Given favourable radio wave propagation, the shortwave and radio amateur band are chock-a-block with SSB (single-sideband) transmissions, which no matter what language they’re in, will fail to produce intelligible speech on an AM radio. SSB is transmitted without a carrier wave. To demodulate an SSB signal (i.e. turn it into intelligible speech) it is necessary to use a locally generated carrier at the receiver side. As most inexpensive SW/MW/LW portable radios (and quite a few more expensive general coverage receivers) still use plain old 455 kHz for the intermediate frequency (IF), adding SSB amounts to no more than allowing the radio’s IF to pick up a reasonably strong 455-kHz signal and let the existing AM demodulator do the work.
The system is called BFO for ‘beat frequency oscillator’. The heart of the circuit is a 455-kHz ceramic resonator or crystal, X1. The resonator is used in a CMOS oscillator circuit supplying an RF output level of 5 Vpp. which is radiated from a length of insulated hookup wire wrapped several times around the receiver. The degree of inductive coupling needed to obtain a good beat note will depend on the IF amplifier shielding and may be adjusted by varying the number of turns. All unused inputs of the 4069 IC must be grounded to prevent spurious oscillation.
Author: D. Prabakaran