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SSB Add-On for AM Receivers Schematic Circuit Diagram

Given favorable radio wave propagation, the shortwave and radio amateur bands 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.

SSB Add-On for AM Receivers Schematic Circuit Diagram

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 ‘the 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.

The beat frequency refers to the rate at which the volume is heard to be oscillating from high to low volume. For example, if two complete cycles of high and low volumes are heard every second, the beat frequency is 2 Hz.
Beats are caused by the interference of two waves at the same point in space. This plot of the variation of resultant amplitude with time shows the periodic increase and decrease for two sine waves.
The beat frequency is when two sound waves with different frequencies come across each other; then, their amplitude gets added and subtracted alternatively for a given time period.

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