Linearized Microwave Power Modules for Communications

JasonYoo

Linearized Microwave Power Modules for Communications [Copy link]

Microwave power modules ( MPMs ) have been widely used in military applications, including radar systems and electronic warfare (EW) systems, but have rarely been used in communications due to linearity limitations. However, if the MPM is combined with a linear circuit, it is possible to apply these robust microwave power amplifiers with output powers up to 250W and wavelengths in the millimeter range to communications applications.

The L-MPM was initially power swept with a phasor network analyzer and adjusted to flat gain and phase to drive the RF input. Then, various frequency bands were tested using different signal sources. Figure 3 shows the C-band transmission response of the L-MPM compared to the MPM itself. The 1dB compression point moves from 5dB to within 2dB of saturation. The phase between the small signal and saturation drops from 45 degrees to less than 1 degree.

Figure 4 shows the transmission response of the X-band. The 1dB compression point is moved from 6dB away from saturation to within 2.5dB. The phase between the small signal and the saturation state is reduced from 45 degrees to less than 2.5 degrees. The transmission response of the Ku-band is: the 1dB compression point is moved from 6dB away from saturation to within 2.5dB, and the phase between the small signal and the saturation state is reduced from 52 degrees to less than 8 degrees.

At 18 GHz, the MPM exhibits some gain overflow, but can be easily linearized (see Figure 5). After adding the linear circuit, the 1 dB compression point is moved from 4 dB away from saturation to within 0.5 dB, and the phase is reduced from 60 degrees to less than 5 degrees.

Figure 6 shows the two-channel carrier-to-modulation ratio curves corresponding to the linearized and nonlinearized responses, respectively. In C-band, X-band, and Ku-band, the linearizer increases C/I by more than 15 dB when the output power backoff (OPBO) is greater than 4 dB, and the increase is greater than 10 dB at DBS frequencies. For all bands, most satellite operations require a minimum C/I of 26 dB. The linearizer will increase active power by 6 dB, and the increase in active power has reached more than 6 dB when the C/I ratio is greater than 30 dB.

Next, we will investigate the cause of the linearization-induced spectral regrowth (SR) or ACLR degradation. SR measurements were performed at X-band and subsequently verified at C-band and Ku-band. The linearizer achieves an SR greater than 26 dB at an OPBO of 0.5 dB. The linearizer achieves an SR of 30 dB at an OPBO of 2 dB. Figure 7 shows not only the SR of the MPM and the linearized MPM, but also the spectral response of the modem/upconverter. In addition, Figure 7 shows that the linearizer actually improves the input signal spectrum under certain frequency conditions. Figure 8 shows the SR vs. OPBO plot for two different conditions with and without the linearizer. The apparent improvement in SR is very close to that obtained with conventional TWTA. We also expect to obtain similar results with OQPSK: the SR performance of BPSK will increase by 1 dB after the linearizer is applied.

The performance of L-MPM with multi-carrier and L-MPM with wideband code-division-multiplexed (WCDMA) signals were measured. The performance of HPA with multi-carrier (more than 10) is usually tested using the noise power ratio (NPR) measurement method. In this test process, white noise is passed through a bandpass filter (BPF) to produce a useful signal with equal bandwidth and approximately square noise floor. Thereafter, the signal is passed through a narrowband stop filter to produce a deep notch in the noise floor. The depth of the notch at the output of the test HPA is the measurement of NPR. NPR can be considered as a measure of the multi-carrier modulation rate (C/I). To estimate the multi-carrier performance of the MPM, a 40-MHz X-band noise floor level (noise pedestal) is usually used, which is the typical bandwidth of most satellite pulse converters. The measurement results are shown in Figure 9. For a linearizer with an NPR of 16 dB, the increase in active power output is 3 dB, while for a linearizer with an NPR of 20 dB, the increase in active power output is 4.5 dB higher than that of a linearizer with an NPR of 16 dB.

In addition to its applications in cellular phones, CDMA technology has also found its way into satellite and communication systems. In these applications, SR (ACLR) is a major concern. We measured the SR generated by L-MPM for 3G WCDMA signals, and the results are shown in Figure 10. The figure shows the SR levels generated by MPM and L-MPM at offsets of 2.5MH and 5-MHz. For a 2.5-MHz WCDMA channel bandwidth, an SR of 30 dB will provide more than 6dB of additional power.

Amplifier power loss is a major factor in communications applications. It is a major cost driver and in some cases determines the feasibility of the project. The efficiency of the tested MPMs is not high and varies with frequency due to the wideband design. In the X-band, the overall efficiency of the MPM in saturation (including: TWT, driver amplifier, power supply) is greater than 35%, but still varies with frequency. For a two-channel 26 dB C/I, the linearization circuit can more than triple the frequency from less than 7% to more than 22%.

These results clearly demonstrate the value of combining an MPM with a linearizer. This combination makes the linearized MPM more attractive for both commercial and military communications applications. The linearized MPM provides more power, higher efficiency, and better linearity than alternative MPMs. The linearizer quadruples the MPM output power at C/I greater than 30 dB, extending the C/I power range by more than 10 dB. The linearizer enables QPSK operation with an SR greater than 30 dB, 50W output power, and 25% overall efficiency, similar to WCDMA. All of this is packed into a small package that measures less than 3.5 lbs (127 cm) and weighs 2.25 kg, and is capable of operating over a multioctave bandwidth.