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Because of nonlinearities, all amplifiers generate intermodulation products, some of which are very difficult to remove. Take the case of a narrow-bandwidth amplifier which must put out a very clean RF signal; that is, any frequencies outside the band due to intermodulation must be greatly attenuated, often by 50 dB or more. For modulation products far away from the desired RF band, it’s often reasonable to filter them out. The well-known modulation products caused by second-order nonlinearities generate unwanted frequencies at DC, at baseband, and at the second harmonics of the in-band signals. All these can be effectively filtered out because of their great distance from the in-band signals.
However, some intermodulation frequencies caused by third-order amplifier nonlinearities are especially difficult to get rid of. These troublesome frequencies are of the form of (2F1 – F2) where F1 and F2 are any two frequencies of the in-band RF signals. Since the RF band is narrow, F1 & F2 are quite close together, so their intermodulation frequencies also lie very close to the RF band (and even within the RF band itself).
Since it is not possible to filter these unwanted close-in frequencies, if the signal level of the RF amplifier is high enough where the third-order modulation products become troublesome, the usual remedy is to back off the RF levels enough to meet stringent linearity requirements. This is a very costly remedy, because higher power amplifiers must be used (at extra cost and extra heat generation) to get the required RF output power.
As a result of this, much effort has been put forth to improve the amplifier linearity by “pre-distorting” the input and/or “post-distorting” the output of the amplifier in order to cancel some of the amplifier nonlinearities and get a bit more output power with the required linearity. Some of these schemes are quite complicated and yet bring only limited improvements.
Self-Cancellation of Third-Order Modulation Products
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