What Is a Distortion Meter?

Distortion meter ^[1] is a special instrument for measuring the distortion of waveforms in the measurement and measurement of low-frequency and ultra-low-frequency standard waveforms. It is often used in power systems and other places that require the purity of signal waveforms to be detected.

Distortion is also called "distortion," which refers to the deviation of a signal from the original signal or standard during transmission. In an ideal amplifier, the output waveform should be exactly the same as the input waveform except for amplification, but in reality, it cannot be done. Until the output and input waveforms are exactly the same, this phenomenon is called distortion.

Distortion meter is a special instrument for measuring the distortion of waveforms in the measurement and measurement of low-frequency and ultra-low-frequency standard waveforms. It is often used in power systems and other places where the purity of signal waveforms needs to be detected.

At present, distortion meters can be roughly divided into two categories according to the measurement principle: fundamental wave elimination method and spectrum analysis method. Generally, the analog distortion measurement instrument uses the fundamental wave rejection. This instrument has a large measurement error. The spectrum analysis method uses a frequency analyzer to measure the content of each harmonic and calculate the waveform distortion. It can analyze and measure the 1-10th harmonic components in the measured signal. This method solves the measurement of ultra-low frequency distortion better. The disadvantages are that the instrument structure is complicated, the price is expensive, and the operation is tedious.

Distortion test

Measurement frequency range: 10Hz-109kHz, divided into four frequency bands

Distortion measurement range:

20 Hz-20 kHz, 0.01 [%]-30 [%]

10 Hz-109 kHz, 0.03 [%]-30 [%]

Distortion measurement error:

300 Hz-5 kHz, ± 7 [%] ± 0.01 [%] of full-scale value (400Hz high-pass, 30kHz low-pass, access in 0.03 [%] range of distortion)

20 Hz-20 kHz, ± 10 [%] ± 0.015 [%] of the full-scale value (in the 0.03 [%] range of distortion, when the fundamental frequency is greater than 10kHz, 400Hz high-pass and 80kHz low-pass are connected. When the wave frequency is less than 300Hz, only 30kHz low-pass is connected)

10 Hz-109 kHz, ± 15 [%] ± 0.025 [%] of the full scale value

Introduction of distortion in the machine:

300 Hz-5 kHz, 0.015 [%] (400 Hz high-pass, 30 kHz low-pass, access when the distortion is 0.03 [%])

20 Hz-20 kHz, 0.025 [%] (in the 0.03 [%] distortion level, when the fundamental frequency is greater than 10 kHz, 400 Hz high-pass and 80 kHz low-pass are connected. When the fundamental frequency is less than 300 Hz, only 30 kHz is connected. through)

10 Hz-109 kHz, 0.035 [%]

Automatic adjustment range of input voltage: more than 10dB

Minimum measurable voltage of distortion: 100mV

Voltage measurement

Voltage measurement range: 300uV-300V

Basic error of voltage measurement: ± 5 [%] of full scale value (1kHz)

Additional error of voltage and frequency:

Input range switch below 100V: 0.5dB at 20Hz-50kHz; 1dB at 5Hz-300kHz

Input range switch 300V: 0.5dB when 20Hz-20kHz; 1dB when 10Hz-100kHz

Voltage noise floor: 50uV

Maximum measurable signal-to-noise ratio: 120dB

Distortion meter input impedance: 100k ± 2 [%], input capacitance 100pF

Output impedance: 600

(1) Power switch and power indicator.

(2) Input rangeattenuates the input signal by 10dB / step.

(3) Over and under voltage indication-When the input voltage is too large, the left indicator lights up; when the input voltage is too small, the right indicator lights up.

(4) Frequency band switch-change the frequency band of distortion measurement working frequency.

(5) Frequency value switch (1)-change the first digit of the distortion measurement frequency.

(6) Frequency tuning indication-when the frequency of the measuring signal relative to the distortion meter is too low, the left indicator lights up; when the frequency of the measuring signal relative to the distortion meter is too high, the right indicator lights up; both indicators are off when properly tuned .

(7) Frequency value switch (2) Change the last digit of the distortion measurement working frequency.

(8) Distortion rangedistortion range control.

(9) Function switch-select the type of work of the distortion meter.

(10) Measurement input-The signal to be measured is sent here.

