RF Power Meter Shows Greatest Signal Detail
Open as PDF
By Wolfgang Damm
Wireless Telecom Group
Abstract
Boonton’s 4540 Series RF Power / Voltage meters provides highly detailed waveform traces for accurate measurements, efficient alignment and detailed analysis of linear and pulsed RF components and circuitry.
Modern communication technologies such as LTE and WiMAX require test and measurement systems that are able to handle complex modulation schemes and high dynamic signal levels. Pulsed RF applications like radar and MRI are also challenging, typically transmitting relatively high power signals but receiving weak signals that face significant ambient noise.
Power measurements are vital for any kind of RF transmission system: too much power and the signal gets distorted, too little power and the signal is submerged in the noise and the information is lost. Spectrum analyzers (SA), vector network analyzers (VNA), and RF power meters all offer power measurement capabilities. Power meters, however, are by far the most accurate way to perform RF power measurements. Their wideband sensors detect all power signals at the measured ports of the DUT in the desired frequency band and in unwanted bands, sacrificing transmission energy and causing interference. Determining sensitivity, maximum output power, or linearity of an RF component are just a few of many parameters commonly analyzed, and fast power meters are the ideal instruments for this.
For pulsed RF signals, critical analysis examines parameters like peak power, average power, rise time, fall time, overshoot, and undershoot (see figure 1). To achieve this, power sensors must be fast, provide a wide bandwidth and must offer a high dynamic range. But the sensors must be matched with an appropriate power meter to produce valuable data. Peak power sensors convert the incoming RF signals into baseband frequencies. Power meters not only process the continuous flow of measurement data, but they need to compile the data and display the signal waveform at their screen in a way that fully matches the actual RF signal.
Power meters with high sampling rate capture signal points in very small increments, which are then used to (re-)build the signal waveform on the screen for analysis. Boonton 4540 power meters not only utilizes fast sampling on the measured signal, but offer an additional feature: sampling performed at random intervals. This sampling technique operates independent of the instrument’s time base and has huge advantages whenever repeating signals are measured, which constitute the vast majority of all signals measured in RF applications. The asynchronous sampling technique is also known as repetitive random sampling or RRS for short. RRS sampling provides a detail depth that reveals much more waveform information than conventional sampling.
Figure 1. RF pulse analysis requires measuring and analyzing a great variety of specific parameters.
Screenshots in figure 2 show three consecutive measurements of a fast signal transition using two power meters. The upper row illustrates a power meter with conventional sampling technique; the lower row illustrates a Boonton 4540 power meter with RRS technology. Both power meters measure the same signal. As one would expect, both power meters are able to measure the fast signal transitions, but using RRS technology Boonton’s 4540 power meter has an effective time resolution of 200ps. This offers two huge advantages: first, it generates significantly more detail in the shape of the displayed waveform, allowing the user to quickly pinpoint possible issues in the RF components.
Second, the very high time resolution in combination with wide bandwidth sensors, offers a more stable display of the waveform, making signal waveform analysis much more efficient.
User experiences show that Boonton 4540 power meters frequently detect many circuitry issues that users were unable to detect with other instruments. With its fine resolution, the 4540 also allows them to calibrate their RF systems in a way that increases efficiency.
Figure 2. Three consecutive measurements of fast signal transitions measured with a conventional RF power meter (top) and with a Boonton 4540 power meter that uses RRS technology (bottom).