Downconversion and Frequency Tuning

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EOVSA has three consecutive frequency-conversion operations required to tune and isolate a clean 500 MHz IF band from the 1-18 GHz RF band. The first is a tunable upconversion to a 20-20.5 MHz band, which is filtered but with a rather gentle roll-off due to the high center frequency. The second is a fixed-frequency downconversion of the 20-20.5 MHz band centered on a sharply filtered IF bandpass from 600-1200 MHz. The third downconversion is that due to the digitizer, whose nominal clock is 1200 MHz, although for practical reasons during the prototype phase we are operating the digitizer at a sub-optimal 800 MHz clock speed. This document describes these frequency conversions and the resultant ordering of frequency channels in the digitized data, both for the production unit with a 1200 MHz digitizer clock, and for this interim period with an 800 MHz clock. We demonstrate that the system corresponds to our expectations by introducing a frequency-swept CW signal into the IF path and observing its effects. We also discuss the linearity of the system, which probably belongs in a separate memo.

Figure1:Schematic representation of frequency tuning for EOVSA, showing tuning to four of the 34 RF bands. From top to bottom they are: 1.5-2 GHz, 3.5-4 GHz, 7.5-8 GHz, and 17.5-18 GHz.

Tuning and First Frequency-Conversion

Second Frequency-Conversion

Digitization and the Third Frequency-Conversion

Linearity and RFI