Recall that frequency modulation (FM) is the process in which the frequency of one signal is controlled by the magnitude of another signal. A yea generates an FM signal, as illustrated in Figure 15-37. In this example, the input to the yea (the modulating signal) ia a ramp voltage whose amplitude increases with time
Consequently, the output of the YCa (the modulated signal) is a signal whose frequency increases with time. In a communications system, the modulated signal is transmitted to a receiver, where it must be demodulated; that is, the original modulating signal must be recovered from the modulated signal. As shown in the example of Figure 15-37, the FM demodulator performs this function. The input to the demodulator is the FM signal and the output is the ramp voltage that created it. An FM demodulator is also called a frequency-to-voltage converter. A phase-locked loop can serve as an FM demodulator by connecting the .FM signal to the input and taking the output from the low-pass filter. As the input frequency changes. the output of the phase comparator and filter changes the same way. For example, an increasing input frequency (originally produced as the result of an increasing modulating signal) will create an increasing error voltage because the frequency of the yea must be increas d to track the input. Thus, the error voltage duplicates the original modulating signal.
A frequency synthesizer is simply a signal generator whose frequency can be readily adjusted. A phase-locked loop can be used as a frequency synthesizer h/ inserting. a frequency divider (counter) in the feedback path to the phase comparator and taking the output from the vca. This arrangement is shown in Figure 15-38. Note that a low-pass filter is not required. The counter is a digital-type device that produces a signal having frequency f/N, where N is an integer, when the input to the ccunter has frequency f. As can be seen in the figure, a signal with fixed reference frequencY./REF,is compared in the phase comparator to the output of the counter. Consider what happens when this arrangement is first connected: The signal fed back to the phase comparator has a frequency that is initially smaller (by the factor 1IN) than/REF’ Consequently, the phase comparator produces an output that causes the YCa to increase its frequency, in the comparator’s usual attempt to bring the two frequencies to equality. The yea increases its frequency until the signal fed back to the comparator has frequency /REF and lock is achieved, But, for the output of the counter to have frequency fREt’> the VCO must have reached frequency NfREF, since the counter divides its input frequency by N. Thus, the output of the phaselocked loop (the output of the VCO) is a signal whose frequency is the multiple NJRF.f’ of the reference frequency.
One advantage of a frequency synthesizer using a PLL is that the signal providing the reference frequency can be very stable, ss, for example, from a crystalcontrolled oscillator. The higher frequency, NfRF.F, will then also be very stable. In some high-frequency applications, it is not possible 10 achieve such ,MntrotJed stability at high frequencies without using such a scheme. We should note that a ” ‘frequency-dividing counter cnn also be connected hetween the free input and the phase comparator. This connection has just the opposite effect of connecting a divider in the feedback: The frequency of the vea is divided by the same factor that the input divider provides.
The 565 Integrated-Circuit PLL
An example of a commercially available, integrated-circuit PLL is the LM565, manufactured by National Semiconductor. The inputs and outputs of the phase comparator and VCO are accessible at separate pins so that external components, such as filters and frequency dividers, can be inserted as required for different applications. Some important specifications of a PLL, and their values for the LM565, are the maximum VCO frequency (500 kHz), the demodulated output voltage (300 mV for a 10% change in input frequency), and the phase comparator sensitivity (0.68 V/radian).