# A Broadband Analog Predistortion Linearizer Based on GaAs MMIC for Ka-Band TWTAs
## 4 Experimental Results  
The proposed APD MMIC was implemented using 0.15 \(\upmu\)m GaAs pHEMT technology. The size was 1.6 mm \(\times\) 2.0 mm, and the operating current was less than 10 mA. The chip is shown in Figure 7. The nonlinear performance of the APD MMIC and a TWTA cascaded with the APD MMIC were investigated. The measurement process was divided into two stages. During Stage 1, measurements were carried out on a wafer for the APD MMIC. A vector network analyzer and power supply were connected to the APD MMIC through a Cascade EPS150 RF probe station. In Stage 2, the APD MMIC was packaged in a Kovar casing and cascaded with a driver amplifier and an adjustable attenuator to drive a Ka-band 65 W TWTA. The output power at 1 dB gain compression (\(\rm P_{1 dB}\)) of the driver amplifier was 18 dBm over the frequency range of 25.1-27.5 GHz, which is far greater than the saturated input power of TWTA. The gain flatness of the driver amplifier and the adjustable attenuator was lower than 1.0 dB in the operating band, which ensures that the TWTA can be pushed to the saturated output power state without affecting the nonlinearity and gain flatness of the system when they are connected to the system. In \"Setup (1)\" of Figure 8, the vector network analyzer ZVA40 was used to perform the gain and phase compression tests of the TWTA and linearized traveling wave tube power amplifier (LTWTA). \"Setup (2)\" shows the measurement process of the third-order intermodulation signal (C/IM3)  
Figure 6: (**a**) Phase expansion difference with \(\rm C_{j}\). (**b**) Gain and phase expansion after adding NFAM to APD MMIC. and the Noise Power Ratio (NPR). The vector signal source SMW200A was employed to provide a dual-tone signal and a broadband noise signal. The signal analyzer FSU was used to perform the C/IM3 and NPR tests of the TWTA and LTWTA. Before testing, a SOLT and power calibration procedure were carried out to remove the influence of the connecting lines. To verify the nonlinear characteristics of the APD MMIC, it was necessary to test its gain and phase expansion at the operating band of 25.1-27.5 GHz in Stage 1. The test results are shown in Figure 9. The gain and phase under small-signal excitation are normalized to 0 dB 0\({}^{\circ}\), respectively. When \(V_{\rm g}\) = 1.1 V and \(V_{\rm p}\) = 3.4 V, a gain and phase expansion of 4.5 dB and 40\({}^{\circ}\), respectively, were realized at the center frequency of 26.3 GHz. The gain expansion of the APD MMIC was 4 dB at the lowest frequency of 25.1 GHz and 5 dB at the highest frequency of 27.5 GHz over the whole operating band, i.e., decreasing as the frequency increased. The phase expansion was 35\({}^{\circ}\) at the lowest frequency and 45\({}^{\circ}\) at the highest frequency, increasing over the range. The phase expansion difference with frequency was consistent with the phase compression change in the TWTA over the whole operating band and is therefore very suitable for compensating broadband phase expansion  
Figure 8: The test platform for APD MMIC and TWTA cascade with proposed APD MMIC. Figure 7: APD MMIC Photo. characteristics of the TWTA. The gain under saturation and small-signal excitation are shown in Figure 9b, which shows that the chip has good gain flatness over the operating frequency band. By comparing the simulated and measured data of MMIC gain under saturation and a small-signal excitation, it was found that the simulated and measured data are in good agreement with an error of less than 5%. During the Stage 1 experiments, the independent tuning of gain and phase conversion was also verified at 26.3 GHz. The test results are shown in Figure 10. For a clear comparison, the gain and phase under large signal excitation are normalized to 0 dB and 0\({}^{\circ}\), respectively.