Downconversion and Frequency Tuning
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.
Tuning and First Frequency-Conversion
Figure 1 shows the basic tuning operation for four different IF bands. Each tuning inverts the 1-18 GHz RF by mirroring it around the LO frequency. To tune to each of the 500 MHz between 1-18 GHz, 34 tunings are required ranging from 21.5 GHz to 38 GHz. The RF frequency at the low end of the 500 MHz band is related by the LO frequency by 20.5GHz. In the figure, the RF frequency scale is shown in black, and the IF frequency scale (after the conversion) is shown in blue. As the LO frequency changes, the mirrored RF band slides to the right with twice the step of the LO frequency change, while the fixed 20-20.5 GHz filter slides to the right with the same step as the LO. Thus, the mirrored RF slides past the filter window at the same rate as the LO frequency change. Note that the RF frequencies are inverted due to the mirroring.
In order to digitize the signal, the high-frequency IF must be converted to a lower frequency. To accomplish this, the EOVSA system mixes the fixed 20-20.5 GHz signal with a fixed 21.15 GHz second LO as shown in Figure 2, whose format is similar to that of Figure 1. The rather gentle roll-off of the 20-20.5 GHz filter is indicated by the sloping sides of the bandpass. Again, the original frequency scale is shown in black, and the mirrored copy of the bandpass is shown in blue, labeled with its lower IF frequency scale in MHz, also in blue. This IF bandpass shape is then filtered with a sharply defined IF filter from 600-1200 MHz (outer dashed lines marked as Digitized Bandpass). This second mirroring of the band causes the previously inverted RF frequencies to again have a direct ordering.