Phase frequency detector

　　A frequency discriminator that uses a linear phase-shifting network to transform a frequency modulated wave into a phase modulated frequency modulated wave. Figure 2 (a) shows the circuit of this frequency discriminator. The first stage circuits L1C1 and L2C2 are both tuned to the center frequency fc of the frequency modulated wave, i.e. fc=f1=f2=f0; The coupling capacitor C0 adds the primary circuit voltage u1 to the center tap of the secondary circuit coil, so the input voltages of the two diode detectors are:

　　The output voltage u of the frequency detector is the voltage difference between the load resistors R1 and R2 of the detector. Figure 2 (b) shows the voltage changes applied to two detectors for three different phases of u2. When the instantaneous frequency of the frequency modulation wave is f=f0, U1 and U2 are perpendicular to each other, so | UVD1 |=| UVD2 |. At this time, the output of the frequency detector is equal to zero. When f>f0, U2 lags behind U1 by more than 90 ° in phase, | UVD1 |<| UVD2 |, and the output of the frequency detector is greater than zero; On the contrary, when f0, the phase lag of U2 relative to U1 is less than 90 °, | UVD1 |>| UVD2 |, so the output of the frequency detector is less than zero. The relationship curve between the output voltage of the frequency discriminator and the input signal frequency f is called the frequency discrimination characteristic curve. The linearity of the phase frequency discriminator is good, and the frequency discrimination sensitivity (output voltage generated when the unit frequency changes) is also high, but the anti parasitic amplitude modulation interference performance is poor. The use of a proportional frequency discriminator with self limiting capability is an ideal solution to this problem

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