QRP Labs QCX 5W CW, Руководство по сборке

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QCX assembly Rev 1.08
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The switch wouldn’t provide enough attenuation to mute an operating receiver; but during receive,
our receiver isn’t operating; all the switch has to do is protect the Quadrature Sampling Detector
from seeing 45V peak-peak which would destroy it.
5.5 Band Pass, Phase Splitter, QSD and pre-amps
Since the band-pass filter, Phase splitter, Quadrature Sampling Detector and pre-amp circuits are
so tied up together, I am going to consider them all together in this section.
This circuit implements an input band-pass filter and double-balanced Quadrature Sampling
Detector with low-noise pre-amps. Yet it does this with a low parts count, and resulting low
complexity and cost. The FST3253 is a dual 1:4 multiplexer which is often seen in QSD circuits. It
has fast switching times and very low on resistance of only a few ohms. The input signal is
switched by the quadrature LO to each of the four integrating capacitors C43-C46 in turn, for 90-
degrees of the RF cycle each. The result is that the audio difference (beat) between the RF input
and LO input appears across each of the four integrating capacitors, with four phases at 0, 90, 180
and 270 degrees.
The operational amplifier IC5a takes the difference of the 0 and 180-degree outputs and amplifies
it, resulting in the I output of the QSD. Similarly, IC5b differences the 90 and 270-degree outputs
to produce the Q output.
The combination of the relatively large 470nF capacitors and the low source resistance results in a
fast roll-off of the audio response. This is effectively a very narrow band pass filter since any
incoming RF more than a few kHz away from the LO frequency is greatly attenuated. The QSD is
therefore inherently a very high performance mixer design with high third order intercept and
dynamic range, and low loss (0.9dB).
The FST3253 dual switch is often connected with the two switches simply paralleled together
(which does half the switch ON resistance). But I prefer the double-balanced mixer configuration
which provides higher performance. The double-balanced configuration requires two RF inputs
180-degrees out of phase (opposite to each other).