Intermodulation suppression in a broad band TWT

Summary form only given, as follows. We examine methods for reducing two-tone intermodulation products in the custom-made XWING broadband traveling wave tube. Harmonic suppression and difference frequency techniques are used to demonstrate intermodulation suppression techniques and the spatial variation of their spectral evolution. Harmonic and difference frequency optimum drive levels and phase are determined which suppress the third order intermodulation product by more than 21 dB in the 2-6 GHz band. The variation of the suppression with drive level near saturation and two-tone difference frequency are examined. IM3 reduction using the harmonic injection technique is experimentally demonstrated in a broadband TWT distributed amplifier. The TWT used in this investigation, termed the XWING TWT (for eXperimental WIsconsin Northrop Grumman TWT), is a research version of a product manufactured by Northrop Grumman. This two-stage, helical TWT provides a moderate gain of 20-30 dB over a frequency range of 2-6 GHz. The upper fundamental and harmonic frequencies were set to 2.00 and 4.00 GHz, respectively. Since the phase of the injected harmonic must be referenced with respect to the higher frequency fundamental, two Agilent 83623B synthesizers were configured to share a common 10 MHz phase reference signal. The 4.00 GHz signal was sent through a Narda 3752 phase shifter, allowing the fundamental-to-second harmonic phase relationship to be adjusted in real-time. The lower fundamental frequency of 1.95 GHz was supplied by a Wavetek 3520 synthesizer, providing a 50 MHz difference between the two drive tones. The experiment was performed for fundamental drive tones of 15 and 18dBm/tone. First, the 1.95 and 2.00 GHz fundamental drive tones were independently set to 15 dBm/tone at the TWT input tap, and the output spectrum was captured on an Agilent E4407B digital spectrum analyzer. Next, the 4.00 GHz second harmonic tone was injected at the TWT input and the phase was varie- , with respect to the 2.00 GHz fundamental, to achieve the lowest IM3 level. With the phase relationship optimized, the injected harmonic amplitude was varied until a maximum suppression in the upper IM3 level was observed. This occurred with injected harmonic amplitude of -2.1 dBm or 17.1 dB below f2. The upper IM3 was reduced by 21.3 dB, yielding an upper carrier to IM3 power ratio of 43.9 dB. The TWT output spectrum with and without harmonic injection is shown below. It can be seen that the upper IM3 is suppressed by 21.3 dB. The experiment was repeated at higher input drive levels of 18 dBm/tone and an IM3 reduction of 24.2 dB was observed. The optimum phase of the 4 GHz injected harmonic was measured at the TWT input tap and was found to lead the 2 GHz fundamental by 47.5 degrees, with respect to the fundamental period.