There is one circuit block which FM stereo radios, modems, musical synthesisers and compact disc players (to name some examples) have in common: It is called the phase locked loop (or PLL), and it must be worth a closer look.
A PLL really comprises two units: A phase comparator and a voltage- controlled oscillator (or VCO): Phase comparators accept an input frequency and a control frequency. A positive voltage is output if the control frequency is higher; a negative voltage is output if the input frequency is higher; a voltage of (almost) zero value is output if the two frequencies are almost the same.
(Reasonably enough, a voltage- controlled oscillator will shift its output frequency in sympathy with some input voltage.)
When the phase comparator output supplies the VCO input voltage, and the VCO completes the loop by providing the control frequency to the phase comparator, the PLL tracks the input frequency.
Why go to such lengths to regenerate a replica of the input frequency, which has been available in the first place? Well, the input frequency for the equipment mentioned at the beginning of this section is likely to be noisy (or even intermittent); the PLL, on the other hand provides a clean signal to synchronise subsequent circuitry.
Discussing principles of operation is well, but it is also useful to point out some of the pitfalls in the implementation: The output voltage from the phase comparator is useless for driving the VCO directly; first, it must be smoothed by a low- pass filter.
Inadequate filtering will result in a noisy VCO output frequency; too much filtering will slow down the PLL's response too much. (Although it may sound contradictory, some latency is sometimes desired: If the VCO output is additionally to be frequency modulated, it is clearly required to stabilise the average frequency only, not the instantaneous frequency.)
Sometimes, the frequency to be controlled is too high to handle comfortably, or a crystal of a convenient value is difficult to procure anyway, and then it usual to introduce a frequency divider (typically) in the path of the frequency to be controlled. It may be that several channels must be stabilised using a single crystal, a situation which clearly requires a programmable frequency divider. If the phase comparator is made of digital gates (as is often the case ), both the input and control frequencies must be amplified up to the level required by logic circuits.
A phase comparator can be built as follows: An Xor gate is fed by the input frequency and a slightly delayed version of it- it then functions as an edge detector. The control frequency is subjected to the same processing, then the two gates drive an S-R flip- flop.
The semiconductor's junction capacitance has a value of a few pF, and can be modulated to a small extent by the voltage across the diode. Thus a VHF oscillator's output is easily frequency modulated. (Of course, this shift percentage would be hopelessly insufficient for the much lower frequency oscillators required by the equipment mentioned above.)
Both the PLL, and the VCO on its own, are available as ready- made blocks; they are very inexpensive, and therefore liable to experimentation without fear. The venerable '565' (or similar) will provide a simple phase comparator individually. It is also possible to save a little money by designing a PLL from scratch, but that really isn't worth the time, is it now?
The high- frequency oscillators page is a related section.
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