FM reception specifications including the FM capture ratio and its associated capture effect, along with the FM quieting figures and facilities including squelch are of great importance to users of FM systems.
Frequency modulation, FM is widely used in radio communications and broadcasting, 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 order to be able to receive FM a receiver must be sensitive to the frequency variations of the incoming signals which 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 and this improves the signal to noise ratio after the point when the signal limits in the IF stages. However the high levels of gain associated with the limiting process mean that when no signal is present, very high levels of noise appear at the output of the FM demodulator.
To overcome the problem of the high noise levels when no signal is present a circuit known as “squelch” is normally used. This detects when no signal is present and cuts the audio, thereby removing the noise under these conditions. The level for this is normally present in domestic radios, but there is often a level adjustment for PMR or handheld transceivers, or for scanners and professional receivers.
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.
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.
By Ian Poole