Boonton 4500B Peak Power Meter - Manual (Page 311)

Boonton 4500B RF Peak Power Analyzer
Application Notes 6-1
6. Application Notes
This section provides supplementary material to enhance your knowledge of Model 4500B operation, advanced features and
measurement accuracy. Topics covered in this section include pulse measurement fundamentals, automatic measurement
principles, and an analysis of measurement accuracy.
6.1 Introduction to Pulse Measurements
Power Measurements
The following is a brief review of power measurement fundamentals.
Unmodulated Carrier Power. The average power of an unmodulated carrier
consisting of a continuous, constant amplitude sinewave signal is also termed
CW power. For a known value of load impedance R, and applied voltage Vrms,
the average power is:
P = Vrms²/R
watts
Power meters designed to measure CW power can use thermoelectric detectors which
respond to the heating effect of the signal or diode detectors which respond to the voltage
of the signal. With careful calibration accurate measurements can be obtained over a wide
range of input power levels.
Modulated Carrier Power. The average power of a modulated carrier which has
varying amplitude can be measured accurately by a CW type power meter with a
thermoelectric detector, but the lack of sensitivity will limit the range. Diode detectors
can be used at low power, square-law response levels. At higher power levels the diode
responds in a more linear manner and significant error results.
Pulse Power. Pulse power refers to power measured during the on time of pulsed RF
signals (Figure 6-1). Traditionally, these signals have been measured in two steps: (1)
thermoelectric sensors measure the average signal power, (2) the reading is then divided
by the duty cycle to obtain pulse power,
Ppuls e:
Ppulse = Average Power/ Duty Cycle (measured)
where Duty Cycle = Pulse Width/Pulse Period
Pulse power provides useful results when applied to rectangular pulses, but is inaccurate
for pulse shapes that include distortions, such as overshoot or droop
(Figure 6-2).