Digital Radio Modulation

Radio amateur transceivers can not be connected directly to terminals or computers to perform digital data transmissions. The binary signal exiting from the serial port of a computer has a rectangular shape, has a strong DC component and a great amount of harmonic frequencies. Transceivers are generally planned for voice (or telegraphy) transmission and admit particularly frequency modulation (F3A), thus they can not transmit DC (frequency leading towards zero) nor frequencies that exceed human voice ran ge (passband being almost 3 KHz wide).

To overcome these and other obstacles you don't have to re-plan or rebuild a transceiver. You can instead interpose between the P.C. and the transceiver a particular modulator/demodulator which transforms digital signals of the P.C. into audio signals bet ween 300 and 3,000 KHz (passband of the transmitter modulator); these signals are thus sent to the transceiver as if coming from a microphone. The device, normally called a modem, generates a continuous sinusoid signal centered in the L.F. passband of th e transmitter, whose parameters (amplitude, frequency, phase) are varied by the serial digital signal coming from the P.C.

The transfer in phonic band is obtained modulating a sinusoid signal, whose frequency is in the L.F. passband of the transmitter (nearly 300 - 3,000 Hz), by means of the digital signal coming from the P.C. The techniques of modulation are basically three :

FM or frequency modulation (for speeds up to 1,200 bytes/sec.)
PSK or phase modulation (for speeds up to 4,800 bytes/sec.)
combined phase and amplitude modulation (for higher speeds)

Frequency modulation pre-sets a given frequency value in the generated sinusoid signal to each of the two logic statuses 0 and 1, as shown in figure:

Phase modulation pre-sets two different phases of the carrier wave to the two logic statuses 0 and 1; as shown in figure below, for level 0 waveform is modulated in phase with the reference mark, that is phase remains unchanged, whilst for symbol 1 there is a leap of 180 ° and thus phase inversion; during reception obviously it is mandatory for the receiver to have a phase reference to discriminate variations.

DIGITAL RADIO MODULATION