Thursday, January 29, 2009


So after all that hubbub about crystal radio, I thought I'd explain what a modern solid state tuner is like. The PLL was invented in 1932, that's after Armstrong's superheterodyne. There is a huge difference in the tuners concepts. A superheterodyne is a tube driven tuner. It was the best of tube technology, but it's still a tube with a filament that eventually burns out. A PLL is a solid-state tuner: no tubes*, no crystals, no nada. It's just a linear circuit.

*While this version used vacuum tubes, it's latter implementation used semi-conductors. That's a diagram of his version to the upper right.

It was first conceived by Henri de Bellescize, (sometimes spelled Bellescise) in the French journal Onde Electrique. he described it in detail, but never implemented the design as far as we know. I have read that some British scientists fleshed out working models, but the first large-scale commercial applications weren't for another 20 years. In the 1940s they were used in AM radio synchronous demodulation. In the 1950s a PLL Circuit was used to recover the color data from the analog TV signal.

So what is it? A PLL is a circuit synchronizing an output signal with another one. In this case the output signal is generated by an oscillator. This is a reference" signal. When these two signals are synchronized [having a phase error of zero or of constant error] we describe them as "locked." If phase error changes, a control mechanism adjusts the frequency of the oscillator
so that error will again approach zero. A phase-locked loop is an example of a control system using negative feedback. it is so elegant yet complex that I can see why Bellescize conceived it but never built it.

There are 3 basic parts:
1. Voltage Controlled Oscillator
2. Phase Detector
3. Loop Filter

WARNING THIS REQUIRES MATH: The Voltage Controlled Oscillator (VCO) operates at an angular frequency. That just means it is the scalar measure of the rotation rate. I.E. this angular frequency sets the frequency with which phase changes. [magnitude of the vector quantity.] The Phase Detector (PD) compares the phase of the output signal with the phase of the reference signal. The out put of this function is a measure of the root error between their phases. The output of the PD has both AC and DC components. The Loop Filter then is applied to remove the undesired AC component. If error is not compensated for in this way, the VCO cannot operate at it's center frequency. The Loop Filter feeds back to the VCO to adjust it's operating frequency to eliminate phase error.Once the center frequency of the input signal is frequency modulated by an arbitrary low frequency signal the output signal of the loop Filter is the demodulated signal. And where did he get thsi crazy idea? He saw his own work as an improvement on the first homodyne receiver designed in 1924 by a British engineer named F.M. Colebrook... there is no end to this.

The first digital PLL appeared in 1969, from a company called Signetics. RCA debuted the CD4046, their own version 3 years later. It's phase detector was digital and all other components were analog. But that was the start. Quickly other components were replaced until we had a fully ADPLL (All Digital Phase Locked Loop. ) Today we're trying for the SPLL, A purely software device.