Using the circuit of 40-metre
band direct-conversion receiver described 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.
