This page contains a Flash digital edition of a book.
Defence & DSEi Special

RF amplifier testing just got easier

Greater accuracy, simple set-up and efficient test automation will make RF amplifier testing vastly more effective in a range of advanced communications applications, as Howard Venning explains to CIE


hen it comes to high reliability applications such as radar, and advanced wireless and satellite communications, accurate and high quality test instrumentation is essential. The roll out of new high performance digital wireless systems has been rapid, and we are increasingly reliant on their reliable and continuous operation, as much today in commercial applications, as we have always been in the traditional hi-rel medical, military, avionics and space sectors.

The specification of high quality RF components, such as RF amplifiers, is a fundamental start point. And for that, the design engineers need the right tools for thorough testing and certification of these devices.

A very recent addition to Aspen

Electronic’s portfolio is an Amplifier Test Bench manufactured by Boonton, part of the Wireless Telecom Group. It is a powerful software tool specifically designed for efficient and accurate, test verification and analysis of high power or small signal RF amplifiers. The software can be run with both Boonton’s fast peak power meters and is supported on a variety of standard, third party signal and pulse generators. The software provides a very easy and convenient method of automating test set- ups for different types of amplifiers and its combination with a peak power meter provides a significantly more accurate test system than conventional test approaches. This is primarily because the Amplifier Test Bench performs actual pulse power measurements that guarantee detailed and accurate amplifier analysis. Amplifier Test Bench supports the testing and analysis of pulsed, continuous or wideband digital (noise like) RF amplifier modulation schemes. In operation, it measures input and output signals simultaneously, and can be set up to measure gain over frequency as well as gain over input power and combinations thereof. Supported frequencies are sensor dependent and range from 10MHz to 40GHz.

Long term stability checks Boonton’s software suite has an additional capability to measure VSWR, return loss

Amplifier Test Bench Typical test set-up for high power, gated pulse amplifier

over time and, with the addition of a directional bridge, return loss over frequency. The VSWR measurement feature makes the Amplifier Test Bench a powerful tool for measuring the long term stability of RF systems. The software-enabled automated

process allows the test system to take multiple measurements over an extended time period of time, and with varying VSWR. As a result, the development engineer is able to fully characterise amplifier design vs temperature, time or other parameters that may affect amplifier performance.

The Boonton Amplifier Test Bench can be adapted to measure the performance of passive components, measuring variations in insertion loss and VSWR against time and, for example, temperature. Analysing the quality and wear of rotary joints, such as used on rotating radar systems, is just one example, monitoring subtle changes in insertion loss due to rotation. For this application, the shaft position pulse provides the 0 degree starting point. This triggers a repeating measurement cycle within Amplifier Test Bench. Time is correlated to angle and based on that, the software plots return loss over angle.

Accurate pulsed measurements It is not unknown for engineers to find accuracy problems when measuring pulsed amplifiers using average power measurement equipment, such as RF detectors or average power sensors. Average measurements require calculating average pulse power based on the duty cycle, which can result in a large variation between the calculated and the actual amplifier response. In addition, to calculate

18 July/August 2011 Components in Electronics

the actual average pulse power, the average measurement is multiplied by the inverse duty cycle. Depending on the duty cycle ratio, the measured average power may be too close to the noise floor of the RF sensor, which can again result in a significant inaccuracy.

Duty cycles of 0.1% (1:1000) or 0.01% (1:10,000) are common in radar technology and other applications that require pulsed signals. A duty cycle of 0.1% reduces the usable dynamic range of any average sensor by 30dB, a duty cycle of 0.01% can reduce it by as much as 40dB. The remaining dynamic range of average sensors might not be sufficient for meaningful measurements.

In addition, the digitizer quantization

error of the reduced dynamic measurement data reduces the accuracy of the measurement yet further. It is unwise to assume that pulses have a perfectly rectangular shape without overshoot, undershoot or ringing. Rising and falling edges often show characteristics that depend on system factors, including performance of the system power supply. The solution is to use high dynamic range and fast peak power measurement equipment. The ability to measure input and output pulses simultaneously is critical, together with high dynamic range to resolve all signal details. Only peak power sensors can measure the actual signal form and provide the most accurate information about true pulse power.

For RF component designers in development mode, the Amplifier Test Bench can be used to test signal sources or transmitters over time, and with a variety of stimuli to simulate a range of potential operating conditions.

Easy set-up

The instrument setting of Amplifier Test Bench is easy. Basic parameters like start, stop frequency and power for the signal generator or pulse period and pulse width for the pulse generator are simply entered via the user’s display screen. Depending on what power meters are used, additional information settings for horizontal and vertical display scale and offset can be entered as well. For analysis of particular portions of a pulse waveform, the software permits the user to position time markers on the pulse to define the measurement interval. The operator specifies whether to run the sequence just one time or multiple times. This can be useful for long term testing or temperature related applications. In operation, the display screen can be

configured to present the measurement data in both tabular and graphics formats. The software can plot frequency vs power with only one or with multiple traces. Precise information of every single measurement point is available in the results table, allowing for very detailed signal analysis.

Another mandatory aspect of characterisation and certification, is the production of quality documentation, which is necessary to prove amplifier performance. The Amplifier Test Bench allows the immediate creation of plots by exporting data to an Excel template, supplied as part of the package. DUT relevant information like the serial number is automatically added to the plot.

Aspen Electronics | Howard Venning is managing director of Aspen Electronics

Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52