Abstract:

Transient electromagnetic emission systems have higher requirements on the waveform of the emission current. The closer the emission waveform is to the ideal square wave, the better the detection effect. However, in actual situations, the emission waveform is difficult to achieve the ideal square wave. Generally, trapezoidal waves are used instead. The waveform of the constant current section of the emission actually rises exponentially rather than the approximate level. Regarding the issue above, an improved transmitter circuit is proposed, that is, two metal oxide semiconductor field effect transistor(MOSFETs) are used to control the turn-off of the resistance circuit, and pulse width modulation(PWM) control is added to make the slope of the transmitter constant current section approximately zero, and the addition of two controllable resistors makes the equivalent resistance of the transmitting circuit more flexible, thus reducing the amplitude of ripple after PWM chopping, making the constant current section of the transmitting current more stable, thereby reducing the impact on the received signal. In order to shorten the turn-off time of the emission current, a constant voltage clamp control circuit is usually added to control, so that the detection effect is better.

Key words: Transient electromagnetic method, emission waveform, PWM chopping, constant voltage clamp