Measurement of Pulsed Signals

Question:

”Will the Smart Fieldmeter detect the periodic pulse signals with 5 mW/cm² average power density and peak power density of 20 W/cm²? Pulse duration can be changed from 10 us to 100 us”.

Answer:

1. In order to measure the amplitude of the pulsed signals for very short MW pulses having low duty cycle:

Tp<<(T,ts)<<th,

Tp and T are the MW pulse duration and period, and ts and th are the sample and hold time constants of the peak detector, the following condition should be met:

tav/ts>T/Tp

From the ratio of peak/average power density we calculate that T/Tp is equal to 4,000. For current production model of Smart Fieldmeter

tav/ts = 40,

and the condition above is not valid. This means, that meter will not display such short pulses with so small duty cycle.

Technically it is possible to increase tav by a factor of 10 or more, and decrease ts by a factor of 10 or more. Such capabilities have already been designed into the product, and can be implemented as a special order. Then tav/ts > 4000, and unit would be able to measure the pulsed MW signals provided that their fields are within device dynamic range.

2. Field strength E can be derived from the power density according to the equation:

E[V/m]=Ö(3.77*P[uW/cm²])

This gives for 20 W/cm² ---> 8,683 V/m, and for 5 uW/cm²---> 137 V/m.

Smart Fieldmeter is designed to measure and display fields up to 600 V/m. Maximum field strength tolerated by probe antenna is about 1,500 V/m.

Therefore, field 8,683 V/m will be underestimated, and meter reading will be lower than expected 137 V/m. The exposure to such strong field may potentially damage the probe, but considering very small duty cycle (1/4000) this shouldn’t happen.

3. Based on maximum field strength of 1,500 V/m, maximum peak power density measured by the device is 0.6 W/cm².