2. 2
Noise-Suppression Effects of FM
• Noise is interference generated by lightning, motors, automotive
ignition systems, and power line switching that produces transient
signals.
• Noise is typically narrow spikes of voltage with high frequencies.
• Noise (voltage spikes) add to a signal and interfere with it.
• Some noise completely obliterates signal information.
3. 3
Noise-Suppression Effects of FM
• FM signals have a constant modulated carrier amplitude.
• FM receivers contain limiter circuits that deliberately restrict
the amplitude of the received signal.
• Any amplitude variations occurring on the FM signal are
effectively clipped by limiter circuits.
• This amplitude clipping does not affect the information
content of the FM signal, since it is contained solely within
the frequency variations of the carrier.
5. 5
INTERFERENCE (capture effect)
• A major benefit of FM is that interfering signals on the same frequency will be
effectively rejected.
• If the signal of one is more than twice the amplitude of the other, the stronger
signal will "capture" the channel and will totally eliminate the weaker,
interfering signal. This is known as the capture effect in FM.
• In FM, the capture effect allows the stronger signal to dominate while the
weaker signal is eliminated.
• However, when the strengths of the two FM signals begin to be nearly the
same, the capture effect may cause the signals to alternate in their domination
of the frequency.
6. 6
• Despite the fact that FM has superior noise rejection qualities, noise still
interferes with an FM signal. This is particularly true for the high-frequency
components in the modulating signal.
• Since noise is primarily sharp spikes of energy, it contains a considerable
number of harmonics and other high-frequency components.
• These high frequencies can at times be larger in amplitude than the high-
frequency content of the modulating signal.
• This causes a form of frequency distortion that can make the signal
unintelligible.
• To overcome this problem a technique known as Pre-emphasis and De-
emphasis is used.
7. 7
Pre-emphasis and De-emphasis.
• At the transmitter the modulating signal is passing through a simple network
which amplifies the high frequency component more the low-frequency
component. The simplest form of such circuit is a simple high pass filter.
• The pre-emphasis circuit increases the energy of the higher content of the
higher-frequency signals so that will tend to become stronger than the high-
frequency noise component. This improves the signal-to-noise ratio.
• To return the frequency response to its normal level, a de-emphasis circuit is
used at the receiver. This is a simple low-pass filter
• The de-emphasis circuit provides a normal frequency response.
8. 8
The combined effect of pre-emphasis and de-emphasis is to increase the high-
frequency components during the transmission so that they will be stronger and not
masked by noise
Pre-emphasis and De-emphasis.
