N6KR's QRP Tech Notes / Sierra ABX Circuit

Here's the adjustable-bandwidth crystal filter circuit from the Wilderness version of the Sierra CW transceiver.  I used a fixed filter (about 400Hz wide) in the original NorCal Sierra, but this circuit can be easily added to the earlier unit to give you a bandwidth range of about 250-1500Hz. You can use a potentiometer on the front panel to control the bandwidth, but be sure to bypass the varactor-diode control voltage close to the filter.

Diodes D13, 14, and 15 and D4 can be either MVAM108 or MVAM109 or equivalent. If you can't find them from Dan's Small Parts or another supplier, check with Wilderness Radio.

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