This page discusses the physics and electromagnetics of signal transmission and reception processing. The application in this page will be for phone relevance but is generally universally applicable.
All transmitters combine a carrier frequency with an audio signal. Combining these to or mixing them is called a process of heterodyne. But before we can address the specifics of this process, we will first have a discussion of the word heterodyne.
But first, pronounce heterodyne as heter – oh – dine.
To heterodyne is to mix two signals so as to produce either a “modulated” signal or a demodulated signal. It the way by means that the signal from an antenna may have its intelligence extracted.
You can mathematically heterodyne a signal using Microsoft Excel for a visual approach to understanding.
In a nutshell, (and a little oversimplified) A transmitter generates a constant RF carrier signal. This signal is constant but changeable by the operator. An audio signal is added to it. The sum signal looks like just the RF signal because the RF signal is at a much high frequency than the audio. This sum signal is then dumped into the atmosphere for a receiver in, let’s say Packistan, to receive. That receiver’s antenna input sends all of the world’s radio stations into the RF preamp. The receiver has a variable local oscillator (VFO) which the operator sets to the frequency which he or she is interested in. The receiver then mixes the VFO signal with that coming out of its preamp to arrive at a difference. This difference is the audio content of that the transmitter on the other side of the globe sent.
That’s it in a nutshell. Yes, oversimplified but the primary way it is done.