Microphonics measurment

1. Measurement method

There is no standard measurement method for cable microphonics noise. Also, little mechanism of microphonic noise is known.

Here we will explain the measurement method used in our factory since the 1970's.

  DUT: Device Under Test (flexible electric cable)
  Rl: Load resistance (from 50 Ohm to 100 MOhm)
  Volt meter: peak volt meter or storage oscilloscope
  Monitor speaker: Audio amplifier and speaker confirm microphonic noise as sound

Figure 1 Microphonics measurement setup for flexiblei electric cable

Basically we measure the voltage of the microphonic noise that occurs when shocking the cable.

The load resistor is a standard resistor that can be varied in the range of 50 Ohm to 100 MOhm, Made with multiple standard resistors and one rotary switch.

In principle, the microphonic noise is a constant current sourece and the generated voltage is proportional to the load impedance. So the choice of load resistance is extremely important, it shoult be meet with the intended use, and it shall be added to the measurement record.

The voltmeter measures the maximum peak-to-peak voltage of the noise waveform and uses an analog peak voltage meter consists of OP Amp and diode.

Alternately, with the storage oscilloscope, Record the noise waveform with one shot trigger, then measure the peak-to-peak voltage.

Also the output of the voltmeter's input amplifier is connected to the monitor speaker and check the noise as a sound.

The connection between the variable resistor unit and the voltmeter must be completely shielded.

As a shock applied to the cable, using a method such as hitting, bending, falling on a floor or a desk, dragging, Record the maximum value of peak-to-peak voltage.

2. Mechanism of microphonic noise

Microphonic noise occurs when electrostatic charge generated when the insulator of the cable and the external conductor are mechanically separated for a moment discharges through the load resistance.

For the circuit, the equivalent circuit shown in Fig. 2 is easy to understand.

Fig 2 Generation mechanism of microphonics

This equivalent circuit was created by Kouichi Hirabayashi (our founder) in the 1970s, and in terms of circuitry, it is a shock wave generating circuit.

Although it is somewhat troublesome, the noise waveform generated in the load resistor can be analytically solved.

An example of the simulation result by SPICE is shown in Fig 3.

Fig 3 one of the Fig 2 SPICE simulation

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There is no English translation though.