Power measurements require care and precision. Some of the leading engineers involved with power measurements discussed their strategies at the most recent ARMMS RF and Microwave Society Meeting, held October 28-29, 2002 in the Tortworth Hotel (Tortworth, South Gloucestershire, England). ARMMS is a UK-based organization dedicated to the design and measurement of devices and products operating at RF and microwave frequencies. Sponsored by the National Physical Laboratory (NPL, Teddington, Middlesex, England), the two-day conference was aptly organized by NPL's David Adamson.
At ARMMS, K.P. Holland, J. Howes, and C. Purser of the NPL's Centre for Electromagnetic and Time Metrology described some of their techniques for improving attenuation measurements by minimizing signal leakage. In their attenuation measurement system, any signals reaching the detector other than the desired signals are considered leakage. A leakage signal 80 dB lower than the signal path can result in an error of 0.001 dB in an attenuation measurement. The system passes test signals through a gauge block attenuator and the device under test (DUT) before being downconverted to a lower frequency of either 10 or 50 kHz. These lower signals are then measured with a calibrated commercial AC digital voltmeter.
After exploring different approaches for detecting leakage, the presenters found that the most effective device for detecting leakage is a phase-sensitive detector (PSD), often referred to as a lock-in amplifier. They can be used as vector voltmeters to measure the amplitude and phase of signals having a coherent phase relative to a reference signal. The approach can reliably measure signals at levels of −150 to −170 dBm, but it requires a reference input. In this case, a 5-kHz reference was used—one-half the frequency of the 10-kHz intermediate frequency (IF). For measurements through microwave frequencies, the system has been able to detect levels as low as −170 dBm and, with a RF or microwave signal source of +20 dBm, the approach has shown a measurement range as wide as 190 dB for signals from 10 to 100 MHz.
Winner of the "Best Paper" award went to Nick Long of Great Circle Design (Somerset, England) for his exploration of using a vector network analyzer for oscillator design. His very practical presentation detailed some of the problems that could be "debugged" with his approach, including oscillators that will run but won't start, overtone crystal oscillators running on the wrong overtone (or fundamental frequency), oscillators that oscillate on more than one overtone simultaneously, spurious oscillations at frequencies well beyond the design frequency, and voltage-controlled oscillators (VCOs) that change behavior as they are tuned. His approach is based on performing S11 measurements on oscillator circuits and making use of Smith Chart plots to interpret the behavior of malfunctioning sources.
In other presentations, Alan Coster of Dowding & Mills Calibration compared different techniques for measuring RF voltage, along with techniques for calibrating an RF voltmeter. Guy Purchon of Anritsu Ltd. (Stevenage, Hertsfordshire, England) detailed accurate power measurements on modern communications systems, concentrating on root-mean-square (RMS) power measurements on code-division-multiple-access (CDMA) and quadrature-amplitude-modulation (QAM) signals.
A.D. Vare and R. Hopper from Roke Manor Research (Romsey, Hantsfordshire, England) explained how the 3GPP specifications for base stations impact the choice of power-amplifier (PA) and its devices, and how measurements can assist in making the choice.
Graham Pearson and Liam Devlin of Plextek Ltd. (Great Chesterford, Essex, England) described a dual-channel 2-to-18-GHz receiver front-end module for Electronic Support Measures (ESM) applications. The module converts signals, received by two external antennas, from anywhere in the 2-to-18-GHz range to an IF suitable for digitization. Frequency conversion is realized by upconverting to an intermediate frequency (IF) at around 22 GHz before filtering and then downconverting to the IF. The module contains 12 GaAs MMICs, a total of five designs, four of which are custom ICs designed by Plextek.
Chris Potter of P&H Technology Consultants (Cambridge, England) examined the predistortion linearization of a multicarrier WCDMA PA, using a predistorter application-specific integrated circuit (ASIC) from Intersil. He examined different biasing schemes (Class AB and Class C) for the best combination of linearity and efficiency.
The next ARMMS meeting is scheduled for April 7-8 at the Hotel Elizabeth (Corby, Northamptonshire, England). The Program Coordinator is Dominic FitzPatrick of Milmega (Ryde, Isle of Wight, England). Those interested in presenting a paper or attending can contact Dominic at: Milmega Ltd., Nicholson Rd., Ryde, Isle of Wight PO33 1BQ, England; (44) 1983-618007, FAX: (44) 1983-616864, e-mail: [email protected].