Features

A National Instruments (NI) PXI-5651 signal generator was used to create the probe signal. Next, Mini-Circuits RBF-272 diplexers were used in a low-pass configurations to linearize the probe signal and filter out any second harmonic generated by the signal generator. To boost the power of the probe signal, a Mini-Circuits power amplifier (PA) was used. The probe signal is amplified to 10 W (+40 dBm). The power amplifier is a nonlinear device and generates significant harmonics that need to be filtered. The NI chassis and RF components are shown in Figure 2.

Figure 2. The Nl chassis with computer, signal generator, and spectrum analyzer.

Since the fundamental frequency power is at 10 W, high-power custom diplexers from Reactel were used. The high-power diplexers are rated up to 100 W continuous wave input power. They are cavity diplexers that provide greater than 80 dB of rejection in the stop band. The passband attenuation is less than 0.4 dB. In addition to the diplexers, an isolator is used to isolate the power amplifier output from the DUT and avoid mismatches. The insertion loss of the isolator is 0.2 dB; thus, the power delivered to the DUT was approximately +39 dBm.

Test results from a single diplexer showed the diplexers’ ability to pass the fundamental frequencies, from 800 to 1000 MHz, and attenuate the second harmonics, from 1600 to 2000 MHz.

One cavity diplexer attenuates all harmonics, from 1600 to 2000 MHz, by at least 80 dB with less than 0.4 dB attenuation at the fundamental frequencies, from 800 to 1000 MHz.

The high-frequency path of the diplexer was also tested. Again, the diplexer attenuates the unwanted fundamental frequencies (here it is from 800 to 1000 MHz) by more than 80 dB and it passes the desired harmonic frequencies, 1600 to 2000 MHz, with less than 0.4 dB of loss. Since the diplexers have very little loss in the pass band, they do not absorb a significant amount of pass band energy.

The receiver hardware is straightforward. The probe signal, at the fundamental frequency, is separated from the second harmonic using another high-power diplexer. The spectrum analyzer has an 80-dBc dynamic range; coupling this with the 80-dB of loss provided by the Reactel diplexer yields a system dynamic range of over 200-dBc; however, the theoretical 200-dBc dynamic range is unachievable, as in practice, the system would be noise-limited before 200-dBc can be achieved. The noise floor of the receiver is measured to be less than 135 dBm. With the probe signal measured to be greater than 40 dBm and the noise floor below 135 dBm, the system's dynamic range is estimated to be greater than 175 dB.

System Test Results

Figure 3. Devices tested for second harmonic characterization.

The high-DR measurement system was used to measure the second harmonic response from a variety of circuit elements. The passive devices tested are shown in Figure 3. The two-port devices were tested for their input and output harmonic generation while the one-port devices could only be tested for their reflected harmonic generation.

Since the spectrum analyzer has one input port, the fundamental and second harmonic at the output of the DUT must be measured via two separate measurements. Therefore, the loading conditions of the two measurements are slightly different. The spectrum analyzer is matched to 50 W throughout the bands of interest with an input reflection coefficient of less than 20 dB, which provides a 99% power transfer. The differences between the loading conditions for the fundamental and second harmonic measurements is therefore expected to be negligible compared to using a 50 W termination.

Conclusions

As the frequency spectrum gets more crowded and the demand for wireless communication increases, nonlinear distortions generated by passive elements become more relevant. Commercially available nonlinear measurement systems are expensive and either lack the dynamic range needed to make the sensitive measurements, or are fixed in frequency and do not provide the flexibility required. Robust feed-forward systems have been constructed that provide both the flexibility and dynamic range needed to make the sensitive measurements, but these systems are complex and require iterative tuning and optimization algorithms.

The alternate method for characterizing nonlinear distortion from weakly nonlinear devices uses absorptive diplexers to separate the harmonic response from the high-power probe signal, thus increasing the system's dynamic range. The method is also capable of measuring the reflected harmonic from devices. The method is relatively simple to implement and it is cost-effective for measuring weak nonlinear responses of circuit elements.

The method demonstrated the capability of achieving 175-dBc dynamic range over a 22% bandwidth. The system is capable of producing over 10W (+40 dBm) of probe signal power while measuring second harmonics generated by a DUT as low as 135 dBm, resulting in the 175 dBc dynamic range. These passive RF circuit elements are not traditionally thought to exhibit nonlinearities, which require a high-DR nonlinear measurement system.

This work was done by Ram M. Narayanan of Penn State University; Kyle A. Gallagher, Anthony F. Martone, and Kelly D. Sherbondy of the US Army Research Laboratory Sensors Directorate; and Gregory J. Maz-zaro of The Citadel. For more information, visit the Army Research Lab Sensors Directorate here.