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INTRODUCTION
1.1
OVERVIEW
The overall performance of a power meter is dependent upon the head employed. AR RF/Microwave
Instrumentation (AR) has addressed this by providing quality power heads to meet virtually all applications.
AR offers a family of heads with frequency ranges spanning 10 kHz to 40 GHz and sensitivity from 0.1 nW
(-70 dBm) to 25 W (+44 dBm). A choice of diode or thermocouple heads with 50 or 75 ohms impedance in
coaxial or waveguide styles are available. This manual covers all PH series models.
1.2
HEAD TRADE-OFF
Both the thermocouple and diode heads offer unique advantages and limitations. Thermocouple heads
measure true RMS power over a dynamic range from 1.0 µW (-30 dBm) to 100mW (+20 dBm), and
therefore, are less sensitive to non-sinusoidal signals and those signals with high harmonic content. The
thermocouple heads 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 heads are very rugged in
terms of short duration overload. For example, a head 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 head is on the low-end sensitivity. Low-end sensitivity of these heads is limited by the
efficiency of the thermal conversion. For this reason, the diode head is used for requirements below 10µW
(-20 dBm).
CW diode heads provide the best available sensitivity, typically down to 0.1 nW (-70 dBm). AR diode heads
are constructed using balanced diode detectors. The dual diode configuration offers increased sensitivity as
well as harmonic suppression when compared to a single diode head. The only significant drawback to Diode
Heads 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 10µW (-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 measure 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.
This non-square law region may be "shaped" with meter corrections, but only for one defined waveform, such
as a CW signal. By incorporating "shaping", also referred to as "linearity calibration", AR offers a dynamic
range from 0.1 nW (-70 dBm) to 100 mW (+20 dBm) with a single head module. For CW measurements, the
entire 90 dB range can be used, however, when dealing with non-sinusoidal and high-harmonic content
signals, the diode head should be operated only within its square-law region (10µW and below).
Although thermal heads provide a true indication of RMS power for modulated (non-CW) signals, they are of
limited use for characterizing the short-term or instantaneous RF power due to their rather slow response
speed. The linearity correction factors are automatically generated by the instrument's built-in programmable
calibrator. With the high head bandwidth, and frequency and linearity correction applied continuously by the
instrument, it is possible to make many types of measurements on an RF signal; average (CW) power, peak
power, dynamic range, pulse timing, waveform viewing, and calculation of statistical power distribution
functions.
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