Signal analyzers are the instruments of choice for characterizing and measuring today's complex digitally modulated signals. Many required measurements relate to the intermodulation performance of the device or system under test. These measurements can be masked by intermodulation products generated within the signal analyzer. Design engineers must be aware of the power levels presented to their signal analyzer and its third-order intercept (TOI) performance. Traditional signal analyzers have often lacked the TOI performance to accurately characterize emerging wireless designs.

By implementing an architecture that eliminates the first IF amplifier, a new signal analyzer from Anritsu delivers +23 dBm (+27 dBm typical) TOI over a 100 MHz to 8 GHz frequency range. This is more than a 3 dB improvement over traditional analyzers.

The RF front end (see Figure) is the key design element of the signal analyzer. The decision was made to extend the upconverted frequency range from 100 Hz to 8 GHz to eliminate the distortion and tracking problems associated with a YIG preselector. This is a departure from traditional signal analyzers, which generally use a combination of upconverters and tracked preselectors to cover the 100 Hz to 8 GHz band. A further decision was made to eliminate any amplification in the 9.55865 GHz first IF to realize high TOI performance.

To achieve high TOI, the 9.55865 GHz to 17.55865 GHz synthesized first local oscillator is amplified to +27 dBm and applied to the LO port of the first mixer, which was designed by Anritsu. The first mixer is a key element to the overall high TOI performance of the signal analyzer. If the first mixer degrades the TOI performance too much, the remaining circuitry in the overall signal path cannot recover the signal integrity. In a similar fashion, the second mixer is driven with the second local oscillator at 8.43 GHz at a level of +25 dBm. The 9.55865 GHz first IF signal is converted down to 1128.65 MHz and applied to the analog IF. Another major consideration that had to be addressed was the high temperature created in the mixers by the high LO powers. For that, the company developed custom beam lead diodes for an 8-diode ring configuration to provide a good RF match and dissipate the heat generated from the high LO drive.

The first time the input signal is amplified is in the 1128.65 MHz second IF. It is then filtered and applied to the third mixer. A high dynamic range FET mixer serves as the third downconverter and mixes the 1053.75 MHz third local oscillator with the 1128.65 MHz second IF signal. The third IF, at 74.9 MHz, is filtered and amplified before it is applied to the fourth FET mixer. The fourth local oscillator, at 85.6 MHz, mixes with the 74.9 MHz third IF signal to generate the final 10.7 MHz IF frequency. Variable gain and variable pre filters are set to optimizie sensitivity and minimize distortion in the digital IF as a function of the instrument state.The 10.7 MHz signal is digitized and sent to the digital IF section for all the resolution bandwidth filtering. Optionally, a 30 MHz bandwidth digital IF path is available at the third IF to demodulate and measure wider bandwidth signals.

Recognizing that active devices are the weak links to strong TOI measurement capability, the architecture eliminates amplifiers and uses mixers with high-frequency spans to generate a cleaner signal. Devices with complementary high performance and an architecture that takes advantage of this performance create a signal analyzer with an industry-best +23 dBm TOI, which is necessary for accurate analysis of high-powered wireless signals.

Roy Keeney is a project manager and Eiji Mori is a senior microwave design engineer for Anritsu Company, Morgan Hill, Calif. For more information on Signature, visit www.us.anritsu.com.