IMD5Fifth-Order Inter-Modulation Distortion
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The output power of IMD3 2[[omega].sub.2] - [[omega].sub.1] can be suppressed to zero by appropriately adjusting [A.sub.21] and [[phi].sub.21], but they are not the optimal parameters to completely suppress IMD5 3[[omega].sub.2] - 2[[omega].sub.1] at the same condition.
It can be seen from (5) that the expression of IMD5 is quite complicated.
[L.sub.out,IM3] and [L.sub.out,IM5] represent the output power of IMD3 and IMD5, respectively; the unit is dBm.
When the optimal operating point of the injected amplitude and phase is found where TF[R.sub.IM3 to IM5] has minimum, the relative power of IMD5 and IMD3 and the difference of output power are minimum.
IMD5 (3[[omega].sub.1]-2[[omega].sub.2] and 3[[omega].sub.2] - 2[[omega].sub.1]) are also in the working frequency band of STWT, the effect on fundamental signal is next to IMD3, and both of them are the main performance of nonlinear distortion in STWT.
The study on signal injection to simultaneously suppress the IMD3 and IMD5 has been held all along to improve linearity performance of STWT significantly.
Simultaneous Suppression for the IMD3 and IMD5. On the basis of the previous research achievements on fundamental signal parameters in STWT, the initial two-tone fundamental frequencies [[omega].sub.1] and [[omega].sub.2] are set as 1.50 and 1.52 GHz; phase and amplitude are 0.0[degrees], -10.0 dBm.
The comparison of computation results of output powers at the fundamental, IMD3, and IMD5 under optimum IM3 injection and without IM3 injection is completed in this paper.
The optimal signal injection conditions for IMD3 and IMD5 are not consistent.
Based on simulation results, IMD3 has larger output power level compared to IMD5 and more close to fundamental frequency, so IMD3 suppression is a more important work for improving the performance of STWT.
This condition represents the IMD3 to IMD5 relative power at corresponding frequency point (higher or lower order) achieving the minimum and making the optimum simultaneous suppression for both IMD3 and IMD5 of STWT; then the IMD3+ and IMD5+ output power is -9.6 dBm and 17.9 dBm, respectively.
Second harmonic (2[[omega].sub.2]) injection can also significantly suppress IMD3 and IMD5 according to the previous research results; then whether the effect of 2HD injection is any better on simultaneous suppression in both IMD3 and IMD5 compared with IM3 injection is an important research topic.