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                                               CCWN 75 3
                                               
ANALOG CIRCUITRY FOR A CCW RECEIVER by Wes Hayward

       Diagram 75:2 presents the RF portion of the circuitry
needed for a fairly simple receiver for the reception of CCW.
This design is committed to the extent that local oscillator
signals are derived from the 4 MHZ standard. However, provision
has been made for classic CW reception. This is necessary for
comparison purposes if meaningful experiments are to result,

       The receiver begins with a two pole bandpass filter at
3550 Khz.  This filter was designed for a bandwidth of 50 KHz and
a Butterworth response. This filter should provide 62 db of
rejection of the first image at 4.450 MHZ--certainly not the sort
of rejection one would expect from a competition grade receiver
but still suitable for preliminary experiments. This exceeds the
usual image rejection specifications of commercially manufactured
receivers. Note: Don't forget that 33K resistor. It has the
purpose of providing a proper termination for the filter. See my
paper in HR in early 75 on Preselector Design.

       The output of the filter is applied to a 40673 mixer. This
stage should provide reasonable conversion gain while maintaining
a useable dynamic range. The output of the mixer is applied to a
2 pole bandpass filter cut for a 10 KHz bandwidth. Amidon toroid
cores were used in this filter instead of the more common BC IF
cans because reasonable Q is needed at this stage of the circuit.
An unloaded Q of 200 was assumed in the design, although I have
never used the Amidon-2 material cores at this frequency and can
not be sure of the actual Q which will be obtained. In any case,
it should be much better than could be obtained with the usual,
"el Cheapo" IF cans that are so available. The T-106-2 cores
exhibit Q well over 300 on the 160 meter band.

       There are two outputs to the 450 KHz bandpass filter. One
of these is for conventional receiver work. Here, the signal is
routed to a CA-3028A product detector. This drives a pair of
audio stages which should provide ample gain for receiving
microvolt signals. This design is virtually identical to many of
the direct-conversion receivers which are popular and should
present no problems. Note that a 4 MHZ crystal oscillator with
the above described circuitry will yield a complete conventional
receiver. This should be built as the first step, in that this
will allow the net members to go ahead and get on the air on 3550
to exchange ideas.

       The other output from the 450 KHz BPF is applied to the
second mixer, another dual-gate MOSFET. The output of this
stage is filtered with a single resonator using an available (I
hope) Miller coil. The 50 KHz output is then applied to a high
gain IF amp
using a pair of inexpensive Motorola IC's. These chips are
virtually identical to the much more expensive MC-1590's, and
feature excellent AGC characteristics as well as a high stable
gain. The available gain in this section should be over 80 db.