# A Broadband Analog Predistortion Linearizer Based on GaAs MMIC for Ka-Band TWTAs
## 4 Experimental Results
As the bias voltage \(V_{\mathrm{g}}\) of the gain nonlinear gradually increased from 0.4 V to 2.0 V, while the bias voltage \(V_{\mathrm{p}}\) of the phase branch remained at 3.4 V, the phase expansion remained unchanged at 40\({}^{\circ}\) and the gain expansion increased from 2 dB to 13 dB. When the bias voltage \(V_{\mathrm{p}}\) of the phase branch gradually increased from 2.2 V to 3.8 V with the bias voltage \(V_{\mathrm{g}}\) of the gain branch steady at 1.1 V, the gain expansion remained unchanged at 5.0 dB while the phase expansion increased from 25\({}^{\circ}\) to 40\({}^{\circ}\). In these experiments, the gain (phase) expansion adjustment was realized through a decrease in the small-signal gain (phase) but the saturated gain (phase) change was very small. The measurement results were consistent with the simulation results. It can be seen that the gain and phase expansions can be independently adjusted by adjusting the bias voltages \(V_{\mathrm{g}}\) and \(V_{\mathrm{p}}\). During Stage 2, the APD MMIC was packaged into a Kovar casing to form a linear module. The nonlinear properties of the TWTA before and after the linear module such as gain compression and phase compression, C/IM3, and NPR were verified. It can be seen from Figure 11 that the gain compression of the TWTA cascaded with the linear module was reduced from 50\({}^{\circ}\) to within 12\({}^{\circ}\) over the whole operating band, while the gain compression was reduced from 8 dB to less than 3 dB, which means that the linear module compensates the nonlinearity of the TWTA very well. The C/IM3 metric is often used to measure LTWTA nonlinearity under narrowband signal excitation. It is necessary to check the factors that affect C/IM3, such as the power clutter, the bottom noise of the spectrum analyzer FSU, and the P1 dB of the drive amplifier before the C/IM3 test. During the test, the amplitude of the spectrometer needs to be set reasonably to ensure that the test data truly reflect the C/IM3 value of the DUT. The frequency interval of the dual-tone signals used in the C/IM3 test was 5 MHz. The measurement results are shown in Figure 12. Due to the adoption of the linear module, the C/IM3 of the LTWTA reached 28 dBc at the 3 dB output power backoff (OPBO) over the whole working band. At 4 dB OPBO, the C/IM3 of the LTWTA was larger than 34 dBc. Figure 9: Measured results of (**a**) gain and phase expansion and (**b**) gain under saturation and small signal. The output power fallback of the LTWTA under the same C/IM3 value of 28 dBc was 5 dB lower when compared with that of the TWTA. That is to say, the LTWTA can operate at a higher power state and its efficiency is also higher. When the output power fell back 3 dB over the whole operating band 25.1-27.5 GHz, the minimum and maximum C/IM3 values were 28 dBc and 33 dBc. The difference was not more than 5 dB, which indicates that the linear module has good broadband adaptability. The NPR index is used to measure the nonlinearity of LTWTAs under broadband signal excitation [15]. The noise signal was generated using the NPR software of Rohde & Schwarz, and the noise width can be set from 50 MHz to 500 MHz. The noise gap bandwidth was 5% of the signal bandwidth, and the gap depth was greater than 45 dB. During the NPR test, the four frequency points of 25.4 GHz, 26 GHz, 26.5 GHz, and 27.1 GHz were used as the center frequencies of the noise signals. The noise signal bandwidth ranged from 100 MHz to 500 MHz to observe the impact of the noise bandwidth change on the NPR index when the output power of the LTWTA fell back to 3 dB. The test results are shown in Figure 13. As the noise bandwidth changed, the NPR remained between 15.8 dBc and 17.1 dBc. Since the random signal adopted by the NPR test added an error  
Figure 11: TWTA gain and phase conversion before and after the proposed APD MMIC circuit. Figure 6:0 Measured results of independent tuning of (**a**) gain conversion and (**b**) phase conversion.