Intermodulation Distortion Analysis in Gridless Inductive Output Tube

Conventional inductive output tubes (IOTs) are operated as class B or class C amplifiers by using a modulation grid. However, this imposes limits on the power and frequency of the device. Grids also add design complexity and thermal issues with grid heating. Gridless IOT offers higher output power, while operating as a class D amplifier using pulse width modulated (PWM) input. Class D operation is inherently nonlinear, and introduces harmonic distortion in the output. Intermodulation (IM) distortion analysis is used to characterize these nonlinear output harmonics. This paper investigates the effects of IM distortion on the output of a 420MHz gridless IOT with 70kV annular beam input and 2kV modulation signal. The system is simulated with two-tone input signals at 387.5MHz and 422.5MHz, and the resulting third-order intermodulation (IM3) products are analyzed. Results show IM3 output harmonic intensity ratios of 0.0041 (-47.7dB) and 0.0034 (-49.3dB).