QRP Labs QCX 5W CW, Assembly Instructions Manual

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QCX assembly Rev 1.08
110
In the real world, nothing is perfect
– there are component tolerances to think about. The
unwanted sideband suppression is maximised when the amplitude of the two paths is equal, and
the 90-degree phase shift is accurate.
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 equalise 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.
5.7 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
midrail at half the supply).
Sidetone is generated by the microcontroller and is a squarewave. To make it sound nice and
clean, the sidetone injection is done at the input to the 200Hz CW filter, which cuts off any of the
squarewave harmonics leaving a clean 700Hz sidetone.
The CW filter also provides a measured 18dB of gain.
5.8 Audio amplifier
The final stage in the receiver signal path is the audio amplifier, to drive earphones at a
comfortable listening level.