![]() The latest Keysight PNA and PNA-X network analyzers have an SA option that works like this as well. The images will shift in the wrong direction when the LO is changed (and second harmonic will shift at twice the rate of the fundamental etc.) so you can filter them out: Essentially you record your spectrum (at least) twice, and offset the LO for the second pass. In principle a single mixer and LO is enough. The "modern" approach is to basically ignore image rejection on the RF side. 2 switchable bandpass filters (for high band) The high band still needs a preselection filter for image (and second harmonic) rejection, but I think two different switchable bandpass filters should be enough in this configuration. ![]() For the 1.5-3GHz high band, you could mix down to 0.7GHz IF2 with 0.8-2.3GHz LO. You could have a <1.5GHz low band and mix that up to maybe 1.7GHz IF1 with a 1.7-3.2GHz LO. With a two band design, frequencies much higher than 3 GHz would not be needed. There are a couple of ways around this, but they come with cost/complexity/performance trade-offs. But you would still need a >6GHz LO (and first mixer). For a classic SA design, I think you would put the first IF somewhat above 3GHz, not at 6GHz. A half-usable 3GHz SA in a similar price range as the S-A-A-2 would sureley sell like hot cakes. Well, I've been thinking about this a bit.
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