Notes on the WJ-643-21
YIG Filter
I wanted to take some high frequency measurements, so bought some
Watkins Johnson YIG oscillators
and filters very cheaply on EBay. The WJ-643-21 was used in a radar
scanner receiver
(RWR) from the
1970's.
A good description of the design and operating principles is at multi stage YIG filters .
It has 3 standard coaxial connectors.
No pinouts etc were available.
The filter is mounted on a small machined aluminium box ptted with a
rubber compound.
First step was to dig out the rubber, which exposed a number of
protection components.
These components seemed to be a surge suppressor and some sort of
temperature sensor, and are not necessary to
use the device.
The name plate reads 24.8 ma to 70.1 ma , and the coil resistance
measures about 12 ohm.
The filters span the range of 2 to 18 gHz over this current
range.
The filter has 4 YIG elements mounted on Beryllia posts. There may be
some deliberate flux planning to shape
the bandpass , but I have not established this.
I used an LM317 regulator to control the magnet current, and did not
bother with the heater (which was
open circuit on the unit pictured). It is easier to correct for
temperature than use a heater.
The filter is a narrow bandpass filter, with the rejected frequencies
passed to a third output.
The filter was used in the RWR to select a band of specific frequencies
for closer attention , while the rejected frequencies
were displayed to the aircraft's pilot on a broadband panoramic display.
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