Boonton 4530 Peak Power Meter User Manual - Manual (Page 144)

Chapter 5 Boonton Electronics
Making Measurements 4530 Series RF Power Meter
5-2
Frequency and linearity correction factors for Boonton CW 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 performed if
desired to enhance the absolute accuracy of the measurement. See the Sensor Connection and Calibration
information in Chapter 3.
5.1.3 RF Voltage Sensors . In some cases, it is necessary to measure the RF voltage without terminating or
significantly loading the source. For these applications, a voltage sensor (or voltage probe) is used. The
voltage probe is very similar to a CW power sensor, but there is no load termination resistor, and the diodes
simply detect the applied RF voltage. Since the voltage probe must have a reasonably high input impedance,
it will appear as a short section of unterminated transmission line to an external source. This open circuit
causes signal reflections that limit the upper frequency range of the voltage probe to no more than several
GHz. The low frequency range can extend as low as 10Hz, depending on the model.
Voltage probes are designed to measure primarily CW voltage, but they can also be used to measure the true-
RMS value of a fluctuating or modulated voltage, provided the peaks stay within the square-law region of the
diodes’ transfer curve. The threshold voltage is about 20mV RMS. Above 20mV RMS the curve is no longer
a square-law function, and at about 200mV it has transitioned into the peak-detecting range.
Boonton RF Voltage Probes do not use frequency correction. Linearity correction factors are stored in the
sensor adapter, and voltage measurements can be taken immediately upon inserting the sensor. For best
performance, zero the sensor before taking any low-level measurements. This is the only calibration proce-
dure applicable to voltage sensors; their gain is factory preset, and they cannot be user calibrated.
5.1.4 Peak Power Sensors . Although they can accurately measure CW power within the square-law region, CW
diode sensors cannot track rapid power changes (amplitude modulation), and will yield erroneous readings if
power peaks occur that are above the square-law region. By optimizing the sensor for response time (at the
tradeoff of some low-level sensitivity), it is possible for the diode detector to track amplitude changes due to
modulation. Peak sensors use a low-impedance load across the smoothing capacitors that discharges them
very quickly when the RF amplitude drops. This, in combination with a very small smoothing capacitance,
permits peak power sensors to achieve video bandwidths of several tens of megahertz, and risetimes in the ten
nanosecond range.
It should be noted that the term video bandwidth implies the frequency range of the power envelope
fluctionations, or the AM component of the modulation. If a signal has other modulation components (FM or
phase modulation), the bandwidth of those modulating signals does not have any direct affect on the video
bandwidth unless it causes additional AM modulation as an intermodulation product. A pure FM or phase
modulated signal contains very little AM, and may be considered a CW signal for the purposes of power
measurement. Power sensors are sensitive to only the amplitude of an RF signal, an not to its frequency or
phase.
Although the output of the sensor tracks the signal envelope, the transfer function is nonlinear - it is
proportional to RF voltage at higher levels, and proportional to the square of RF voltage at lower levels. By
sampling the sensor output and performing linearity correction on each sample before any signal integration
or averaging occurs, it is possible to calculate average and peak power of a modulated signal even if the input
signal does not stay within the square-law region of the diode. Additionally, a large number of power samples
can be analyzed to yield statistics about the signal’s power distribution, as well as assembled into an oscillo-
scope-like power-vs-time trace.
Boonton peak power sensors employ proprietary signal processing circuitry to compress the dynamic range
of the sensor before the signal is sampled. This compression is later removed digitally, but it permits the
sensors to maintain very high modulation bandwidths over a wide dynamic range. Conventional peak power
meters must set the gain (range) of the input channel to match the signal level being measured. This has
several undesirable side-effects including a bandwidth reduction as the input gain is increased, and brief
periods where measurements are invalid whenever the amplifiers must switch between gain ranges. Range-