Distortion rate measurement notch filter that enables high-speed measurement without automatic tuning

Function of the circuit

Most distortion rate measuring instruments use CR tuning. If automatic tuning is used, it is quite time-consuming and not suitable for IC test instruments. Non-tuning methods include: using an offset filter to only allow high-order harmonics to pass through and using a notch filter with a 1KHZ±1% attenuation bandwidth like this circuit to filter out the fundamental wave and then measure the high-order harmonics containing power supply hum and noise. The former needs to filter out the power supply noise in the measurement signal , so it is called THD, and the latter is reflected as noise distortion rate. Which method to use can be determined according to the use requirements.

How the Circuit Works

Input amplifier A1 is a buffer composed of low-distortion OP amplifiers. When the switch is short- circuited , the measured low-level signal of several 100MV can be increased by 20DB. If the input signal contains a DC component, a 1UF capacitor can be connected in series at the input end .

The two stages A2 and A4 of the notch filter are tuned in series to have a bandwidth of FO=1KHZ±10HZ. When selecting the tuning resistor , it should be ensured that A2 and A4 have oscillation frequencies of FO*0.99 and FO*1.09 respectively.

Feedback the signal at the notch output to the Wien bridge circuit A2 and A4 to give it a sharp passband characteristic. Without feedback, the 2nd harmonic 2KHZ characteristic curve near 1KHZ will drop, and accurate measurement cannot be performed. The feedback amount is related to the frequency characteristic and is adjusted with variable resistors VR1 and VR2.

The distortion rate is measured by the ratio of the input signal to the notch output. The voltage RMS is accurate and is usually measured with an average detection level meter. If the input level is 1V and the notch output is 1MV, the distortion rate is (1*10-3/1)*100%=0.1% (-60DB). The input level is changed. Since the ratio is often taken, level calibration is not required, which is suitable for high-speed automatic measurement.

Electrical characteristics

Photo A shows the characteristics of the input frequency in the range of 500 Hz to 1.5 kHz and the characteristics when the center frequency is ±50 Hz. The resonant frequency is slightly narrower than ±1%, but the frequency of -70 dB is 28 Hz (±1.4%), which is wider than the target value.

Component Selection

When selecting components, special attention should be paid to C and R that determine the notch frequency. The component error must be within ±1%. High-precision capacitors are difficult to obtain, so they can be selected from ±5% components. The goal is not to find components with zero error, but to calculate the resistance R=1/2πOFC based on the C value, and then use two resistors in series to make it close to the calculated value.

The ratio of resistors R5 and R6, R9 and R10 is very important and should be 1:2, with a ratio of ±0.5%. If the attenuation characteristics have been adjusted, the variable resistors VR1 and 2 of the high-speed feedback amount can be replaced with fixed resistors.