What Do Clocks, Carriers, Local Oscillators, and FM Have in Common?

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The master timing control circuit for most electronic applications is a precision signal source. For digital applications, it’s an accurate clock. In RF applications, it’s a carrier source for a transmitter or a local oscillator (LO) for a receiver. And if frequency modulation (FM) is involved, a modulator and demodulator are needed along with a carrier source.

In all of these cases, a phase-locked-loop (PLL) frequency synthesizer is an excellent choice. It provides not only the precision and accuracy needed, but also a flexible way to change frequency.

PLL 101

Just as a review, a PLL synthesizer is a closed-loop controller with feedback (Fig. 1). A voltage-controlled oscillator (VCO) generates the output signal. Frequency is determined by an LC resonant network, but controlled by a dc input voltage derived from a phase detector and a low-pass loop filter. 

1. The basic configuration of a PLL features a VCO output, controlling circuits such as the phase detector, and feedback dividers.

The input to the phase detector is usually a stable reference signal from a crystal oscillator. The second input to the phase detector is a signal derived from the VCO output that’s usually divided down by a variable-frequency divider. The two inputs to the phase detector must be the same frequency.

The phase detector compares the stable reference input to the divided VCO output. If the output frequency should change, the phase detector recognizes the variation and produces a dc voltage change to adjust the VCO to maintain a constant output frequency. This closed-loop operation keeps the output frequency steady despite any variations. The output essentially has the precision and stability of the input reference.

To change the frequency, you adjust the divide ratio N. For example, if the VCO output is 480 MHz and you’re using a 1-MHz input crystal reference, you will need a divider that divides by 480. The divider is variable. Changing the division factor to 481 will produce a frequency and phase change recognized by the phase detector. As a result, the VCO output will be changed to 481 MHz.

Note that the output frequency changes in increments equal to the input reference frequency. That reference frequency can be varied by inserting a divider (and/or multiplier) between the reference oscillator and phase detector.

FM Modulation Methods

Frequency modulation has been around for over 80 years thanks to inventor and EE Edwin Armstrong. FM is a popular modulation scheme used to transmit information on a high-frequency radio carrier by varying the carrier frequency. It’s still widely used today in both analog and digital formats. You see analog versions in the form of FM broadcast radio or two-way mobile radio. You see it in digital form as frequency shift keying (FSK), like that in Bluetooth and some ISM band radios. In both cases, you will find PLLs involved.

There are a couple of FM implementation techniques, each with different characteristics. In general, you can generate FM in either an analog or digital way. Regardless of technique, you will need a frequency synthesizer to generate the high-frequency radio carrier.

One analog technique is to directly modulate the VCO. A common arrangement is to use varactor diodes (voltage-variable capacitors) to change the capacitance value in the VCO LC tuned circuit. Another method uses the modulating signal to modulate the reference oscillator. In this case, the reference isn’t a precision fixed-frequency crystal oscillator. It would be an LC oscillator whose frequency is changed by the variable capacitance of one or more varactors.

A digital method of modulating a PLL is to rapidly change the divider value. Switching between two divider values generates standard FSK. This approach can be used to generate 4FSK or 8FSK, which boosts bit rate by creating four or eight “symbols”; each symbol frequency represents multiple bits. Using 4FSK, with four frequencies each representing two bits, achieves a doubling of the data rate. With eight frequencies, three bits are represented per symbol.

A key factor in using FSK is to reduce the bandwidth of the transmitted signal by shaping the modulating signal. This is usually done by filtering the binary signal and rounding its edges, which has the effect of reducing the number of harmonics in the signal. This, in turn, reduces the number of sidebands produced by the FM signal.