Boonton 4540 Peak Power Meter User Manual - Manual (Page 232)

Boonton 4540 Series RF Power Meter
Application Notes 6-4
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 6-3), 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 6-3), 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.
6.1.3 4540 Series Features
The 4540 Series design incorporates several significant features to reduce measurement error, simplify operation, and speed
internal processing. These features include:
Balanced diode sensors enhance error performance by increasing signal-to-noise and suppressing even-
order signal harmonics.
Random sampling achieves wide measurement bandwidth at moderately high sampling speeds. Waveforms
can be displayed for repetitive signals when the trigger event is stable.
Smart Sensors (sensor-mounted EEPROM) store sensor frequency calibration and temperature
compensation data, eliminating operator entry.
Floating Point Digital Signal Processors for each channel provide high speed
processing for near real-time measurements.
A built-in programmable calibrator which creates a unique calibration table for each sensor as well as
pulsed RF test signals.