High-Precision Millimeter-Wave Spectrometer: Last Improvements

It is that the wideband output spectrum of a DDS (direct digital synthesizer) contains a lot of rather intense spurious components that is why high factor multiplication (in our case it may be up to 600) of the DDS output signal usually produces too spurious spectrum. In order to overcome this obstacle we applied narrow-band adaptive filtering by means of narrow-band phase-lock loop of the klystron (its closed loop bandwidth is about 1 - 2 kHz). In order to provide wide-range continuous spectra record the klystron is equipped with special system which provides mechanical adjustment of its resonator. As it is shown in even small distortion of modulation signal leads to additional errors in the frequency determination. That is why additional improvement deals with application of direct digital synthesizers (DDS) AD9834 in the modulation system of the spectrometer. We applied two phase-shifted DDS with controlled phase shift. Application of DDS allows us to minimize distortions of modulation signal as well as to improve resolution during phase shift adjustment.