(11) Relative adjustment-applied when the function switch is in the "relative level" position. When it is necessary to measure the signal-to-noise ratio or frequency characteristics of the amplifier, and the meter head of the signal under test indicates the degree of dissatisfaction, you can adjust the potentiometer to make the meter head indicate the degree of fullness, which is convenient for reading the relative value of the level.

(12) FilterWhen measuring a signal with small distortion, connect the corresponding filter according to the working frequency of the signal under test, press the button to connect, and disconnect it when raised.

(13) Oscilloscope socketWhen you need to observe the harmonic waveform of the signal under test, you can connect it to the oscilloscope from this socket.

(14) 300V attenuation switch and indication-press the switch when the measurement signal is between 100V-300V; lift up when less than 100V.

Turn on the power. Warm up for 10 to 15 minutes. Set the input range switch (2) to the left. Distortion range (8) and filter button (12) are all raised.

Distortion meter distortion measurement

The function switch (9) of the distortion meter is set to "distortion".

Place the signal source frequency switch according to the working frequency to be measured, and adjust the signal source output amplitude according to the input size requirements of the audio equipment under test. Change the input range (2) of the distortion meter so that the over- and under-voltage lamps are off. Put the working frequency (5) and (7) of the distortion meter on the working frequency of the signal source. If you find that the frequency tuning indicator (6) is on and the pointer indication cannot be reduced, you can change the working frequency of the distortion meter or the signal source and change it gradually. The distortion meter range makes the meter head indicate the most convenient position for reading, and the degree Ki can be measured in combination with the distortion range.

In the previous figure, the data measured by the distortion meter is the distortion of the signal source. If the distortion of the signal source is Ci, the distortion K of the device under test is obtained as follows: K2 = Ki2-Ci2

In the case of CI Ki / 3, it can be considered that the distortion Ki measured by the distortion meter is the distortion generated by the device under test.

Distortion meter voltage measurement

The function switch (9) is set to "input level", and the filter (12) is all raised.

Connect the signal under test to the input terminal of the distortion meter (10), change the input range of the distortion meter (2) so that the meter head is indicated at the most convenient position for reading. Combine the input range and the meter's indicated value to read the measured voltage. .

Distortion meter SNR measurement

The function switch (9) is set to "relative level".

According to the operating frequency, place the frequency band switch and frequency value switch of the oscillator. Set the oscillator output attenuator and adjust the oscillator output level according to the input size requirements of the device under test. Properly place the input range (2) of the distortion meter according to the output size of the audio device under test, so that the pointer of the meter does not exceed full scale.

For example, the measurement frequency is 1kHz, and the signal-to-noise ratio when the amplifier outputs 1V-3V: Set the distortion meter input range (2) to 3V (10dB) (set the dB number to b1), and send the output signal of the amplifier under test to the distortion meter input terminal. (10), adjust the relative adjustment knob (11) so that the meter head indicates fullness, then disconnect the audio oscillator output and short the input of the amplifier under test, keep the position of the relative adjustment knob (11) of the distortion meter unchanged, and change the distortion Instrument range (2) (set the number of dB to b2), make the meter head indicate the most convenient reading position, read the meter to indicate the number of dB (set to a), then the measured signal-to-noise ratio is b1-b2-a.

If the 300V attenuation switch is pressed while measuring the output signal of the amplifier and raised when measuring the short-circuit noise at the input of the amplifier, the signal-to-noise ratio obtained above should also be increased by 10dB.

Frequency measurement of distortion instrument amplifier

The function switch (9) is set to "relative level".

Set the frequency of the distortion meter and the signal source to 1kHz, set the output attenuation of the signal source according to the input size requirements of the device under test, set the standard voltage meter with a particularly good frequency response to the corresponding range, and adjust the signal source output level to make the standard meter full. The output of the device under test is sent to the input (10) of the distortion meter, change the input range (2) of the distortion meter, and adjust the relative adjustment knob (11) so that the distortion meter head indicates fullness.

Then change the working frequency of the audio signal source, adjust the output level of the signal source, still make the standard voltmeter full, keep the distortion meter relative to the adjustment knob (11), read the dB number indicated by the distortion meter head, and its size reflects The frequency characteristics of the device under test, the absolute value of the dB number is small, the frequency characteristics are good, otherwise it is poor. Of course, this also includes the frequency additional error of the distortion meter voltage measurement. To accurately reflect the frequency characteristics of the device under test, the frequency additional error of the distortion meter voltage measurement should also be subtracted.