The phase-locked loop is composed by three basic parts,namely the discriminator,the ring circuit filter and the pressure control the oscillator to discriminator functin produces an output voltage,this voltage mean direct ratio and two input signals phase ring circuit low pass filter function is filters in the discriminator output current the useless component and other disturbance component,achieved the ring cricuit requests the preformance ,and guarantees the ring circuit the pressure controls the oscillator the function has the frequency along with the control voltage change oscillating filter are in rely on process the signal above the frequency foundation one kind of electric circuit active filter only to bo able to have an effect in the operational amlifier normal work the electronic instrumentation aspect,the phase-locked loop in instruments and so on in frequency synthesizer and phase meter played the vital direct digital frequency synthesis technology is one new frequency synthe tic method,is a frequency synthesis technology revolution,along with the electronic engineering domain actual need as well as the digital integrated circuit an the microelectronic technology development,the DDS technology reveals its superiority day ba word:Phase-locked loop;Oscillator;Ring circuit filter;Frequency synthesizer