Measure power in complex RF signals

June 16, 2021 Editor

Average and peak power measurement require that you use a broadband or narrowband configuration.

Power level is an important parameter of an RF transmitter. In wireless communications, we might just think more power is better but modern wireless systems require accurate control of power levels to avoid unnecessary interference with other nodes in the system and to maximize battery life. We need to know a transmitter’s power level and accurately control it.

The fundamental unit of power is the Watt, corresponding to the rate of energy transfer, with 1 W equal to one joule per second. Power measurements are often expressed using decibels with dBW being referenced to 1 Watt and dBm being referenced to 1 mW.

CW Measurement
The most basic power measurement is determining the power present in a steady continuous wave (CW) signal. Figure 1 shows how a classic RF power meter is configured. The key component is the power sensor which produces a DC voltage proportional to the power present in the signal. There are a wide variety of sensor technologies used for power measurement, including thermistors, thermocouples, and diode detectors. Instrument vendors usually offer a choice of power sensors, varying in frequency range, sensitivity, dynamic range, and cost. The output of the sensor is usually just a DC voltage, measured by a voltmeter circuit and presented to the user on a display. (See page 2 for a sampling of RF power meters and sensors.) The processor shown in Figure 1 can perform measurement corrections and analysis of the power readings.

Figure 1. A classic RF power meter measures the power in a CW RF signal.

Fig. 1 is drawn generically, without specifying how the power meter is configured mechanically. The classic RF power meter is a small bench instrument that accepts different types of power sensors and displays the measured results. Today, we also have compact power sensors with USB connectors that connect to a computer, which acts as our processing and display device. Power sensors can also attach to larger test equipment such as spectrum analyzers or network analyzers to enhance their measurement capability.

Some power sensors measure CW signals only but most can measure more complex signals. Modulated waveforms of all types are important in wireless communications and most power sensors will accurately measure the average power of these signals. Note that these measurements are inherently broadband, so everything present in the signal (carrier, harmonics, modulation, noise) will be included in the measurement.

Pulsed RF measurement
A simple RF power measurement need not be particularly fast if the signal is stable and we just want to know the average power level. For more complex signals, however, we might also be interested in the dynamic power characteristics. Figure 2 shows the measurement of a pulsed RF signal. In this case, the power sensor must respond quickly enough to track the signal power. The output of the sensor is p(t), a voltage proportional to the average power in the signal, as a function of time. This signal is captured by a high-speed analog-to-digital converter (ADC), manipulated by the processor, and displayed by the meter. Fig. 2 is also drawn generically, and the processor and display functions may be done by a dedicated instrument or by a computer.

Figure 2. A peak power meter can measure power in a pulsed pulsed RF signal.

There is a limit to how fast the power meter can track pulsed RF signals. This is normally described as the video bandwidth of the meter, which refers to the bandwidth of the system after the sensor. The meter may also specify the minimum rise and fall time of the measurement.

With p(t) captured, we can treat it like a pulsed waveform and extract waveform parameters from it. Fig. 2 shows the pulsed waveform with nice clean edges, but a more realistic view with finite rise/fall times and overshoot is shown in Figure 3. Standard waveform characteristics can be computed from the power waveform, similar to high-speed digital measurements.

Figure 3. This plot of instantaneous power shows various parameters of the signal power. (Graphic: Keysight Technologies)

The peak-to-average power ratio (PAPR) can be calculated from the waveform’s peak and average power. See How to calculate RF power amplifier efficiency. The pulse repetition interval (PRI), also called pulse repetition time (PRT), is the time between pulses. The pulse repetition frequency (PRF), or pulse repetition rate (PRR) is the reciprocal of the PRI.

Peak power meters can capture the power in the signal at a particular point in time. This usually involves a triggering system so that the measurement time is controllable and repeatable.

Broadband vs. Narrowband
The power meters shown in Figs. 1 and 2 are broadband devices that measure the power across the entire frequency range of the sensor. The system in Fig. 2 can evaluate the signal at a particular point in time and can compute various waveform parameters. The power sensor is, however, always reading the power across a wide frequency spectrum.

