Boonton 4500B Peak Power Meter - Manual (Page 313)

Boonton 4500B RF Peak Power Analyzer
Application Notes 6-3
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.