Sunday, March 8, 2015

BFO for SSB Reception

This post is about beat frequency oscillator (BFO) module which is used to receive SSB and CW signals from ordinary shortwave receiver. The original circuit which is related to this post is obtained through the internet and we modify it to get more precise output with commonly available electronic components. (Unfortunately at the time of writing this original link is no longer accessible due to dead link.)

This module consists with two general purpose NPN transistors and we select 2SC828 because of its availability in local marketplace. For the crystal we use commonly available 455 kHz ceramic resonator.

Assembled 455 kHz BFO module.

This BFO unit is easy to construct and may not need any test instrument for calibration. To receive the SSB signals, power this unit with regulated 5V or 6V DC power source and place BFO module near to the shortwave radio receiver. To get good results place output wire of BFO near to the IF amplifier section of the target receiver. Now switch receive into the shortwave band and tune it until you hear a whistling sound from receiver speaker(s). If whistling sound exists, tune receiver into amateur radio bands to get some SSB/CW radio broadcasts. To get clearer SSB reception try to change the distance between receiver and BFO unit, and change the output wire location also.

We try this BFO unit with several shortwave receivers which including National Panasonic RX-5090T, National R-218, National R-314, Panasonic RX-CS780, Kaito KA-11 and some unbranded Chinese receivers, etc. In most of the receivers this unit produces good results and in some of the receivers it becomes useless due to differences in intermediate frequency.

With proper tuning of trimmer capacitor this unit may be able to produce exact 455.000 kHz output and its waveform is closer to 2Vpp square wave (on 5V DC power source).

In our testing we got quiet good results with this BFO on National Panasonic RX-5090T and some of the recorded samples are available at this soundcloud profile. Our modified schematic and PCB design is available to download at google drive.
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