6 Modulation Index and Sidebands FM Signal BandwidthExample:If the highest modulating frequency is 3 kHz and the maximum deviation is 6 kHz, what is the modulation index?mf = 6 kHz/3 kHz = 2What is the bandwidth?BW = 2fmNWhere N is the number of significant* sidebandsBW = 2(3 kHz)(4) = 24 kHz*Significant sidebands are those that have an amplitude of greater than 1% (.01) in the Bessel table.
7 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.
8 Continued 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.
9 Continued 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.
11 Nonlinear system to reduce envelop variations (AM) Nonlinear Distortion and LimiterNonlinear system to reduce envelop variations (AM)
12 Hard LimiterFM with unwanted amplitude variation A(t). Those variation can be flattened out by an ideal hard limiter or clipper.Clipper circuit uses a comparator or high gain operational amplifier such that any input voltages greater or less than zero cause the output voltage to reach either positive or negative power supply rails.Clipper circuit employing back to back Zener diode with break down voltage at the output of a high gain amplifier.
16 ContinuedThe Limiter plus BPF in (a) removes unwanted amplitude variation from AM or PM wave and would be used in receiver.
17 DeemphasisNoise can interfere with an FM signal and particularly with the high-frequency components of the modulating signal.Noise is primarily sharp spikes of energy and contains a lot of harmonics and other high-frequency components.Detected FM interference is most severe at large value of fi.Post detected filtering improves the performance of the system.
19 ContinueddeemphasisA simple low-pass filter can operate as a deemphasis circuit in a receiver.Demodulator is followed by a low pass filter having a amplitude ratio that begin to decrease below W; this will de-emphasis the high frequency portion of the message band there by reduce the more serious interference.A deemphasis circuit returns the frequency response to its normal flat level.
20 PreemphasisObviously de-emphasis filtering also attenuate the high frequency component of the message itself.But it is simple matter to compensate for deemphasis distortion by preemphasizing the modulating signal at the transmitter before modulation.
21 ContinuedTo overcome high-frequency noise, a technique known as preemphasis is used.A simple high-pass filter can serve as a transmitter’s pre-emphasis circuit.Pre-emphasis provides more amplification of only high-frequency components.
22 ContinuedFigure 2.Preemphasis and deemphasis. (a) Preemphasis circuit.
25 ContinuedThe combined effect of preemphasis and deemphasis is to increase the signal-to-noise ratio for the high-frequency components during transmission so that they will be stronger and not masked by noise.
26 Capture EffectIn referring to radio, capture effect, or FM capture effect, is a phenomenon associated with FM reception wherein only the stronger of two signals at, or near, the same frequency will be demodulated.Capture effect is defined as the suppression of the weaker signal at the receiver limiter; where the weaker signal is not amplified, but attenuated.
27 Continued Demodulated Signal with Interference The presence of Фi(t) in the above equation indicates potentially intelligent interference (or cross talk)
29 ContiuedWhen more than one signal is nearly equal in strength, or the signals are fading independently, the receiver may switch from one signal to the other and exhibit picket fencing.
30 Picket FencingPicket fencing is slang for the chopping effect sometimes heard by cell phone users at the edge of a cell's coverage area, or (more likely) by the landline user to whom the cell phone is connected."Picket fencing" refers to the way portions of speech are stripped from the conversation, as if the listener was walking by a picket fence, and hearing a conversation on the other side that changes audibly depending on the position of the pickets relative to the listener.
31 How do we measure it?Capture Effect is measured as the lowest ratio of the power of two signals that will result in the suppression of the smaller signal.