The frequency discriminator is a circuit whose output voltage corresponds to the frequency of the input signal.It can be divided into two categories according to its use.The first type is used for demodulation of FM signals.The common ones are slope frequency discriminator, phase frequency discriminator, proportional frequency discriminator, etc. The requirements for such circuits are small nonlinear distortion and low noise threshold.The second type is used for frequency error measurement, such as the frequency discriminator used to generate error signals in the automatic frequency control loop.The zero drift of such circuits is strictly limited, and the requirements for nonlinear distortion and noise threshold are not high.
Chinese name
Frequency discriminator
Foreign name
frequency detector
Role
Make output voltage correspond to input signal frequency
The circuit that makes the output voltage correspond to the frequency of the input signal.It can be divided into two categories according to the purpose: one is used for demodulation of FM signals, and the common ones are slope discriminators, phase discriminators, proportional discriminators, etc. The requirements for such circuits are mainly small nonlinear distortion and low noise threshold.The other is used for frequency error measurement, such asAutomatic frequency controlFrequency discriminator that generates error signals in the loop.The zero drift of such circuits is strictly limited, and the requirements for nonlinear distortion and noise threshold are low.The circuit of slope frequency discriminator is relatively simple, but its resonance characteristic curve is not straight when the circuit is out of tune, so the linearity of frequency discriminator is poor.The linearity of the phase frequency discriminator is good, and the frequency discriminator sensitivity is also high.
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Frequency discriminator
Frequency discriminator
To realize FM signal demodulationFrequency discriminator circuitIt can be divided into three categories. The first category isfrequency modulation--AM and FM conversion type.In this type, the equal amplitude FM wave is first transformed into an AM FM wave whose amplitude is proportional to the instantaneous frequency of the FM wave through a linear network, and then the amplitude detector is used for amplitude detection.The second type is the phase shift multiplication frequency discrimination type.This type is to change the frequency modulation wave into the phase modulation frequency modulation wave through the phase shifting circuit. The change of its phase is just linear with the change of the instantaneous frequency of the frequency modulation wave. Then, compare the phase of the phase modulation frequency modulation wave with the original frequency modulation wavewave filterTake out demodulationsignal。Because the phase comparator is usually composed of a multiplier, it is called phase shifted multiplication frequency discrimination.The third type is the pulse mean type.This type is to transform the FM signal into a unipolar constant amplitude pulse sequence with the same repetition frequency as the instantaneous frequency of the FM signal through a zero crossing comparator, and then take out the average value of the pulse sequence through a low-pass filter, which restores the signal proportional to the change of the instantaneous frequency.The frequency discriminator is a kind of phase shift frequency discriminatorceramicsFilter element, mainly used inTelevisionorvideo recorderIn the audio intermediate frequency amplification or demodulation circuit, andFM radioFrequency discriminator circuit.It can be divided into two types: balanced type and differential type. The former is used for balanced demodulation of synchronous phase discriminator, and the latter is used for differential differential demodulation of differential peak discriminator.DSK FM audio narrowband JTCV10.7M series chip frequency discriminator is used with a variety of ICs for FM program inspection to convert frequencies into useful audio signals.
The characteristics of FM wave areamplitudeIt remains unchanged, while the instantaneous frequency changes linearly with the size of the modulated signal. The modulated signal represents the information to be transmitted. In analysis or experiment, it is often represented by low-frequency sine wave.The purpose of frequency discrimination is to detect the low-frequency modulation signal from the frequency modulation wave, that is, to complete the frequency voltage transformation.The circuit that can perform this function is called frequency discriminator.
Frequency discriminator
Phase modulation waveOfDemodulation circuitIs to take the original modulation signal from the phase modulation wave, that isoutput voltage It is proportional to the instantaneous phase offset of the input signal, also known as the phase detector.For the demodulation circuit of the FM wave, the original modulated signal is taken from the FM wave, that is, the output voltage is proportional to the instantaneous frequency offset of the input signal, also known as the frequency discriminator.
Phase detection circuits are generally divided into analog circuit type and digital circuit type.In the integrated circuit system, the commonly used circuits are product phase discrimination and gate circuit phase discrimination.In addition to demodulating the phase modulation wave, the phase discriminator can also form a phase shift frequency discriminator circuit.Especially inPhase-locked loopAs a major component, the Chinese language has been widely used.
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Slope discriminator
Frequency discriminator
：Where transistor andLCThe loop is essentially aTuned amplifier, but the resonant frequency of the loopf0 and the center frequency of the frequency modulated signalfC YesDetuningOf.Once the instantaneous frequency of the modulated signal changes, the amplifier will output an AM - FM wave corresponding to it.After diode detection processingRThe output on L is the same as that of the original modulation signal.The circuit of slope frequency discriminator is relatively simple, but its resonance characteristic curve is not straight when the circuit is out of tune, so the linearity of frequency discriminator is poor.
Phase discriminator
：The primary and secondary circuits are tuned at the center frequency of the FM wavefC, i.ef0=fc。capacitanceC0 Set the primary voltageu1 is coupled to the central tap of the secondary coil, so that the voltage applied to the detection diodes D1 and D2 are
Frequency discriminator
Frequency discriminator
Frequency discriminator
And frequency discriminatoroutput voltage uIs the detection loadR4 andR3.Primary and secondaryVoltageu1 anduThe phase difference between 2 varies with the instantaneous frequency of the frequency modulation wave.When instantaneous frequencyf=fC,u2 ratiou1 lags 90 °, but | UD1 |=| UD2 |, at this time, the discriminator output isFatal Frame。Whenf>fC,u2 lagging behinduThe phase angle of 1 is less than 90 °, | UD1 |<| UD2 |, and the output of frequency discriminator is greater than zero.Whenf<fC,u2 lagging behinduThe phase angle of 1 is less than 90 °, | UD1 |>| UD2 |), and the output of frequency discriminator is less than zero.The linearity of the phase frequency discriminator is good, and the frequency discriminator sensitivity is also high.
