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40-Metre
Direct Conversion Receiver
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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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