and the RF reference frequency (+
or - any deliberate offset). This error is fed back to the main electromagnet driver
to force the filter onto the reference frequency and keep it there under all static
and dynamic conditions of temperature, vibration, driver component aging, etc.
Figure 1 shows the block diagram of the basic FERRETRAC device. Note that the closed-loop
circuits are independent of the filter, allowing complete flexibility for design
and optimization of the filter.
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