Suppression of Third-Order Intermodulation in a Klystron by Third-Order Injection

Abstract

The first observations and measurements are reported on suppression of the third-order intermodulation (IM3) product arising from nonlinear mixing of two drive frequencies in a klystron, by externally injecting a wave at the IM3 product frequency. Optimum amplitude and phase of the injected wave for maximum suppression are examined. Results indicate that suppression of the IM3 product by as much as 30 dB can be achieved. Experimental results compare favorably with predictions of a 1D simulation code that takes into account all kinematical and dynamical effects including charge overtaking and space charge forces.

Nonlinear Space Charge Wave Theory of Distortion in a Klystron

“…The generation of these distortions is undesirable since it degrades communications using the carriers. It has been shown experimentally and numerically for steady state inputs [4] that by including the intermodulation frequency as an input with proper amplitude and phase, that the intermodulation frequency can be canceled in the output. Furthermore, for nonsteady state inputs as found in communications, injection of the third-order intermodulation spectrum is the basis for predistortion linearizers [18].…”

“…For comparison with [4] we consider two closely spaced input frequencies ( MHz, MHz), the second harmonics and sum frequency ( MHz, MHz, MHz), the 3IM products ( MHz, MHz), and cancellation of the 3IM with an injected modulation at the intermodulation frequency. The amplitude of the velocity modulations are set such that the square of the 3IM beam current modulation, which is proportional to cavity power, is about 30-dB down from the fundamental at as in [4].…”

“…For a two tone input we show that the generation of 3IM distortions (e.g., ) comes from mixing second-order distortions (sum frequency and harmonics) with the linear space charge waves. Furthermore, suppression of the generated 3IM by injecting a signal at the 3IM frequency [4] is approximately described as a superposition of the nonlinearly generated 3IM and linear space charge waves due to the (small) injected 3IM. This view sheds light on the operation of predistortion equalizers since they inherently work on the principle of 3IM injection [18].…”

“…NDERSTANDING the generation and mitigation of harmonic and intermodulation distortion in linear-beam microwave vacuum electronic devices (MVEDs) is currently an active area of research, e.g., [1]- [4]. The use of modern largesignal MVED codes in the design process (e.g., [3], [5], and [6]) has resulted in improved device performance, including enhanced performance in the nonlinear regime of the device transfer characteristics.…”

“…In Section IV we consider the two-frequency input problem. The Eulerian model is first compared to the conventional Lagrangian model to determine validity for a case that has been studied experimentally 0018-9383/$20.00 © 2005 IEEE [4], then analytic solutions of nonlinear space charge waves are compared to simulations of the Eulerian model for insights into the mechanisms of distortion generation and mitigation. The paper is concluded in Section V.…”