Introduction to the circuit and working principle of the CR scanning oscillator with a large variation range

Circuit working function

The sweep oscillator with a large frequency variation range is used for automatic determination of frequency characteristics. The sweep range needs about 3 grids on the logarithmic axis. This circuit can obtain a frequency proportional to the control voltage VC. In the logarithmic sweep, an anti-logarithmic circuit is added. When a sawtooth wave is input, it can also perform automatic sweep and can be used as a low-frequency sine output VCO.

[page]Circuit working principle

This circuit is composed of a CR Wien bridge oscillator circuit and a voltage-controlled variable resistor circuit. The Wien bridge circuit is a frequency-selective CR circuit network. The gain of amplifier A1 is about 3, the loop gain is 1, and the oscillation frequency is FO=1/2πCO.RO.

If the oscillation amplitude is unstable, the output waveform is easy to saturate or stop oscillating when A≤3. Diode D2 performs half-wave rectification on the output and compares the base average current with the reference current generated by the voltage regulator diode. A2 integrates the comparison result and controls the gate voltage VCS of the J-FET to change the resistance between the drain and the source. If the oscillation output level increases, IIN≥IE, and the output of A2 is negatively integrated, which increases the resistance value of TT1, reduces the gain of A1, and IIN≈IE reaches stability.

The variable resistor circuit uses feedback optocoupler PC3 to improve linearity. Adjust PC3 so that the control input voltage VC and the output of OP amplifier A4 have opposite polarities and the same potential. Let the voltage generated by A3 be +10V. If VC=+10V, then due to the OP amplifier, the resistance value of PC3 should be equal to R14.

If R14=2K, when VO=1V, the resistance of PC3 is 20K, and when VO=0.1V, it is 200K, which is inversely proportional to the voltage. Therefore, the oscillation frequency can obtain an output proportional to the voltage. This is based on the premise that the characteristics of PC1~PC3 are the same, so three photoresistors with similar characteristics should be prepared.

Component Selection

When the oscillation frequency range is reduced, resistors R1 and R2 can be added to the Wien bridge circuit. Capacitor C1=C2, and their capacity is determined by the oscillation frequency. Assuming the highest frequency FMAX=20KHZ, R1=R2=0, and the resistance value of the CDB optical coupling is 1.6K, a polystyrene capacitor with a capacity of C1=C2=1/2πFMAX.R14=4976PF (4700+2700PF) can be selected.

Optocouplers PC1 and PC2 must be selected with the same characteristics. The selection method is: when 100UA, 1MA, and 10MA currents flow through them, the equivalent resistance of the two optocouplers should be basically the same.