Boonton 4530 Peak Power Meter User Manual - Manual (Page 145)

Boonton Electronics Chapter 5
4530 Series RF Power Meter Making Measurements
5-3
less operation allows continuous measurement of signals without the loss of data that would be incurred
whenever the signal crosses a power range threshold (typically every 10dB). The top and bottom of pulse
transitions can be measured without worrying about artifacts introduced by input amplifier saturation, and a
single-shot burst of samples can be acquired without the need to set the correct range in advance. And range
switching makes statistical sample analysis impossible, since it causes signal-related “holes”, clipped inter-
vals, or synchronization in the sampling. When this happens, the sample population can becomes somewhat
or even strongly correlated with the signal rather than completely random. Statistical analysis requires a
completely random sample population for the results to be valid.
5.2 SELECTING THE RIGHT SENSOR
5.2.1 CW Signals . The absolute or relative power of CW Signals may be accurately measured using any type of
Boonton power sensor: CW Diode Sensors, CW Thermal Sensors, or Peak Power Sensors. The choice
depends mainly upon the signal’s power range. If the signal is always unmodulated or if the power level never
exceeds -20dBm, a CW diode sensor is the best choice due to its wide dynamic range. Although thermal
sensors and peak power sensors will work fine for many CW applications, they offer no distinct advantages,
and have somewhat less sensititivity for low-level signals.
5.2.2 Modulated Signals . For low-level modulated signals, a CW sensor is often the best choice. But whenever
the peak power of a modulated signal exceeds about -20dBm, the average power reading from a CW diode
sensor becomes unreliable due to its peak-detecting characteristics at higher power levels.
In these cases, a peak power sensor offers accurate average power measurements over the sensor’s entire
dynamic range, plus the ability to fully characterize the time-domain or statistical power characteristics. Note,
however, that peak sensors must possess sufficient video bandwidth to track any modulation-induced signal
variations. If the signal modulation bandwidth is above the sensor bandwidth, the sensor cannot track the
fastest signal changes, and the measurement accuracy is degraded.
In these cases, a thermal sensor may offer the best choice. The detector accurately integrates (averages) the
effect of any modulation present over a period of several tens or hundreds of milliseconds, regardless of
power level or frequency content. This type of sensor will measure the true, average power of any signal
within its power and frequency range. The primary drawback is slow response time, and lack of sensitivity for
low-level measurements.
5.3 MEASUREMENT MODES
The 4530 Series RF Power Meter can accept any type of Boonton power or voltage sensor, and each sensor type can
operate in one or more measurement modes.
5.3.1 CW Mode . CW diode sensors, thermal sensors and voltage probes all operate in CW mode. CW mode uses
a low-noise, high-resolution analog channel to process and digitize the sensor voltage. A wide dynamic-
range sigma-delta A/D converter allows elimination of the ranging that plagues conventional power meters.
Range changing causes the hardware to switch to different gain settings for different signal levels. Each time
this is done, there is an interruption in the measurement process, and an additional settling time for the
amplifiers to stabilize at the gain setting. Additionally, it is difficult to get the gain settings to “splice
together” smoothly so that a signal measured at the top of one range is exactly equal to that same signal
measured at the bottom of the next range. This results in linearity errors, or steps in the reading as the power
level changes. Eliminating ranging allows the full dynamic range of a thermal sensor or the entire square-law
region of a diode sensor may be measured without the need for changing ranges. To optimize performance
and measurement speed at higher levels, the instrument switches to a higher-speed low-gain setting for the
upper region of diode sensors.