How does that Work?

Multicarrier power amplifiers are critical components of wireless-communications systems and must limit distortion levels to no more than –80 dBc to meet W-CDMA, cdma2000, and other next-generation specifications (Figure 1). Consequently, a test system for 3G amplifiers must accurately measure a DUT's intermodulation distortion (IMD) and adjacent-channel power ratio (ACPR).

Ideally, a test system such as the one in Figure 2 would be adequate for making these measurements. A signal source would provide a pure, in-channel test signal to the DUT, which in turn would drive a test load. A directional coupler would divert a calibrated portion of the DUT output to a spectrum analyzer, which would then determine the in-channel as well as the adjacent-channel power the DUT produces.

Figure 1

Figure 2

Figure 3

The N4256A employs a two-path signal-cancellation scheme (Figure 3) to effectively increase the dynamic range of an existing test system. In the two-path scheme, a splitter routs a portion of a source signal to the DUT and the remaining portion to a phase shifter. In part one of a two-part measurement process, the phase-shifter path is disabled, allowing measurement of a DUT reference power-output level. In part two, the phase-shifter path is enabled, delivering a 180° out-of-phase version of the input signal to a combiner. The combination process cancels out distortion due to the original test signal so any remaining distortion transmitted to the spectrum analyzer is distortion generated by the DUT. In addition, the operation of the combiner cancels out 25 dB of the DUT output power, effectively lowering the noise floor of the measurement system.

"How does that work?" is a bimonthly column covering test or measurement products that take advantage of a novel effect or unusual technology.