How does a digital radio tuner work?

As a kid (late 70’s, early 80’s) I tore apart a few busted-up clock radios. These had analog tuners as part of the radio: when you tured the dial, a system of strings and pulleys moved one set of metal plates into increasing or decreasing intermesh with another set of metal plates, like this. This altered their capacitance, changing the resonant frequency of the tuner and enabling it to be dialed in to one frequency or another.

So how does a digital tuner work? If I push a button on the front of my car/home stereo and select 101.5 MHz, how does the system end up getting tuned into that particular frequency? Is there a whole bank of tiny caps that get added in binary fashion by connecting/disconnecting subcircuits so as to obtain the desired net capacitance?

I remember the buzzword on digitally tuned radios in the 1980s was “PLL synthesized”.

There is some discussion here but it is on a higher frequency than my brain…

A radio requires the precise selection of a narrow frequency band or channel. In old radios, the channel was selected using a “tuned circuit” consisting of an inductor and a capacitor. The values of these two components must be precisely adjusted so the circuit resonates at a particular frequency, like adjusting the tension of a violin string. The OP’s link shows a mechanically variable capacitor, which was used to adjust the resonance and tune in the desired channel.

Most radios (even old ones) used the superheterodyne principle in which the received signal is converted to an “intermediate frequency” (IF). A narrow band filter at this intermediate frequency provides the selectivity. The advantage is that the IF filter is always the same, it does not have to be tuned for each channel. The trick is how to do the frequency conversion.

You convert a signal from one frequency to another by driving a nonlinear device (like a diode) with a combination of your signal and a sine or square wave at another frequency. This is called a “mixer.” The output of the mixer will have components at frequencies that are at the sum and the difference of the two signals that are combined in the mixer. In a typical radio, it is the difference frequency that is utilized. So, to tune the radio, you just have to change the frequency of the oscillator that drives the sine wave or square wave, the so-called “local oscillator.” If you are just trying to create a sine or square wave, there are all kinds of fancy techniques that can be used. One way is to generate a square wave of the desired frequency using digital techniques, then compare it to the output of a “voltage controlled oscillator” (VCO) that generates a sine wave whose frequency can be controlled by a voltage. A phase comparator measures the phase difference between the digitally-generated square wave and the VCO output. The difference signal is used in a feedback loop to correct the VCO to force a nearly zero phase error. This is a “phase locked loop.” A picture and a few diagrams are necessary to really understand this.

With this approach, you no longer need a fancy precision variable capacitor. You build one really good filter at the intermediate frequency, then use a tiny and inexpensive electrically variable capacitor to make the voltage controlled oscillator