caused by a first-order SSB term along with the carrier. The small amount of distortion caused by this effect is generally quite low and acceptable. The Kahn CSSB method was also briefly used by Airphone as the modulation method employed for early consumer telephone calls that could be placed from an aircraft to ground. This was quickly supplanted by digital modulation methods to achieve even greater spectral efficiency. While CSSB is seldom used today in the AM/MW broadcast bands worldwide, some amateur radio operators still experiment with it. Demodulation The front end of an SSB receiver is similar to that of an AM or FM receiver, consisting of a superheterodyne RF front end that produces a frequency-shifted version of the radio frequency (RF) signal within a standard intermediate frequency (IF) band. To recover the original signal from the IF SSB signal, the single sideband must be frequency-shifted down to its original range of baseband frequencies, by using a product detector which mixes it with the output of a beat frequency oscillator (BFO). In other words, it is just another stage of heterodyning. For this to work, the BFO frequency must be exactly adjusted. If the BFO frequency is off, the output signal will be frequency-shifted (up or down), making speech sound strange and "Donald Duck"-like, or unintelligible. For audio communications, there is a common agreement about the BFO oscillator shift of 1.7 kHz. A voice signal is sensitive to about 50 Hz shift, with up to 100 Hz still bearable. Some receivers use a carrier recovery system, which attempts to automatically lock on to the exact IF frequency. The carrier recovery doesn't solve the frequency shift. It gives better S/N ratio on the detector output. As an example, consider an IF SSB signal centered at frequency = 45000 Hz. The baseband frequency it needs to be shifted to is = 2000 Hz. The BFO output waveform is . When the signal is multiplied by (aka heterodyned with) the BFO waveform, it shifts the signal to , and to , which is known as the beat frequency or image frequency. The objective is to choose an that results in = 2000 Hz. (The unwanted components at can be removed by a lowpass filter; for which an output transducer or the human ear may serve). There are two choices for : 43000 Hz and 47000 Hz, called low-side and high-side injection. With