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A Complete FM Radio on a Chip AN192

Page 9 of 11

Circuit with variable-capacitance diode tuning Since it is only necessary to tune the local-oscillator coil, it is very simple to modify the circuit of Fig.1 for variable- capacitance diode tuning. The modifications are shown in


Fig.14. A circuit board layout for the modified receiver and a photograph of a complete laboratory model are shown in .Fig. 15

    TDA 7000 ( Fig. 1)

Fig.14 Variable-capacitance diode tuning for the local-oscillator. Additonal measures must be taken to ensure temperature stability

Fig.15 Circutt board layout and complete model of a TDA7000 radio with varlable-capacitance diode tuning. This is the same pcb as shown in Fig.1 2

Narrow-band f.m. receiver

The TDA7000 can also be used for reception of narrow- band fm. signals. In this case, the local-osciflator is crystal- controfled as shown in Fig.16 and there is therefore hardly any compression of the i.f. swing by the FLL. The deviation of the transmitted carrier frequency due to modulation must therefore be limited to prevent severe distortion of , the demodulated audio signal.
The component values in Fig.16 result in an i.f. of 4.5 kHz and an if. bandwidth of 5 kHz (Fig. 17). If the i.f.


is multiplied by N, the values of capacitors C l -L and C18 in the all-pass filters and the values of filter capacitors C7, C8, C 10, C l 1, and C 12 must be multiplied by l /N. For improved i.f. selectivity to achieve greater adjacent channel attenuation, second-order networks can be used in place of Clo and Cl 1.
In this circuit the detuning noise generator is not used. Since the circuit is mainly for reception of audio signals, the audio output must be passed through a low-pass Chebyshev filter to suppress i.f. harmonics.