Hi Paul,
An AM radio transmitter transmits a central carrier signal and two sidebands. The sidebands contain all of the intelligence in the transmitted signal but the carrier signal uses a large part of the transmitters power and is required by the AM receiver as a reference to demodulate the information in the sidebands. Only one of the sidebands (either one) is required,since they both contain the same information. Since an AM receiver uses the transmitters carrier signal as its demodulation reference, you can tune an AM receive to any frequency relatively close to the center frequency and easily understand what is being transmitted. If you eliminate the transmitters carrier signal and one of the sidebands, you save a large percentage of the power required to transmit the original AM signal but the remaining sideband becomes unintelligible when listening with a standard AM receiver. The intelligence is still there however and if you re-insert a carrier signal at the correct frequency, the the transmitted signal becomes intelligible again. This is exactly what is done in a single sideband system. You start out with a standard AM signal, eliminate one or the other of the two sidebands as well as the carrier signal and only transmit the remaining sideband. If you decide to transmit the sideband on the high side of the carrier frequency, you are transmitting an "UPPER SIDEBAND" signal The alternative would be the "LOWER SIDEBAND". The carrier, then needs to be re-inserted at the receiver end of the system. This process saves a tremendous amount of power at the transmitting end but greatly increases the complexity of the receiver.
When you listen to an AM signal with a SSB receiver, you essentially have one sideband ( the other sideband is filtered out in your receiver) and two carrier signals, the carrier transmitted by the radio station and the locally generated carrier generated inside your SSB receiver. Your receiver hears the AM transmitters carrier just like any other radio signal and that carrier signal will mix with the receivers locally generated carrier in a circuit called the product detector and produce a tone in your speaker. The frequency of that tone in your speaker is equal to the difference in frequency between the broadcast stations carrier frequency and the frequency of the locally generated carrier frequency inside your receiver. If the broadcast station is transmitting on 93 KHZ and your receiver is tuned to 92 Khz and you have selected Lower Sideband, you will hear a 1Khz tone in your speaker. If you slowly retune your receiver closer and closer to 93 Khz, the difference between the two carrier signals decreases and the "DIFFERENCE" tone goes down in frequency until the two carriers are exactly the same frequency at which point the "DIFFERENCE" frequency goes to "ZERO" and you receiver is said to be "ZERO BEAT" with the transmitters carrier frequency. If your receiver is properly aligned, the frequency display on your radio will be the same as the transmitted frequency of the broadcast transmitter.
Older model receivers used band switching and complex mixing schemes with many separate oscillators to accomplish this Since they had several oscillators involved, it was necessary to align the oscillators for each band individually so that the frequency display on your radio would be correct on all bands. On an older radio like that you couldn't zero beat a signal on 93 Khz and know that if your receiver displayed 93 KHZ, that it would also be accurate when listening to a signal on 14 MHZ because when you changed your receiver to listen to 14 Mhz, you were selecting a different crystal oscillator inside your receiver to serve as the basis for your locally generated carrier.
Modern radios use a different method of generating all of these frequencies which only relies on one crystal oscillator. In this case if your radio displays the correct frequency at 93 Khz, it will more than likely display the correct frequency at 14 Mhz as well. This isn't absolutely true all the time, but you can pretty much depend on it.
In your statement, you indicated that you thought you needed another oscillator to "ZERO BEAT" with the local AM radio station. You are correct and the point of this discussion is to let you know that you already have that oscillator built in to your radio.
Your locally generated carrier oscillator will not only mix with the radio stations carrier oscillator, it will also mix with the sidebands transmitted by the radio station. Only when your radio is tuned to the correct frequency will the audio coming from your receiver sound normal to you. Normal sounding speech is also an indication of "ZERO BEAT". In fact, some communications receivers use filters to eliminate the very low audio tones and make it very difficult to listen for a good zero beat. In this case, you may have to turn the audio volume up very high to hear the zero beat or just say "This is good enough, if I'm within a few Hz, I'm happy"
Rich NN7D
Flagstaff, Arizona USA.