Unlocking SSB Transmissions
In areas where radio waves propagate effectively, the shortwave and radio amateur bands are filled with SSB (single sideband) transmissions. Regardless of the language used, these signals, when received on an AM radio, fail to produce understandable speech. This is because SSB is transmitted without a carrier wave. To transform an SSB signal into coherent speech, it is imperative to employ a locally generated carrier at the receiver side for the demodulation process.
Unlocking SSB with BFO Circuit
Many affordable SW/MW/LW portable radios (and even some high-end general coverage receivers) utilize the standard 455 kHz intermediate frequency (IF). Introducing SSB into these radios essentially involves enabling the radio’s IF to catch a strong 455 kHz signal, letting the existing AM demodulator handle the rest. This system is known as BFO, short for ‘the beat frequency oscillator.’ At the core of this circuit lies a 455 kHz ceramic resonator or crystal, X1. This resonator operates within a CMOS oscillator circuit, generating an RF output level of 5 Vpp. This signal is transmitted via a length of insulated hookup wire wrapped multiple times around the receiver. Achieving an effective beat note requires a specific degree of inductive coupling, dependent on the IF amplifier shielding, and can be fine-tuned by adjusting the number of turns. To prevent unwanted spurious oscillations, all unused inputs of the 4069 IC must be grounded.
Understanding Beat Frequency
Beat frequency refers to the oscillation of volume from high to low and vice versa. For instance, if two complete cycles of high and low volumes occur every second, the beat frequency is 2 Hz. Beats occur when two sound waves with different frequencies overlap. During this overlap, their amplitudes alternate, adding and subtracting within a specific timeframe. This phenomenon results in periodic variations in the overall amplitude, creating the characteristic beat frequency pattern.