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Fig. h Dual-conversion design. The first VTO mixes with the downlink signal into the first i-f amp////er The second VTO is fixed. Gain distribution may vary in different designs. The 70-MHz bandpass filter must be flat in the pass band for best results.

noting here are conversion to a lower frequency where we can crank in lots of easy-to-find gain and sticking to the rules. But why do that? Well, conversion makes a lot of sense After all, the incoming signal may very well be around —50 to — 60dBmat 4 GHz and a 564 PLL detector chip wants something quite different before it will deliver pictures.

But who made the rules about dividing by 10 and that nonsense? The guys that got there first, of course. They did some arithmetic and made a discovery of some importance. A single-conversion design has problems if you chose a first (and only) LO just 70 MHz from the desired signal You get down to baseband a lot quicker, but you get something else in the bargain the »mage signal as seen in Fig 2.

Remember from last month's Satellite Central that the satellite band (3.7 to 4.2 GHz) is 500 MHz wide. So even an LO spaced some distance away, whether above or below the

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