Detailed analysis shows the feasibility of improvements by increasing the basic interaction efficiency between the beam and the wave, in the delay line. Another major challenge is increasing collector efficiency.

Higher efficiency also means that the thermal control system has to deal with less dissapated energy.

Another important factor in maintaining effieciency is tube-to-tube reproducibility. At THALES, we produce more than 1000 space TWT´s per year, and the performance spread is very low. For guaranted efficiency, the typical deviation must not exceed 1%.

To transmit digitally encoded TV signals and multimedia channels, TWTA channelds must operate in multicarrier mode.

In this case, the amplifier has to meet strikt liniearity requirments to keep the intermodulation products of the carriers wthin acceptable limits. This can be achieved by operating the TWTA in "backoff" mode (typically, 7-8 dB output power backoff). The drawback of this mode, however, is a loss of efficiency.

Using a linearizer, intermodulation products can be kept at the same level with only 3-4 dB output power backoff, wich means the TWT consums less power.

The linearizer is a preamplifier with a distortion network. The network generates a signal similar to the nonlinearities of the TWTA. The signal is attenuated and mixed with the input signal of the TWTA, with a 180 phase shift.

This method compensates for interference signals such as CI3 and higher-order intermodulation products.

The noise-to-power ratio (NPR) is used to predict the multicarrier performance of a TWTA. Meassurements are based on a white noise RF signal, with a notch in a given bandwith (e.g., 200 MHz) to simulate multicarrier operation.

This signal is amplified by the TWT, generating intermodulation products.

The measured NPR is a figure of merit for the amplifier.