The transmission of information over a high-frequency channel initially began with amplitude modulation. This was a simple procedure but was quite susceptible to atmospheric disturbances. From 1945 onwards, more and more frequency modulation was used. This was also the first method of transmitting digital information. In 1G systems, digital information was encoded by assigning a frequency to “zero” and a different frequency to “one”. On the receiver side, all you had to do was differentiate between the frequencies using filters.

How to convert digital information into radio signals

Phase Modulation

However, phase modulation is now used for particularly fast and efficient transmission of digital information. But how can you change or detect high frequency phases?

To explore this further, lets hav a look at trigonometric functions. A high-frequency signal, an electromagnetic wave, corresponds to a sine or cosine function.

The following graphic shows a sine function and a cosine function.

Sine- and cosine signal and addition of the signals

Blue is the sine function, orange is the cosine function. It should be noted that the sine function differs from the cosine function only by a 90° phase shift. If you add both functions together, you also get a sine function (gray) which is 45° out of phase with the original sine function.

The next graphic shows signals for (sin + cos), (sin – cos), (- sin – cos) and (- sin + cos). The result are four sine functions shifted by 90° from each other.

Four signals that belong to additions and subtractions of sine and cosine.

Not only phase jumps of 90° can be set, but any phase. If we designate I as the amplitude of the sine signal and Q as the amplitude of the cosine signal, any phase can be set using I and Q. This is shown in the following I/Q diagram.

How can phases be created/changed in a transmitter? For this purpose, two transmission paths are created instead of one. The transmitter mixers are powered by the same oscillator, but one of the transmitters receives the signal with a 90° phase shift.

The signals are added after the mixer and form, for example, the IF signal of the transmitter. With such an I/Q modulator, any phases can be set with I(t) and Q(t). The signal can likewise be demodulated at the receiver.

Quaternary Phase Shift Keying

An example of digital phase modulation is quaternary phase shift keying, also known as QPSK. The I and Q inputs are digitally controlled with 1 or -1. Each change results in a change in phase, either 90° or 180°. In this way, two bits are transmitted simultaneously per transmission cycle. A “symbol” is transmitted.

This makes it possible to efficiently modulate binary data and transmit it via radio.