The phase discriminator consists of two parts: frequency phase converter and phase detector.The frequency phase converter is composed of capacitance coupled (CM) dual tuning loop, whose function is to convert the instantaneous frequency change of input FM wave V12 into the phase change of Vab.
The phase detector is also divided into two parts: first, the voltage vector is superimposed at the secondary pole of the coupling circuit, and the change of Vab's phase difference relative to V12 is converted into the change of the amplitude of the synthetic voltage VD, which further converts the change of the instantaneous frequency of the FM wave into the envelope change of VD, and then the low-frequency modulated signal V0 is taken out by differential output through two envelope detectors.
Ceramic frequency discriminator
Frequency discriminator
It is a kind of ceramic filter element with phase-shifting frequency discrimination characteristics, which is mainly used in the audio IF amplification or demodulation circuit of TV or video recorder and the frequency discriminator circuit of FM radio.It can be divided into two types: balanced type and differential type. The former is used for balanced demodulation of synchronous phase discriminator, and the latter is used for differential differential demodulation of differential peak discriminator.
Ceramic frequency discriminatorThe text symbols and circuit graphic symbols ofCeramic filterSame as.Ceramic frequency discriminators used in TV sets or video recorders include JT4.5MD, JT5.5MB, JT6.0MB, JT6.5MD, JT6.5MB2, CDA6.5MC, CDA6.5MD, etc.For FM adjustmentradioThe ceramic frequency discriminator in is JT10.7MG3 and other models.
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Adjust the frequency discrimination characteristics of the frequency discriminator with a frequency sweeper.Test conditions: connect the three contacts E, F and G in the circuit with semi adjustable capacitors CT1, CT2 and CT3 respectively.Connect the output signal of the frequency sweeperCircuit inputThe output signal of terminal IN should not be too large. Generally, a 30db attenuator is used. The sweep frequency standard uses an external frequency standard. The external frequency standard source uses a high-frequency signal generator whose output frequency is adjusted to 6.5MHz.
(1) Adjust the loop frequency of the waveform conversion circuit
Frequency discriminator
Insert the sweep frequency detector into the test hole A,Coupling capacitanceWhen CT3 is set to the minimum, the display screen will display the resonance curve graph.Adjust CT1 so that the resonant frequency of the resonant curve is 6.5MKZ. At this time, the frequency standard should be on the peak of the curve, and then increase the coupling capacitance of CT3capacityInput the detector head and insert it into the test hole B. At this time, the display screen shows the coupled resonance curve with pits. Adjust CT1, CT2 and CT3 to make the curve 6.5MHz frequency scale appear at the center point, and the frequency bands on both sides of the center point are symmetrical.
(2) Adjust the frequency discrimination characteristic S curve
The input detector probe of the frequency sweeper is replaced by a double clip cable, which is connected to the output terminal OUT of the frequency discriminator to see the S-shaped curve. If the curve is not ideal, adjust CT1 appropriately to make the upper and lower symmetrical;Adjust CT2 so that the center frequency of the curve is 6.5MHz;Adjust CT3 to make the linearity near the center of f0.After adjustment, record the frequency of the upper and lower two peak points and the grid number of the height of the two peak points, namely fmax, fmin and Vm, Vn.
(3) Using high-frequency signal generator to measure frequency discrimination characteristics point by point
Frequency discriminator
Connect the high-frequency signal generator, the input voltage is about 50mv, use a multimeter to measure the output voltage of the frequency discriminator, and measure the corresponding output voltage Uo at 0.2MHz per grid within the range of 5.5MHz~7.5MHz.
(4) Observe the influence of loop CT1, CT2 and CT3 on S curve
1 Adjust the influence of capacitance CT2 on frequency discrimination characteristics, record the change of CT2>CT2-0 or CT2<CT2-0 and compare it with the curve of CT2=CT2-0, then adjust CT2 to the normal position of CT2-0. Note: CT2-0 meansLoop resonanceCapacitance at.
2 Adjust CT1 to repeat the experiment of 1.
Frequency discriminator
3 Adjust the smaller position of CT3,Fine tuningCT1 and CT2 get the S curve, record the frequency f0, fmin, fmax at the midpoint of the curve and the upper and lower peaks, and the height of the upper and lower points Vm, Vn, then adjust CT3 to the maximum, and adjust the S curve again as the best, record:
f0′,f’min, f′max,And Vm ', Vn'.
Definition: peak bandwidth BW=fmax - fmin
Curve slope S=(VM Vin)/BW
Compare BW and S with maximum and minimum CT3.
(5) Connect the FM circuit with the frequency discrimination circuit
takeFrequency modulation circuitThe center frequency of the frequency discriminator is adjusted to 6.5MHB, and the center frequency of the frequency discriminator is also adjusted to 6.5MHz. The FM output signal is sent to the input terminal of the frequency discriminator, f=2KHz, Um=400mVAudio modulation signalAdd to the input end of the frequency modulation circuit for frequency modulation.
Simultaneous observation of modulated signals andDemodulated signalCompare the similarities and differences between the two. For example, the output waveform of the adjustable frequency discriminator CT1, CT2, CT3 is not ideal. Increase the audio signal to Vm=800mV, 1000mV.... Observe the waveform changes and analyze the reasons.
