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A Complete FM Radio on a Chip AN192

Page 4 of 11

Corralation muting system with open FLL
A well-known difference between f.m. and a.m. is that, for f.m., each station is received in at least three tuning positions. Fig.6 shows the ftequency spectrum of the output ftom the demodulator of a typical portable f.m. radio receiving an r.f. carrier ftequency-modulated with a tone of constant frequency and amplitude. In addition to the audio response at the correct tuning point in the centre of Fig. 6, there are two side responses due to the flanks of the demodulator S-curve. Because the flanks of the S-curve are non-linear the side responses have increased harmonic distortion. In Fig.6, the ftequency and intensity of the side responses are functions of the signal strength, and they are

Fig.6 Audio signal of a typicel portable radlo " a furmtIon of tuned frequency with r.f. Input os a peremeter. The modu- lation frequency and amplituds, are both oonstant


separated ftom the coirect tuning point by amplitude minima. However, in practice, the amplitude minitna are not well defined because the modulation frequency and index are not constant and moreover, the side responses of adjacent channels often overlap.

High performance f.m. radios incorporate squelch systems such as signal-strength-dependent muting and tuning-deviation-dependent muting (Ref.1) to suppress side responses. They also have a tuning meter to facilitate correct tuning. Although the TDA7000 is mainly intended for use in portables and clock radios, it incorporates a very effective new correlation muting system which suppresses interstation noise and spurious responses due to detuning to the flanks of the demodulator S-curve. The muting system is controlled by a circuit which determines the correlation between the waveform of the i.f. signal and an inverted version of it which is delayed (phase shifted) by half the period of the nominal if. (180*). A noise generator wor~s in conjunction with the muting system to give an audible indication of incorrect tuning.

FIg.7 Function of the corretation muting system

Figure 7 diustrates the function of the muting system. Signal 117' is derived by delaying the i.f. sipal by half the period of the nominal i.f. and inverting it. With correct tuning as shown in Fig.7(a), the waveform of the two signals are identical resulting in large correlation. In this situation, the audio signal is not muted. With detuning as shown in Fig.7(b), signal IF' is phase-shifted with respect to the i.f. signal. The correlation between the two waveforms is therefore small and the audio output is muted. Figure 7(c) shows that, because of the low Q of the i.f. filter, noise causes considerable fluctuations of the period of the i.f. signal waveform. There is then small correlation between the two waveforms and the audio is muted. The correlation muting system thus suppresses noise and side responses due