If a signal generator can deliver a pure, accurate signal at a sufficient power level, the instrument ensures improved measurement accuracy. It also enables testing with greater dynamic range and at extreme or unusual operating conditions. In a five-page application note titled, "Generating and Applying High-Power Output Signals," Agilent Technologies explains how its E8257D PSG analog signal generator can deliver capabilities that include at least +25 dBm output power across its operating range and +30 dBm or more power across much of the operating range, thanks to an ultra-high output-power option.
With the 10-MHz-to-20-GHz Option 521, the PSG promises to ensure measurement accuracy by addressing spectral purity, impedance matching, and automatic level control (ALC). That option enables the signal generator to produce typical output power above +30 dBm to 14 GHz and greater than +27 dBm output power to 20 GHz. To reduce the level of harmonics, Option 521 also includes selectable harmonic filters for carrier signals below 2 GHz. This filtering approach does lower the maximum output level, however.
Despite the fact that this application note focuses on a specific product and option, it offers a lot of solid advice for engineers performing RF testing. When higher output power is the main goal, for example, harmonic filters should be placed at the input of the high-power amplifier stage. Yet the level of the harmonic components will be higher than it would be if the filtering was performed closer to the power amplifier's (PA's) output. Adding a high-power amplifier to the signal generator also affects the instrument's broadband noise floor. Several decibels of additional phase noise will be seen at offsets greater than 1 MHz from the carrier frequency. Spurious response also will be affected.
Often, matching issues arise because of the structure of the combiner network for the multiple amplifiers that comprise a PA. These problems may be handled by using parallel RC circuits at the PA's input and output to create closer impedance matches. Alternatively, the engineer can amend the combiner network with additional segments that accommodate the long wavelengths of lower frequencies. The note next delves into the optimization of level accuracy via static and dynamic leveling. It ends with examples that illustrate how the E8257D PSG with Option 521 can help to simplify the testing of high-power amplifiers, overcome losses with automated-test-equipment
(ATE) systems, and address the attenuation of signals with long cable runs. agilent technologies, inc., 5301 stevens Creek Blvd., santa Clara, Ca 95051; (408) 345-8886, Fax: (408) 345-8474, internet: www.agilent.com.