Boonton 4530 Peak Power Meter User Manual - Manual (Page 143)

Boonton Electronics Chapter 5
4530 Series RF Power Meter Making Measurements
5-1
5. MAKING MEASUREMENTS
5.1 SENSOR TYPES
The 4530 Series RF Power Meter can operate with any type of Boonton sensor to measure CW or modulated RF power
or voltage over a wide range of operating frequencies. Refer to Appendix A for a description of the most popular
sensors. Those sensors listed cover the great majority of applications used. For special requirements, contact
Boonton Electronics for a complete listing of available sensors. There are four key types of sensors:
5.1.1 Thermal RF Power Sensors . Thermal sensors use the RF energy to produce a temperature rise in a thermal
detector that produces a voltage output which is proportional to the applied RF power. Since the detector
output is very linear with input power over its entire operating range, and the thermal time constant is longer
than most signal fluctuations due to modulation, a thermal sensor may be used to accurately measure the
average RF power of both CW and modulated signals. Even very narrow duty-factor pulse signals can be
accurately measured, since the detector senses the long-term heating effect of the input signal. Thermal
sensors are very linear, and generally offer up to 50dB of dynamic range. They can be optimized to measure
as low as about one microwatt (-30dBm), but are sometimes combined with input pads (attenuators) to allow
measurement of higher power levels.
Frequency and linearity correction factors for Boonton Thermal Power Sensors are stored in the sensor
adapter, and power measurements can be taken immediately upon inserting the sensor. For best performance,
zero the sensor before taking any low-level measurements. A single- or multi-point calibration can be per-
formed, if desired, to enhance the absolute accuracy of the measurement. See the Sensor Connection and
Calibration information in Chapter 3.
5.1.2 CW Dual-Diode RF Power Sensors . CW Diode sensors use high-frequency semiconductor diodes to
detect the RF voltage developed across a terminating load resistor. Two diodes are used so both the positive
and negative carrier cycles are detected; this makes the sensor relatively insensitive to even harmonic distor-
tion. The diodes’ output signals are filtered by smoothing capacitors, and the resulting DC output voltage is
proportional to power at low signal levels and proportional to voltage at higher levels. To achieve high
sensitivities, the load resistance driven by the diode’s output is typically several megohms.
Below about -20dBm, the RF voltage is not high enough to cause the diodes to fully conduct in the forward
direction. Instead, they behave as non-linear resistors, and produce a DC output that is closely proportional
to the square of the applied RF voltage. This is referred to as the “square-law” region of the diode sensor.
When operated in this region, the average DC output voltage will be proportional to average RF power, even
if modulation is present. This means a CW sensor can be used to measure modulated signals, provided the
instantaneous (peak) power remains within the square-law region of the diodes at all times.
Above about 0 dBm, the diodes go into forward conduction on each cycle of the carrier, and the peak RF
voltage is held by the smoothing capacitors. In this region, the sensor is behaving as a peak detector (also
called an envelope detector), and the DC output voltage will be equal to the peak-to-peak RF input voltage
minus two diode drops. If modulation is present, the output voltage will rapidly slew to the highest peaks,
then slowly decay once the signal drops. Since the input signal could be at any amplitude during the time the
capacitor voltage is decaying, it is impossible to deduce the actual average power level of a modulated signal
once the peak RF power gets into this peak-detecting region of the diode.
CW Diode sensors can offer up to a 90dB dynamic range, and are extremely sensitive; some can measure
signals as low as about 100pW (-70dBm). They are also available with built-in input attenuators for calibrated
measurement of higher power signals.