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Traveling Wave Tubes

Suppression of Intermodulation Products in a Broadband TWT

Suppression of intermodulation products is a crucial communications and ECM area. The suppression of nearest neighbor nonlinear products in a broadband TWT could allow one to fully utilize the bandwidth of a TWT for a variety of applications. We have focused on new techniques to accomplish intermodulation suppression and the underlying spectral evolution in our specially designed Northrup-Grumman research TWT.

A. Highlights of 2001-02 Research

B. Intermodulation suppression technique using Harmonic Injection

Traveling Wave Tube (TWT) amplifiers are used for their high power output but the non-linear behavior of these tubes leads to intermodulation interference when closely spaced multiple tones are amplified. The suppression of these intermodulation products is a crucial research topic for communication as well as ECM applications.

Growth of non-linear distortion products along the helix of the tube is measured using XWING, a custom-modified research tube jointly developed by UW and Northup-Grumman researchers that has sensors along the helix.

Figure 1: Spatial evolution of two-tone spectrum

A linearization scheme was investigated where the harmonic (4.00 GHz) of one of the tones was injected along with the two fundamental tones (2.00 and 1.95 GHz). Phase locking of the harmonic to the fundamental was achieved by sharing a 10 MHz reference signal.

Figure 2: Harmonic Injection set-up diagram

The amplitude and phase of the injected harmonic were adjusted by repeated iterations to achieve a maximum suppression of 24 dB for the upper IM3 [1]. The lower IM3 can be similarly suppressed by injecting harmonic of the other tone.

Figure 3a: TWT output spectra without harmonic injection

Figure 3b: TWT output spectra with harmonic injection

Amplitude and Phase sensitivity plots of IM3 suppression were obtained by varying the phase and amplitude of the injected harmonic and observing the IM3 level. It was found that IM3 power goes up by 3 dB for 0.5 dB change in amplitude or about 3 degrees change in phase of the injected harmonic.

Figure 4a: IM3 level sensitivity to injected amplitude

Figure 4b: IM3 level sensitivity to injected phase

Using the XWING, spatial evolution of the suppression mechanism was studied using the sensors along the helix. It was observed that a particular value of injected harmonic amplitude and phase is needed to achieve maximum suppression of the IM3 at a particular axial position. The required level of the injected harmonic was found to be very small compared to the fundamental level. For the optimum case, an IM3 reduction of 24.2 dB was observed for an injected harmonic drive level of only 0.9 dBm, or 17.1 dB below f2.

The spatial spectrum shown in Fig. 5 indicates that the upper IM3 (2f2-f1) initially grows from sensor 1 to sensor 4. When the IM3 due to beating of the injected harmonic (2f2) with the other fundamental (f1) becomes strong enough in amplitude with an opposing phase, the resultant IM3 starts to decay and is a minimum of 46 dBc at the output of the tube.

Figure 5: Spatial evolution of upperIM3 suppression

C. Current/Future objectives:

Relevant Publications