EET260 FM: FM Noise Analysis

1. EET260 FM:FM Noise Analysis

2. 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?

3. 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

4. Edwin Armstrong: First regularFM radio broadcasting. FM history 1941 1837 1880 1922 1965 2007

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

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

7. 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

8. resultant N  S 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. resultant resultant vector (S + N) vector representing noise signal (Nin) N  S vector representing FM signal (Sin)

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

10. 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 fd (max) is the maximum frequency deviation.

11. Example Problem 1 Broadcast FM (fd (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.

12. Example Problem 2 Narrowband FM (fd (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.

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

14. Noise suppression for various modulation formats

15. 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 DC90.9 MHz Baltimore90.9 MHz

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

17. 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.

18. Preemphasis • Returning to the expression for frequency error, we can see that larger error is induced for higher modulating frequencies (fm) • 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.

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

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

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