Figure 4 shows the frequency domain representation of an RF signal that has multiple frequencies and noise present. A broadband power sensor will measure all the power in the signal, across all frequencies, including harmonics, modulation sidebands, spurious responses and noise.

Figure 4. Power measurements can be either broadband or narrowband, depending on the type of meter and application.

In contrast, a narrowband power measurement zooms in on one particular frequency band to measure the power within that band. As shown in Fig. 4, we can use a narrowband measurement to capture the power in just one spectral line, such as the carrier frequency. This type of measurement will ignore the other frequencies present at the power sensor input.

Figure 5 shows the block diagram of a frequency selective power sensor [Ref. 4]. This measurement device is similar to a tuned receiver used in a spectrum analyzer. The input signal is mixed with a local oscillator to produce a lower frequency signal which is sampled and digitally processed to achieve the desired measurement. You can select the frequency and bandwidth of the measurement to make any desired narrowband measurement (within the limits of the power sensor).

Figure 5. Block diagram of the R&S NRQ6 Frequency Selective Power Sensor (Graphic: Rohde and Schwarz)

Narrowing the frequency range improves an instrument’s sensitivity. Broadband noise is rejected, and the effective noise floor of the measurement drops dramatically. Narrowband capability also supports measurements that are frequency dependent. For example, adjacent channel leakage ratio (ACLR), is an important and common wireless measurement. ACLR describes how much of the desired channel power “leaks” into the adjacent channels. Using narrowband measurements, the instrument must capture the power in the adjacent channels and compare it to the power in the desired channel. ACLR is often measured using a spectrum analyzer, but it can also be measured with a frequency-selective power meter.

So which power measurement approach is better: broadband or narrowband? It depends on the parameters you are trying to measure. If you want to capture the complete power in a signal, use a broadband measurement. If you want to selectively measure specific frequencies, choose a narrowband measuring device. Some instruments provide both broadband (using conventional power sensors) and narrowband power measurements (using a spectrum analyzer type receiver) [Ref 5].

Conclusion
At first glance, RF power measurements seem simple: just hook up the power meter and sensor to the signal being tested and read the result. In reality, we need to be concerned with the specifications of the power sensor and meter. This is especially true if we are measuring peak power and other dynamic characteristics. The introduction of narrowband power sensors provides even greater measurement flexibility, while blurring the line between what is consider a RF power meter measurement and a spectrum analyzer measurement.

References
1. “Principles of Power Measurement,” Boonton, Wireless Telecom Group, 2015, https://boonton.com/resource-library/principles-of-power-measurement-guide
2. “Fundamentals of RF and Microwave Power Measurements (Part 1),” Application Note AN 1449-1, Keysight Technologies, Publication Number 5988-9213EN, 2017.
https://www.keysight.com/us/en/assets/7018-01150/application-notes/5988-9213.pdf.
3. “Fundamentals of RF and Microwave Power Measurements (Part 2),” Application Note AN 1449-2, Keysight Technologies, Publication Number 5988-9213EN, 2014.
https://www.keysight.com/us/en/assets/7018-01151/application-notes/5988-9214.pdf.
4. “R&S NRQ6 Frequency Selective Power Sensor,” Rohde&Schwarz, Product Brochure, 2020.
https://www.rohde-schwarz.com/us/product/nrq6-productstartpage_63493-533642.html.
5. “Techniques for Precise Power Measurements in the Field,” Application Note, Keysight Technologies, Publication Number 5991-0423EN, 2015.
https://www.keysight.com/us/en/assets/7018-03481/application-notes/5991-0423.pdf.

See page 2 for a sampling of RF power meters and sensors

Bob Witte is President of Signal Blue LLC, a technology consulting company. Bob has held many positions in R&D, technology planning, strategic planning, and manufacturing for Keysight Technologies, Agilent Technologies and Hewlett-Packard Company. Inside, he is just an engineer that loves to see innovative products solve real customer problems. Bob is the author of two books on test and measurement instrumentation: Electronic Test Instruments (2nd Edition) and Spectrum and Network Measurements (2nd Edition).