Peak Power Meters: Essential Instruments for Radar Power Amplifier Measurements
The pulsed signals of radar systems present unique measurement and characterization challenges as their signals may only be “on” for a short time, followed by a long “off” period. During the “on” time, the system transmits high levels of RF power that can overstress power amplifiers during both on/off transitions and prolonged “on” periods. As peak power meters measure, the power envelope of an RF signal in the time domain, they are an essential tool for analyzing and characterizing the anomalies and behavior of amplifiers used in pulsed radar systems.
Figure 1 shows a simplified block diagram of a benchtop peak power meter. The RF front end is an RF envelope detector housed in a power sensor. The detector removes the RF carrier and generates an analog waveform representing the envelope of the RF input signal. The most critical detector specification is its response time to a pulsed RF signal (or its rise time) because if the detector does not have the bandwidth to track the envelope of the signal, the accuracy of measurements, including peak, pulse, and average power is compromised as shown in Figure 2.
The detector output is then digitized by an analog-to-digital converter (ADC) whose digitized samples are processed by a digital signal processor for linearization and measurement analysis. The processed waveform is displayed in the time domain as power versus time, along with automated pulse and marker measurements. In Boonton’s peak power meters, for example, power (or voltage) is displayed in watts, volts, or dBm on the vertical axis along with the ability to change the vertical scale and reference value.
On the horizontal axis, the time base can be set as low as 5 ns per division (50 ns span) to zoom in to a specific portion of the waveform, such as the rising or falling edge, to observe fine details of the waveform. Boonton’s peak power meters use the Random Interleave Sampling (RIS) technique that yields 100 ps resolution on repetitive waveforms. The benefit of this approach is shown in Figure 3, in which Figure 3a shows a time domain diagram of a conventional sampling and interpolation method and Figure 3b shows the RIS method.
Peak power meters can be triggered by the incoming RF signal or by an external gating (baseband) trigger signal applied to auxiliary inputs. Real-time power processing in USB peak power sensors, such as Boonton’s RTP5000 series’ allows the sensor to perform up to 100,000 measurements per second, which makes it the most capable for capturing each pulse and glitch event. Advanced triggering, such as trigger holdoff, delays the re-arming of the trigger, which is useful when working with identification friend or foe (IFF) or other radar signals.
Peak power meters perform numerous manual or automated marker and automated pulse measurements. Automated pulse measurements provide measurement values of critical parameters that help characterize the performance of the power amplifier in the radar. Rise and fall time indicate the amplifier’s ability to output a pulsed RF signal with the necessary signal integrity. Overshoot pinpoints potential ringing problems. Droop shows the amplifier’s power supply limitations or the impact of thermal effects with prolonged pulse widths.
Pulse width, period, pulse repetition rate and duty cycle measurements provide other time domain characteristics of the signal. There are also several automated marker measurements that enable time-gated measurements. These measurements are performed between two markers and provide average, peak, minimum and maximum power readings, as well as peak-to-average ratio, a delta marker of power level, and delta time marker measurements.
A significant advantage power meters have over other measurement instruments is the size of the power sensor. It is small and light enough to be directly connected to the measurement port without the need for an RF cable that can degrade measurement accuracy because of impedance mismatches and cable loss, the latter increasing with frequency. Peak power meters such as those from Boonton for automated test environments can be remotely accessed via interfaces such as USB, LAN (TCP/IP), RS-232, and GPIB.
Test Set-ups
Although there are numerous pulsed RF amplifier test techniques, two types are considered here. The first test setup includes an amplifier in which the input is CW and the output is pulsed RF, in which a gating signal modulates the incoming signal to achieve the desired pulsed RF signal (Figure 4). A dual-channel meter measures the input signal power and the reflected power in the time domain for return loss calculation and monitoring anomalies of the reflected signal. A second dual channel power meter is used to measure the output of the amplifier to calculate parameters, such as gain, as well as monitor the reflected power of the load. The gating signal that modulates the RF input signal can also trigger the power meter that enables delay and latency measurements.
