Analogue PLL as Circuit Basis of FM Radio Receivers

Demodulators of broadband frequency modulated (FM) signals are characterized by noise immunity threshold — the phenomenon of rapid deterioration of signal-to-noise ratio on its output with the slight deterioration of this ratio on its input.

The classic elements of the block diagram of FM signals receiving devices are the amplitude limiter and frequency detector, where the last one is practically implemented by transforming frequency modulation into phase modulation and phase modulation into amplitude modulation. But nowadays there are many other methods of receiving and demodulating of FM signals that provide greater noise immunity. These known noise immune systems with revers control are described by differential equations, which is a particular case of phase-locked loop (PLL) device equations, or are driven to them. The optimum filter to track the frequency that is slowly changing, is the PLL system with second-order proportional- integrating filter. This conclusion is approved by the theory of optimal linear filtering.

With minor interference and relatively small signal jitter comparatively to own generator frequency accept that working area of phase detector characteristics is linear, and transfer ratio coefficient in a locked loop is defined by a formula known from the theory of linear reverse connection. The final results of the calculation of amplitude-phase-frequency characteristics of PLL device are listed in the normalized parameters as frequency hodograph. In the practical implementation of the PLL its parameters adequacy must be checked by calculations. Such verification performed at frequencies with maximum transfer rate is equal to 1 and 0,7.

The presence of parasitic frequency dependent elements in a reverse locked loop circuit results in the deterioration of the real system functioning. Since one of the noise immunity characteristics of the device is its noise band, the requirement of slight increase of this band was adopted as the noise immunity criterion in circuit with real elements comparing to theoretical model.

The bridge phase detectors with diodes, with diodes and additional resistors, and parametric phase detectors with transistors are considered. For these circuit schemes, input and output resistors, transfer coefficients ant thermal currency instability were defined. Further the low-pass filters schemes, that have minimal phase shift at high frequencies and contribute to the stability conditions are considered. Also circuit voltage controlled oscillator for LC and RC-elements by their frequency generation and temperature stability are analyzed.