Boonton Power Sensor - Manual (Page 6)

1-1 Overview
Introduction
1
Power Sensor Manual 1
The overall performance of a power meter is dependent upon the sensor employed.
Boonton Electronics (Boonton) has addressed this by providing quality power sensors
to meet virtually all applications. Boonton offers a family of sensors with frequency
ranges spanning 10 kHz to 100 GHz and sensitivity from 0.1 nW (-70 dBm) to 25 W (+44
dBm). A choice of Diode or Thermocouple Sensors with 50 or 75 ohms impedances in
Coaxial or Waveguide styles are available.
Both the Thermocouple and Diode Sensors offer unique advantages and limitations.
Thermocouple Sensors measure true RMS power over a dynamic range from 1.0 µW (-30
dBm) to 100 mW (+20 dBm), and therefore, are less sensitive to non-sinusoidal signals
and those signals with high harmonic content. The Thermocouple Sensors also provide
advantages when making pulsed RF measurements with extremely high crest factors.
While the headroom (the difference between the rated maximum input power and burnout
level) for CW (continuous wave) measurements is only a few dB (decibels), Thermocouple
Sensors are very rugged in terms of short duration overload. For example, a sensor that
operates up to 100 mW average power (CW) can handle pulses up to 15 watts for
approximately two microseconds. One of the major limitations to the Thermocouple
Sensor is on the low-end sensitivity. Low-end sensitivity of these sensors is limited by
the efficiency of the thermal conversion. For this reason, the Diode Sensor is used for
requirements below 10 µW (-20 dBm).
CW Diode Sensors provide the best available sensitivity, typically down to 0.1 nW (-
70 dBm). Boonton Diode Sensors are constructed using balanced diode detectors. The
dual diode configuration offers increased sensitivity as well as harmonic suppression
when compared to a single diode sensor. The only significant drawback to Diode
Sensors is that above the level of approximately 10 µW (-20 dBm), the diodes begin to
deviate substantially from square-law detection. In this region of 10 µW (-20 dBm) to
100 mW (20 dBm), peak detection is predominant and the measurement error due to the
presence of signal harmonics is increased.
The square-law response can be seen in Figure 1-1, where a 100% amplitude modulated
signal is shown to have virtually no effect on the measured power at low levels. Of
course, frequency modulated and phase modulated signals can be measured at any
level, since the envelope of these modulated signals is flat. Frequency shift keyed and
quadrature modulated signals also have flat envelopes and can be measured at any
power level.
1-2 Sensor Trade-offs