# Hardware Design of a High Dynamic Range Radio Frequency (RF) Harmonic Measurement System
## 4 System Hardware Design  
Using the information from the previous sections, a higher power, high dynamic range harmonic measurement system was constructed. The measurement system was developed to measure second harmonic, \(n=2\), responses from weakly nonlinear targets. The system also collects data on the linear, fundamental (\(n=1\)) responses from DUTs. The system was designed to measure both the pass through and reflected, linear and second harmonic response. This allows for full characterization of devices. The linear frequency range spans 800 to 1000 MHz and the second harmonic frequency range is 1600 to 2000 MHz. A block diagram of the measurement system is shown in Figure 8. Note that the second diplexer is flipped to give the harmonics traveling in the reverse direction a path to a 50-\(\Omega\) termination. Wherever possible, all inputs and outputs are terminated in 50 \(\Omega\) at both the fundamental frequency and second harmonic. A NI PXI-5651 signal generator is used to create the probe signal. Next, Mini-Circuits RBF-272 diplexers are used in a low pass configurations to linearize the probe signal, filter out any second harmonic generated by the signal generator. To boost the power of the probe signal, a ZHL-30W-252-S+ Mini-Circuits power amplifier (PA) is 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 filtered. Since the fundamental frequency power is at 10 W, high power custom diplexers, manufactured by Reactel, are 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. Section 3.2 demonstrated the need for a highly linear probe signal; therefore, two such diplexers are cascaded to attenuate the self-generated harmonics below the noise floor. 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 are presented in Figure 9. The data in Figure 9 show the diplexers' ability to pass the fundamental frequencies, from 800 to 1000 MHz, and attenuate the second harmonics, from 1600 to 2000 MHz. Figure 9a shows that 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 and the results are shown in Figure 9b. 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. Figure 8: Block diagram of the linear/nonlinear measurement system with specific test locations marked. A measured link-budget for a 900-MHz tone, used as the probe signal, is presented in Table 2. Note from Table 2 that going from D to E for the second harmonic shows a loss of 113.4 dB. However, according to Figure 9a, the loss is seen to be around 95 dB. This discrepancy is probably due to the fact that the \(|S_{21}|\) measurements are made on a network analyzer and the results in Figure 9a are likely limited by the network analyzer's noise floor or the internal coupling. Photographs of the NI chassis and the RF components (Mini-Circuits and Reactel diplexers and power amplifier) are shown in Figures 10 and 11, respectively. The receiver hardware is straightforward. The probe signal, at the fundamental frequency, is separated from the second harmonic using another high power diplexer. The fundamental frequency passing through the DUT is measured at port 3 in Figure 11. The output second harmonic is measured at port 4.