First and second harmonic generation
The root cause of harmonic generation is caused by nonlinear loads. When current flows through the load, it is not linearly related to the applied voltage, and a non-sinusoidal sinusoidal current is formed, thereby generating harmonics. The harmonic frequencies are integral multiples of the fundamental frequency. According to the French mathematician M. Fourier, the principle of any repetition can be decomposed into a sine wave containing the fundamental frequency and a series of harmonics that are multiples of the fundamental. Component.
The second harmonic of Band 40 TDD-LTE requires -30dBc or less. In order to ensure the test at high and low temperatures, the terminal manufacturer will ensure that the second harmonic is below -36dBc at room temperature. The harmonics generated by the Band 40 TDD-LTE amplifier are as follows: :
The typical suppression of the second harmonic in the B40 filter on the market today is about 24 dB. Theoretically, this filter is sufficient to suppress the second harmonic. In the actual B40 TDD-LTE terminal project, the insertion loss of the power amplifier output to the antenna output port is 4dB, then when the antenna output is 23.5dBm, the output power of the power amplifier is 27.5dBm, and the second harmonic at this time can fully satisfy - 30dBc standard.
But in fact, because the device is non-linear, the filter switch also has such a problem, plus the irregularity of the PCB Layout trace, the actual test of the project, the second harmonic result is -28dBc, can not meet the standard.
Second, LC filter simulation and debugging
This project uses the LC passive low-pass filter to effectively suppress the second harmonic of the signal. The filter is constructed only by the principle that the reactance of the capacitor and the inductor component changes with the frequency, and the advantage is that the circuit is relatively simple, does not require a DC power supply, and has high reliability. In the design of this project, the π-type matching of the filter backend is shown in Figure 3.
After ADS simulation, the suppression of the second harmonic of the filter is shown in the curve shown in Figure 4: the red curve indicates the insertion loss of the L-type matching at 2.0 GHz to 5.5 GHz, and the blue curve π type matches the insertion loss at this frequency interval. . We can compare that π-type matching has better in-band insertion loss and out-of-band rejection, which is the best choice for this project.
Using the π-type matching scheme provided by the simulation, after actual debugging, when C1 is 1.2pF, L1 is 2.4nH, and C2 is 1.2pF, the second harmonic is optimally suppressed. The data is shown in Figure 5.
Third, the conclusion
This paper mainly solves the second harmonic problem of Band 40TDD-LTE by adopting LC filter and ADS simulation and adopting effective matching method.
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