Basic method for adjustment and testing of sine wave oscillation circuit

Test - Uppercase JPG - Memory 146K

Kaixin Micro Test

Test Probe P100-M3

Crystal Oscillator

1. Amplitude Adjustment and Measurement: After checking the wiring of the oscillating circuit and confirming there are no errors, ensure that the selected components are appropriate. Once the DC power supply is turned on, oscillation may occur. Use an oscilloscope to observe the voltage waveform at the output. If the positive feedback circuit is disconnected at this point and the waveform disappears, it indicates that the signal observed on the oscilloscope is not interference or parasitic oscillation. If no waveform appears, the circuit is not oscillating. In such cases, check if the positive feedback circuit is properly connected, if the feedback polarity is correct, and whether the gain-bandwidth product of the operational amplifier meets the requirements. Then proceed with the following adjustments:
(1) Increase the positive feedback coefficient to boost the positive feedback level.
(2) Reduce the negative feedback amount to increase the amplification factor.
(3) For a transistor-based oscillating circuit, you can increase the amplification factor by adjusting the quiescent operating point, increasing the collector load resistance, or improving the quality factor of the oscillating circuit. These methods can also help increase the output amplitude if it's insufficient, or reduce it if necessary. 2. Adjustment and Measurement of Oscillating Waveform: The output waveform of the oscillating circuit should be a clean sine wave. If the observed waveform is severely distorted, first reduce the positive feedback and increase the negative feedback to lower the loop gain. Then check if the quiescent operating point of the transistor, the quality factor of the LC resonant tank, and the slew rate of the operational amplifier meet the required specifications. 3. Frequency Adjustment and Measurement: You can measure the frequency using an oscilloscope, but for accurate results, a digital frequency meter is recommended. The main method for frequency adjustment involves changing the parameters of the frequency-selective network in the oscillating circuit. For example, in an RC oscillator, adjust the resistance and capacitance values in the RC network. For an LC oscillator, change the capacitance of the variable LC circuit, the position of the core, or the number of turns in the coil. However, during the adjustment process, be mindful of the impact of device characteristics, parasitic capacitance, and the input impedance of testing instruments. 4. Measurement of Amplitude and Frequency Stability:
(1) To measure short-term stability, vary the DC power supply voltage by ±10% and change the load impedance. Measure the changes in output voltage and frequency to determine the amplitude and frequency stability, represented as ΔU0/U0 and Δf0/f0.
(2) To evaluate long-term stability, monitor the relative changes in amplitude and frequency over a period of one hour (or half an hour). This helps assess how stable the circuit performs under continuous operation.