Classical modulation procedures - Signals, Processes, and Systems - page 244

Frequency modulation FM
Apart from amplitude, frequency and phase also offer the possibility of having meaning
superimposed on them. This means changing frequency or phase “in the rhythm” of the
source signal.

Frequency modulation FM and phase modulation PM hardly differ at first sight as with
every (continuous) phase displacement the “instantaneous frequency” is changed simul-
taneously. If the zero crossing of the sinusoidal carrier is displaced, the period length T*
of the instantaneous frequency changes.

In the case of the FM things are easy at first. In Chapter 7 “Linear and Non-linear Proces-
ses” the VCO (Voltage Controller Oscillator) was already dealt with in connection with
telemetrics. This VCO is simply an FM modulator.

As almost always in microelectronics there are analog and digital VCOs and various
representatives within these two categories. Here is a brief survey:

• Analog VCOs for the range 0 to 20 MH

These are ICs with internal voltage - controlled source of current. An (external)
capacitor is charged and subsequently discharged linearly via an adjustable constant
current generator whereby a near-periodic triangular voltage arises. The higher the
current is set, the more rapidly the capacitor charges and discharges. The frequency
of this triangular voltage is proportional to the current of the constant current
generator. This triangular voltage is distorted by a transistor or a diode circuit in such
a non-linear way that a sine-like voltage arises. Given the appropriate function gener-
ators this can be seen in the output signal: the sine has a small bend in the upper and
lower areas. In the final analysis the linear curve of the triangular voltage is rounded
off by a non-linear distortion.

• Analog VCOs for the range 300 kHz to 200 MHz

Capacity diodes have the property of changing their capacity slightly depending on
the voltage applied. This property is exploited in high frequency analog VCOs. A
capacity diode lying parallel to an LC resonant circuit is activated by an alternating
current, whereby the diode-capacity changes in the rhythm of the source signal. As a
result the resonance frequency/ natural frequency of the resonant circuit changes
minimally. To put it clearly: we have an FM signal.

• Digital VCOs and FM modulators with DASYLab

DASY Lab has two modules with which FM signals can be generated directly.
These signals can be outputted by means of the D/A converter of a multi-function
card. With traditional cards it is only possible to generate FM signals in the LF
domain in a real sense. We use these modules only to simulate, to establish the
essential characteristics of FM signals.