Experimental principle:
The phase-sensitive detection circuit is shown in Figure (7): ① is the input signal terminal, ② is the AC reference voltage input terminal, ③ is the detection signal output terminal and ④ is the DC reference voltage input terminal.
When the control voltage signal is input at terminals ② and ④, D and J are turned on or off under the action of difference channel, thus converting the sinusoidal signal input at terminal ① into a full-wave rectified signal.
(Figure 7)
Components required for the experiment:
Common circuit module (2) (phase sensitive detector, phase shifter, low-pass filter), audio signal source, DC regulated power supply, voltmeter and oscilloscope.
Experimental steps:
1. Connect the power cord between the host and the experimental module, and the audio signal output is connected to the phase-sensitive detection input ①.
2. The 2V output (positive or negative) of the 2.DC regulated power supply is connected to the ④ terminal of the phase sensitive detector.
3. The two channels of the oscilloscope are connected to the phase-sensitive input and output respectively, and the phase relationship and amplitude relationship of the input and output waveforms are observed.
4. Change the polarity of the reference voltage at terminal ④ and observe the relationship between the phase and amplitude of the input and output waveforms. It can be concluded that when the reference voltage is positive, the input and output are in phase, and when the reference voltage is negative, the input and output are in phase.
5. Send the output signal of the audio oscillator 00 to the input end of the phase shifter. The output of the phase shifter is connected to the reference input end ② of the phase-sensitive detector, and the signal input end ① of the phase-sensitive detector is connected to the audio 00 output.
6. Observe the waveforms of additional observation sockets ⑤ and ⑤ with a two-channel oscilloscope.
It can be seen that the role of the shaping circuit in the phase-sensitive detector is to convert the input sine wave into a square wave, so that the electronic switch in the phase-sensitive detector can work normally.
7. Connect the output end of the phase-sensitive detector with the input end of the low-pass filter, and the low-pass output end is connected with the 20V file of the digital voltmeter.
8. The two channels of the oscilloscope are respectively connected to the input and output ends of the phase-sensitive detector.
9. Adjust the amplitude knob of the audio oscillator and the "phase shift" knob of the phase shifter appropriately, observe the waveform change and voltmeter voltage change in the oscilloscope, and then switch the input end ① of the phase-sensitive detector to the output port of the audio oscillator 1800 to observe the changes of the oscilloscope and voltmeter.
As can be seen from the above, when the input signal of the phase-sensitive detector is in phase with the switching signal, the output is a full-wave rectified signal with positive polarity, and the voltmeter indicates the maximum value of the positive polarity direction; Otherwise, the full-wave rectified waveform with negative polarity is output, and the voltmeter indicates the maximum value of negative polarity.
10. Adjust the "phase shift" knob of the phase shifter, and measure the relationship between the input peak-to-peak value and the output DC voltage of the phase-sensitive detector with an oscilloscope and voltmeter.
1 1. Change the phase of the input signal and the reference signal by 1800, and get the experimental results.
Function and principle of phase-sensitive detection
1, what is a phase-sensitive detection circuit?
Phase-sensitive detection circuit is a kind of detection circuit with the ability to distinguish the phase of modulation signal and select frequency.
2. Why use phase-sensitive detection?
There are two problems in envelope detection: first, the main process of demodulation is half-wave or full-wave rectification of amplitude modulation signal, so it is impossible to distinguish the phase of modulation signal from the output of detector. Secondly, the envelope detection circuit itself does not have the ability to distinguish different carrier frequency signals. For signals with different carrier frequencies, it rectifies them in the same way to recover modulated signals, which means that it has no ability to distinguish signals. In order to make the detection circuit have the ability to distinguish the phase and frequency of the signal and improve the anti-interference ability, it is necessary to adopt the phase-sensitive detection circuit.
3. What is the main difference between phase-sensitive detection circuit and envelope detection circuit in function and circuit composition?
The main functional difference between the phase-sensitive detection circuit and the envelope detection circuit is that the phase-sensitive detection circuit can identify the phase of the modulation signal, thus identifying the direction of the measured change, and at the same time, the phase-sensitive detection circuit also has the ability of frequency selection, thus improving the anti-interference ability of the measurement and control system. From the circuit structure, the main feature of the phase-sensitive detection circuit is that besides the amplitude modulation signal to be demodulated, a reference signal is also input. With the reference signal, it can be used to identify the phase and frequency of the input signal.
4. What are the similarities between the phase-sensitive detection circuit and the amplitude modulation circuit in structure? What is the difference between them?
Multiplying the modulation signal ux by the carrier signal with the amplitude of 1 can get the double-sideband amplitude modulation signal us, and multiplying the double-sideband amplitude modulation signal us by the carrier signal can get the low-pass filtered modulation signal ux. This is why the phase-sensitive detection circuit is similar to the modulation circuit in structure.
The main difference between them is that the amplitude modulation circuit multiplies the low frequency modulation signal with the high frequency carrier signal to output the high frequency amplitude modulation signal; The phase-sensitive detector multiplies the high-frequency amplitude modulation signal with the high-frequency carrier signal, and outputs the low-frequency demodulation signal after filtering. This makes their input and output coupling loops and filters different in structure and parameters.
(3) Frequency selection and phase discrimination characteristics of phase-sensitive detection circuit.
1, frequency selection characteristics of phase-sensitive detection circuit
What is the frequency selection characteristic of phase-sensitive detection circuit?
The frequency selection characteristic of phase-sensitive detection circuit means that it has different transfer characteristics for input signals with different frequencies. Taking the reference signal as the fundamental wave, the average output of all even harmonics in a period of the carrier signal is zero, that is, it has the function of suppressing even harmonics. For odd harmonics such as n= 1, 3, 5, the amplitude of the output signal is correspondingly attenuated to 1/ n of the fundamental wave, that is, the transmission coefficient of the signal is attenuated with the increase of the number of harmonics, which has a certain suppression effect on higher harmonics.
2. The phase discrimination characteristics of the phase-sensitive detection circuit.
What is the phase discrimination characteristic of the phase-sensitive detection circuit?
If the input signal us and the reference signal uc (or Uc) are the same frequency signals, but there is a certain phase difference, then the output voltage uo=Usm/2cos∮, that is, the output signal changes with the cosine of the phase difference ∮.
When the input signal and the reference signal have the same frequency but a certain phase difference, the magnitude of the output signal has a definite functional relationship with the phase difference, so the value of the phase difference can be determined according to the magnitude of the output signal. This characteristic of phase-sensitive detection circuit is called phase discrimination characteristic.