PMX40 RF Power Meter – INSTRUCTION MANUAL
5-4
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.
Frequency Response
. The carrier frequency response of a diode detector is determined mostly by the
diode junction capacitance and the device lead inductances. Accordingly, the frequency response will
vary from detector to detector and cannot be compensated readily. Power measurements must be
corrected by constructing a frequency response calibration table for each detector.
Mismatch
. Sensor impedance matching errors can contribute significantly to measurement uncertainty,
depending on the mismatch between the device under test (DUT) and the sensor input. This error
cannot be easily calibrated out, but can be minimized by employing an optimum matching circuit at the
sensor input.
Signal Harmonics
. Measurement errors resulting from harmonics of the carrier frequency are level-
dependent and cannot be calibrated out. In the square-law region of the detector response (Region A,
Figure 5-3), the signal and second harmonic combine on a root mean square basis. The effects of
harmonics on measurement accuracy in this region are relatively insignificant. However, in the linear
region (Region C, Figure 5-3), the detector responds to the vector sum of the signal and harmonics.
Depending on the relative amplitude and phase relationships between the harmonics and the
fundamental, measurement accuracy may be significantly degraded. Errors caused by even-order
harmonics can be reduced by using balanced diode detectors for the power sensor. This design responds
to the peak-to-peak amplitude of the signal, which remains constant for any phase relationship between
fundamental and even-order harmonics. Unfortunately, for odd-order harmonics, the peak-to-peak
signal amplitude is sensitive to phasing, and balanced detectors provide no harmonic error
improvement.
Noise
. For low-level signals, detector noise contributes to measurement uncertainty and cannot be
calibrated out. Balanced detector sensors improve the signal-to-noise ratio by 3 dB, because the signal is
twice as large.
5.2
Pulse Definitions
IEEE Std 194™-1977 Standard Pulse Terms and Definitions “provides fundamental definitions for general
use in time domain pulse technology.” Several key terms defined in the standard are reproduced in this
subsection, which also defines the terms appearing in the PMX40 text mode display of automatic
measurement results.
