To improve the accuracy of the 90-degree phase shift, R17 and R24 allow adjustment of
the phase shift at higher and lower audio frequencies respectively.
R27 allows adjustment of the balance between the I and Q channels, to equalize the
amplitude from each path.
This CW transceiver kit includes built in alignment and test equipment, with a signal
generator that can inject a test signal into the receiver input. It makes it easy to perform
these adjustments, as described previously.
Note the 2-pin header pads JP9, JP10 and JP11 at the output combination potentiometer
which allow experimental modifications or connections.
4.7 CW filter
The CW filter used in this receiver has a 200Hz bandwidth. The circuit is based on the HI-
PER-MITE CW filter kit design by David Cripe NM0S, available from the Four State QRP
group: http://www.4sqrp.com/HiPerMite.php (thanks David for permission to use it here).
This is a high performance circuit specifically designed to avoid objectionable ringing.
There are three stages of low-pass filtering and one stage of high-pass filtering. The first
three stages retain the 2.5V “midrail” bias all the way through from the input transformer T1.
The final stage IC9A is biased using the 5V supply (avoiding a few extra components to
create a real 6V mid-rail at half the supply).
The CW filter also provides a measured 18dB of gain.
Sidetone is injected at the input to the CW filter, so as to make is sound nice and clean by
cutting off the squarewave harmonics leaving a clean 700Hz sidetone.
4.8 Audio amplifier
The final stage in the receiver signal path is the audio amplifier, to drive earphones at a
comfortable listening level.
90
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