Until now, the almost total integration of an FM radio has been prevented by the need for LC tuned circuits in the RF, IF, local oscillator and demodulator stages. An obvious way to eliminate the coils in the IF and demodulator stages is to reduce the normally used intermediate frequency of 10.7MHz to a frequency that can be tuned by active RC filters, the op amps and resistors of which can be integrated. An IF of zero deems to be ideal because it eliminates spurious signals such as repeat spots and image response, but it would not allow the IF signal to be limited prior to demodulation, resulting in poor signal-to-noise ratio and no AM suppression. With an IF of 70kHz, these problems are overcome and the image frequency occurs about halfway between the desired signal and the center of the adjacent channel. However, the IF image signal must be suppressed and, in common with conventional FM radios, there is also a need to suppress interstation noise and noise when tuned to a weak signal. Spurious responses above and below the center frequency of the desired station (side tuning), and harmonic distortion in the event of very inaccurate tuning must also be eliminated. We have now developed a mono FM reception system which is suitable for almost total integration. It uses an active 70kHz IF filter and a unique correlation muting circuit for suppressing spurious signals such as side responses caused by the flanks of the demodulator S-curve. With such a low IF, distortion would occur with the ±75kHz IF swing due to received signals with maximum modulation. The maximum IF swing is therefore compressed to ±15kHz by controlling the local oscillator in a frequency-locked loop (FLL). The combined action of the muting circuit and the FLL also suppresses image response.

Using the TDA7000 results in significant improvements for all classes of FM radio. For simpler portables, the small size, lack of IF coils, easy assembly and low power consumption are not the only attractive features. The unique correlation muting system and the FLL make it very easy to tune, even when using a tiny tuning knob. For higher-performance portables and clock radios, variable-capacitance diode tuning and station presetting facilities are often required. These are easily provided with the TDA7000 because there are no variable tuned circuits in the RF signal path. Only the local oscillator needs to be tuned, so tracking and distortion problems are eliminated.

Part Value Description Quantity
Resistors 1/4W 5%
R1 22k Red, Red, Orange, Gold 1
R2 10K Brown, Black, Orange, Gold 1
R3 5.6K Green, Blue, Red, Gold 1
R4 82 Gray, Red, Black, Gold 1
R5 1.5K Brown, Green, Red, Gold 1
R6 100k Brown, Black, Orange, Gold 1
R7 270K Red, Violet, Yellow, Gold 1
R8 4.7 Yellow, Violet, Gold, Gold 1
R9 150K Brown, Green, Yellow, Gold 1
R10 100 Brown, Black, Brown, Gold 1
R11 560 Green, Blue, Brown, Gold 1
C1 47p (47) Ceramic capacitor 1
C2 2.2n (222) Ceramic capacitor 1
C3 39p (39) Ceramic capacitor 1
C4, C17, C21, C24 100n (104) Ceramic capacitor 4
C5 220p (221) Ceramic capacitor 1
C6 150n (154) Ceramic capacitor 1
C7, C12 330p (331) Ceramic capacitor 2
C8, C13, C19 10n (103) Ceramic capacitor 3
C9 180p (181) Ceramic capacitor 1
C10, C11, C15 3.3n (332) Ceramic capacitor 3
C14 150p (151) Ceramic capacitor 1
C16 1.8n (182) Ceramic capacitor 1
C18 220n (224) Ceramic capacitor 1
C25 220µ/12V Electrolytic capacitor 1
C20 1000µ/16V Electrolytic capacitor 1
C22, C23, C17 10µ/16V Electrolytic capacitor 3
C26 22n (223) Ceramic capacitor 1
C27, C28 27p (27) Ceramic capacitor 2
IC1 TDA7000 CI FM – DIL18 1
IC2 78L05 5V positive voltage regulator 1
IC3 LM386N-1 (lm386) CI Audio Amplifier 1
D1 bb105 or BB809 or equivalent Varicap Diode 1
D2 Zener Diode of 2.7V or 3.3V Zener Diode 1
T1 BC558 PNP Transistor 1
T2 2SC3355 NPN Transistor 1
ANT, ANT1 Connect Antenna 1
DC Terminal block 2 pins Power supply 1
OUT Terminal block 2 pins Audio output 1
L1 4,5 turns 10 mm diam. Coil 18 AWG wire 1
L2 5 turns 10 mm diam. Coil 18 AWG wire 1
P1 10K (103) Potentiometer 1
P2 100K (104) Potentiometer 1