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Using the
circuit of 40-metre band direct-conversion receiver descr- ibed
here, one can listen to amateur radio QSO signals in CW as well as
in SSB mode in the 40-metre band. The circuit makes use of three
n-channel FETs (BFW10). The first FET (T1) performs the function
of ant./RF amplifier-cum-product detector, while the second and
third FETs (T2 and T3) together form a VFO (variable frequency
oscillator) whose output is injected into the gate of first FET
(T1) through 10pF capacitor C16. The VFO is tuned to a frequency
which differs from the incoming CW signal frequency by about 1 kHz
to produce a beat frequency in the audio range at the output of
transformer X1, which is an audio driver transformer of the type
used in transistor radios. The audio output from transformer X1 is
connected to the input of audio amplifier built around IC1
(TBA820M) via volume control VR1. An audio output from the AF
amplifier is connected to an 8-ohm, 1-watt speaker. The receiver
can be powered by a 12-volt power-supply, capable of sourcing
around 250mA current. Audio-output stage can be substituted with a
readymade L-plate audio output circuit used in transistor
amplifiers, if desired. The necessary data regarding the coils
used in the circuit is given in the circuit diagram itself.
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