Low frequency small drift polarity conversion type same frequency detection circuit

Function of the circuit

The operating frequency of the synchronous detection circuit using anti-phase and common-phase switching is less than tens of kilohertz. Its DC stability is much better than that of the double-balanced differential circuit and can be applied in the entire low-frequency band.

The analog switch of this circuit uses a general N-channel J-FET. The smoothing circuit adds a 12DB/OCT low-pass filter to shorten the response time. The detection adopts a full-wave rectification method, which can easily eliminate high-order harmonics. This circuit is widely used in the lock-in amplifier detection circuit for measuring small voltages.

How the Circuit Works

OP amplifier A1 is an impedance buffer. If the output impedance of the previous stage is also low like the OP amplifier, A1 can be removed. A2 is an amplifier that switches between inverting and non-inverting. TT2 is turned on, the non-inverting input terminal is grounded, and the amplification factor is -1 (-1=R3/R2).

After TT2 is turned off, the input signal is input to A2 through R4. Since the input resistance is very high, it becomes equal potential, and the inverting input terminal must follow it, so TT2 acts as a follower with an amplification factor of 1.

The analog switch circuit composed of TT2 must swing the gate-source voltage VOS in the negative direction to near the pinch-off voltage VP when it is cut off. In order to use TTL level drive , +5/0V is replaced with -VCC/+5V. It can also be replaced with a CMOS analog switch, but the component cost must be considered. TT1 is a PNP transistor with an emitter voltage of -VCC. When the input TTL level is "L", TT1 generates base current, TT1 is turned on, the collector voltage swings to near +5V, and the diode D1 is cut off, so ROS≈0, TT2 is turned on.

Capacitors C1 and C2 are added to reduce spike pulses. When the REF waveform that generates the reference phase is in phase with the input waveform, the output waveform of the OP amplifier A2 is a positive full-wave rectified wave, and when it is in reverse phase, it is a negative full-wave rectified wave. When there is a 90-degree phase difference, the polarity is changed at 90 degrees and 270 degrees of the input waveform to form an S-shaped positive and negative symmetrical wave. If it is smooth, the output is 0, regardless of the size of the input signal.

If the peak value of the input voltage is set to E1, the average value of the full-wave rectification is 2E1/π=0.636E1. If the gain of the filter is 1.56, a DC voltage basically equal to E1 can be obtained at the output end.

The low-pass filter is constructed using the same parameter method, and the cut-off frequency is determined by the output response. Here, FO is 10HZ and the attenuation gradient is 12DB/OCT.

Component Selection

In order to ensure accurate in-phase and anti-phase conversion, R2 must be equal to R3, and the on-state resistance TOE of TT2 should be equal to zero. However, in fact, there is a resistance of several ohms. This can be solved by increasing the resistance of R4 as much as possible or connecting a parallel resistor to reduce RON.

The level switching circuit has a large number of components. For simplicity, VOB=0V, a cut-off N-channel MOS FET or a C-MOS analog switch can be used.