Frequency modulation is widely used, particularly on frequencies above 30 MHz. It offers many advantages, particularly in mobile radio applications where its resistance to fading and interference is a great advantage. It is also widely used for broadcasting on VHF frequencies where it is able to provide a medium for high quality audio transmissions.

In view of its widespread use, scanners and many other receivers are able to demodulate these transmissions. Naturally there are specifications and figures that receiver manufacturers quote for the performance of their sets when receiving FM.

What is FM?
As the name suggests frequency modulation uses changes in frequency to carry the sound or other information that is required to be placed onto the carrier. As shown in Figure 1 it can be seen that as the modulating or base band signal voltage varies, so the frequency of the signal changes in line with it. This type of modulation brings several advantages with it. The first is associated with interference reduction. Much interference appears in the form of amplitude variations and it is quite easy to make FM receivers insensitive to amplitude variations and accordingly this brings about a reduction in the levels of interference. In a similar way fading and other strength variations in the signal have little effect. This can be particularly useful for mobile applications where charges in location as the vehicle moves can bring about significant signal strength changes. A further advantage of FM is that the RF amplifiers in transmitters do not need to be linear. When using amplitude modulation or its derivatives, any amplifier after the modulator must be linear otherwise distortion is introduced. For FM more efficient class C amplifiers may be used as the level of the signal remains constant and only the frequency varies.

Figure 1 Frequency modulating a signal

Wide band and Narrow band
When a signal is frequency modulated, the carrier shifts in frequency in line with the modulation. This is called the deviation. In the same way that the modulation level can be varied for an amplitude modulated signal, the same is true for a frequency modulated one, although there is not a maximum or 100% modulation level as in the case of AM.

The level of modulation is governed by a number of factors. The bandwidth that is available is one. It is also found that signals with a large deviation are able to support higher quality transmissions although they naturally occupy a greater bandwidth. As a result of these conflicting requirements different levels of deviation are used according to the application that is used.

Those with low levels of deviation are called narrow band frequency modulation (NBFM) and typically levels of +/- 3 kHz or more are used dependent upon the bandwidth available. Generally NBFM is used for point to point communications. Much higher levels of deviation are used for broadcasting. This is called wide band FM (WBFM) and for broadcasting deviation of +/- 75 kHz is used.

To receive FM a scanner may have two different modes, one labelled WBFM and the other NBFM. The correct mode must obviously be selected for correct reception. Also if it is anticipated that reception of both modes is required, then the receiver must have the capability of receiving both of them.

Receiving FM
In order to be able to receive FM a receiver must be sensitive to the frequency variations of the incoming signals. As already mentioned these may be wide or narrow band. However the set is made insensitive to the amplitude variations. This is achieved by having a high gain IF amplifier. Here the signals are amplified to such a degree that the amplifier runs into limiting. In this way any amplitude variations are removed.

To convert the radio frequency signals appearing at the output of the IF stages of the receiver into audio voltage variations to be amplified by an audio amplifier an FM demodulator must be used. This converts the frequency variations of the carrier into audio voltage variations. This is done using a circuit where the output voltage is dependent upon the input frequency. The linearity of the response is obviously important otherwise distortion is introduced. A number of circuits can be used to do this. Two popular circuits that use discrete components are the ratio detector and the Foster-Seeley detector although today FM demodulators are contained within integrated circuits and the only requirement is for a coil and capacitor to connected to the chip to provide the frequency dependent circuit.

Another method is to use a phase locked loop. The way in which this circuit operates to demodulate FM is very simple. The circuit is set up to operate as shown in Figure 2. The FM signal from the IF stages of the set is connected to one of the phase detector inputs as shown, and the output from the VCO is connected to the other.

Figure 2 A phase locked loop FM demodulator

With no modulation applied and the carrier in the centre position of the pass-band the voltage on the tune line to the VCO is set to the mid position. However if the carrier deviates in frequency, the loop will try to keep the loop in lock. For this to happen the VCO frequency must follow the incoming signal, and for this to occur the tune line voltage must vary. Monitoring the tune line shows that the variations in voltage correspond to the modulation applied to the signal. By amplifying the variations in voltage on the tune line it is possible to generate the demodulated signal.

Squelch
When a receiver switched to FM and no signal is present it is found that high levels of audio noise are heard. This is very unpleasant and to overcome this most receivers have what is called a squelch circuit. This cuts off the audio when no signal is present. Many scanners and hand-held VHF or UHF transceivers have a control to adjust the signal level below which the audio cuts off. By using this control, the set can be adjusted to detect very low level signals if necessary.

Quieting specification
One of the advantages of FM is its resilience to noise. This is one of the main reasons why it is used for high quality audio broadcasts. However when no signal is present, a high noise level is present at the output of the receiver. If a low level FM signal is introduced and its level slowly increased it will be found that the noise level reduces. From this the quieting level can be deduced. It is the reduction in noise level expressed in decibels when a signal of a given strength is introduced to the input of the set. Typically a broadcast tuner should give a quieting level of 30 dB for an input level of around a microvolt.

Capture effect
Another effect that is often associated with FM is called the capture effect. This can be demonstrated when two signals are present on the same frequency. When this occurs it is found that only the stronger signal will heard at the output This can be compared to AM where a mixture of the two signals is heard, along with a heterodyne if there is a frequency difference.

A capture ratio is often defined in receiver specifications. It is the ratio between the wanted and unwanted signal to give a certain reduction in level of the unwanted signal at the output. Normally a reduction of the unwanted signal of 30 dB is used. To give an example of this the capture ratio may be 2 dB for a typical tuner to give a reduction of 30 dB in the unwanted signal. In other words if the wanted signal is only 2 dB stronger than the unwanted one, the audio level of the unwanted one will be suppressed by 30 dB.