40-Metre Direct Conversion Receiver

• Using the circuit of 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 note 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.

• Audiooutput 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.

• Using the circuit of 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 note 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.

• Audiooutput 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.