EET260 FM: FM Noise Analysis

Why use FM? Characteristics of AM  Easy to modulate and demodulate  Narrow bandwidth requirement Characteristics of FM  More complex modulation and demodulation required  Potentially very large bandwidth requirement (ex. AM radio 10 kHz vs. FM radio 200 kHz) So why do we bother with FM?

Noise Suppression Noise suppression is the most important advantage of FM over AM.  Static noise is almost never heard on FM.  Static noise is almost always present on AM. The difference in performance between AM and FM is due to how the two modulating schemes carry information.  AM in amplitude  FM in phase

Edwin Armstrong: First regular FM radio broadcasting FM history

Noise in an AM signal “clean” AM signal at transmitter additive noise in transmission “noisy” AM signal at the receiver (S/N = 10 dB) +

Noise in an FM signal + “clean” FM signal at transmitter additive noise in transmission “noisy” FM signal at the receiver (S/N = 10 dB)

FM Limiter Circuit Since amplitude should be constant for an FM signal, anything larger is due to noise and can be removed. This is the action of the limiter circuit in the receiver. Noise also causes undesired phase errors and the limiter cannot remove these. Limiter

Phase error analysis Consider a phasor representation of the FM signal (vector lengths determined by S/N ratio) Worst case phase error (  ) occurs when N is perpendicular to the resultant. S resultant N vector representing FM signal ( S in ) vector representing noise signal ( N in ) resultant vector ( S + N )  S resultant N 

Phase error analysis Magnitude of maximum phase error (  ) is given The phase error induces a frequency error (  n ) given where  n is phase error (in radians) S resultant N nn

Phase error analysis Since frequency conveys information in FM, this frequency error  n will translate into an amplitude error in the demodulated signal. The S/N ratio of the output is given where f d (max) is the maximum frequency deviation.

Broadcast FM ( f d (max) = 75 kHz ): Determine the worst-case output S/N for a broadcast FM program that has a maximum modulating frequency of 5 kHz. The S/N ratio of the received signal is 2. Example Problem 1

Narrowband FM ( f d (max) = 10 kHz ): Determine the worst-case output S/N for a narrowband FM receiver with a maximum modulating frequency of 3 kHz. The S/N ratio of the received signal is 3. Example Problem 2

FM Noise Analysis The previous two examples show that noise reduction is improved when the modulating frequency ( f m ) is reduced. Noise can also be reduced by increasing m f at the cost transmitted bandwidth. Example 1: m f =5 Example 2: m f =3 Input S/NOutput S/N (~12 dB gain) (~5 dB gain)

Noise suppression for various modulation formats

Capture effect The ability of FM to minimize the effect of noise also explains a phenomena known as the capture effect. In the presence of two FM stations on the same frequency, an FM receiver will lock-onto or “capture” the stronger signal and ignore or suppress the weaker. This is unlike AM, where it is common to hear two stations simultaneously. Washington DC 90.9 MHz Baltimore 90.9 MHz

Threshold The threshold is the input S/N ratio below which a sharp deterioration in output S/N ratio occurs.

Threshold The effect of input S/N ratio decreasing below the threshold value is observed when driving in a large city with a weak FM signal. Multi-path reflections off buildings cause the S/N ratio to dip below the threshold which causes FM radio output to completely blank out.

Preemphasis Returning to the expression for frequency error, we can see that larger error is induced for higher modulating frequencies ( f m ) This is compounded by the fact that higher frequency components tend to have smaller amplitudes. To improve noise suppression for high frequencies, preemphasis is applied prior to modulation.

Preemphasis Preemphasis is the process in an FM transmitter of artificially amplifying high frequency components to reduce the effect of noise prior to modulation.

Deemphasis Deemphasis is reverses the effect of preemphasis by reducing the amplitude of high frequency components in the receiver after demodulation.

Dolby System Harman Kardon CAD-5 Cassette Deck (1970). KLH Model 40 reel-to-reel recorder (1968).