The idea (roughly) is to use the output (low pass filtered) directly for 2-4 GHz and mix with 2 GHz (also filtered) from the second one for 0-2GHz.Īll those things sure can be solved. But it should be possible with two ADF4350, a mixer and some switches and filters. At least I think this does not exist yet. But I think the second option eventually will be.Īnother niche that I hope will be filled sooner rather than later is a cheap 3 or 4GHz signal generator with sine wave(-ish) output. You will also very likely need to calibrate against a levelled signal generator at the factory. In the end, some things you can get away with in a VNA will not give good performance in a SA. So is this economically viable right now? Maybe not, and I suspect Gabriel has already considered these options as well. (btw, the Signal Hound SA44B does something similar, but with two mixers and two slow ADCs here is the patent: ) But then again you need a couple of MHz IF bandwidth anyway if you don't want analog RBW filters and still have a reasonable sweep speed. I think at least twice the bandwidth of the signal you want to look at, otherwise the image will overlap. The downside is, there are cases where this will go wrong and also you need a relatively fast IF ADC. 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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