Reverese engineered

The TDA7000 uses the PLL and VCO to reduce the FM deviation. A stereo decoder and even an RDS decoder will work. I built such a radio some years ago using a Philips RDS decoder and PIC programmed to take the data and display on an LCD as well as the Philips stereo decoder IC. The TDA7000 and stereo decoder used SMD to DIL header PCBs.

One of the trickiest parts of the TDA7000 design is how it manages to use an intermediate frequency of just 70 kilohertz. The problem is that broadcast FM has a "modulation frequency deviation" of 75 kHz, which means that the broadcast frequency varies by up to ±75 kHz. The mixer shifts the broadcast frequency down to 70 kHz, but the shifted frequency will vary by the same amount as the received signal. How can you have a 70 kilohertz signal that varies by 75 kilohertz? What happens when the frequency goes negative?

The solution is that the local oscillator frequency (i.e., the frequency that the radio is tuned to) is continuously modified to track the variation in the broadcast frequency. Specifically, a change in the received frequency causes the local oscillator frequency to change, but only by 80% as much. For instance, if the received frequency decreases by 5 hertz, the local oscillator frequency is decreased by 4 hertz. Recall that the intermediate frequency is the difference between the two frequencies, generated by the mixer, so the intermediate frequency will decrease by just 1 hertz, not 5 hertz. The result is that as the broadcast frequency changes by ±75 kHz, the local oscillator frequency changes by just ±15 kHz, so it never goes negative.

From Ken Shirrif's blog with photos of the naked IC

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Actually it was a later version of the radio IC we used. But the TDA7000 is similar.

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