Boonton 4540 Peak Power Meter User Manual - Manual (Page 231)

Boonton 4540 Series RF Power Meter
Application Notes 6-3
6.1.2 Diode Detection
Wideband diode detectors are the dominant power sensing device used to measure pulsed RF signals. However, several diode
characteristics must be compensated to make meaningful measurements. These include the detector’s nonlinear amplitude
response, temperature sensitivity, and frequency response characteristic. Additional potential error sources include detector
mismatch, signal harmonics and noise.
Detector Response. The response of a single-diode detector to a sinusoidal input is given by the diode equation:
where:
i = diode current
v = net voltage across the diode
Is = saturation current
= constant
An ideal diode response curve is plotted in Figure 6-3.
Figure 6-3.
Ideal Diode Response
The curve indicates that for low microwave input levels (Region A), the single-diode detector output is proportional to the
square of the input power. For high input signal levels (Region C), the output is linearly proportional to the input. In between
these ranges (Region B), the detector response lies between square-law and linear.
For accurate power measurements over all three regions illustrated in Figure 6-3, the detector response is pre-calibrated over
the entire range. The calibration data is stored in the instrument and recalled to adjust each sample of the pulse power
measurement.
Temperature Effects. The sensitivity of microwave diode detectors (normally Low Barrier Schottky diodes) varies with
temperature. However, ordinary circuit design procedures that compensate for temperature-induced errors adversely affect
detector bandwidth. A more effective approach involves sensing the ambient temperature during calibration and recalibrating
the sensor when the temperature drifts outside the calibrated range.
This process can be made automatic by collecting calibration data over a wide temperature range and saving the data in a
form that can be used by the power meter to correct readings for ambient temperature changes.