Boonton’s Model 4500C benchtop peak power analyzer has waveform math capabilities and can display gain and return loss in the time domain. The instrument is also equipped with two scope channels. When the gating signal to the amplifier triggers the peak power meter, the gating signal and the output of the amplifier can be displayed, enabling timing measurements as well as detecting if any amplifier anomalies are caused by the gating signal.
The RTP5000 Series power meters can measure and display up to eight channels on a single GUI window, and when using three or four USB sensors, amplifier input and output power, reflected input power, and reflected load power can all be measured and displayed on the same trace window, or via remote programming for automated testing.
The second type of test setup incorporates an amplifier in which the output is an amplified version of the pulsed RF input signal (Figure 5) with no gating signal supplied to the amplifier. The configuration is well suited for analyzing fully assembled amplifiers as well as subassemblies such as the driver stage or the final stage of the amplifier, or even a power transistor. The setup uses three USB peak power sensors and a directional coupler to make scalar-like gain and return loss measurements of the amplifier.
As the power rating of a typical sensor is about +20 dBm, the output of the power amplifier is attenuated to protect the power sensor while making the output power measurement. Before measurements can be taken, a thorough calibration procedure is required at the frequencies in which the amplifier is going to be tested to account for losses in the signal path. The losses that need to be calibrated out in the test setup and the calculations required to compute gain and return loss are provided below:
- L1: Loss from signal generator output to the forward port of the directional coupler
- L2: Loss from signal generator output to the power amplifier input
- L3: Loss from the amplifier output to the 40 dB attenuator output
- L4: Loss from amplifier input into the reverse port of the directional coupler.
Once the losses are measured, the input, output, and reflected power measurements can be made:
- P1: Power reading at the forward port of the directional coupler
- P2: Power measured at the 40 dB attenuator output
- P3: Power measured at the reverse port of the directional coupler
Power amplifier input power equals P1+L1-L2, output power equals P2+L3, and input reflected power equals P3+L4.
All Boonton peak power meters (and USB power sensors) are capable of adding an offset to the measurements, so the math above can be done by the meter once the losses are measured and entered into each channel as an offset.
The input, output, and reflected power measurements can be used to compute gain (S21) and input return loss (S11).
Amplifier gain in dB equals the amplifier’s output power—amplifier input power in dBm
Power amplifier input return loss in dB equals the input power in dBm minus input reflected power.
The 4500C can perform these measurements in the time domain using waveform math.
It is important to note that in both test setups the directional couplers must have a high level of directivity to make accurate power measurements, especially for return loss calculations. Unused ports of the couplers must be terminated with 50 ohms during the measurements.
Figure 6 shows three waveforms measured using the test setup in Figure 5 using the RTP5000 Series sensors. The input waveform is displayed in yellow (channel 1), the reflected waveform on channel 3 (purple), and the output on channel 2 (blue). The automated measurements performed on all three channels are displayed to the left of the trace display window and can be transferred to a spreadsheet to perform the necessary gain and return loss calculations as well as other parameters of interest. In an automated test environment, the same measurements can be accessed through remote programming to perform gain and return loss computations.
When evaluating GaN power devices and other advanced semiconductor devices, monitoring power droop across the pulse width is a critical metric as it can indicate the limitations of the thermal properties of the device and its package. Time domain peak power measurements can be taken at the output of the amplifier.
Droop measurement capabilities are shown in Figure 7. Power droop can be measured either using automated pulse measurements or using automated marker measurements as well as horizontal markers. The automated marker measurements can display the droop by placing markers at the desired points on the waveform. Reference lines can also be placed on the vertical axis at the desired high and low points of the pulse to measure the droop. Pulse measurements are computed automatically based on pulse definition, irrespective of marker or reference line placements.
Conclusion
Regardless of the technology used in a radar’s power amplifier, high-resolution precision time domain power measurements are critical to understanding amplifier performance and behavior. Consequently, peak power meters and USB peak power sensors are essential measurement tools for time-domain power analysis to test radar power amplifiers for R&D, quality, manufacturing, field support, and system calibration.
Boonton has been developing power meters for more than seven decades, and they have dramatically evolved to include more features, sensors that are connected by USB rather than a coaxial cable, much greater precision, the use of automation, and many others. Boonton peak power meters are used in a broad array of both commercial and defense radars, electronic warfare, and wireless communication systems.
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