Spectrum analyzer range extension and usage
I want to learn to design RF circuits. However, I can't afford a decent spectrum analyzer but 1-2Ghz range cheap ones. (Rigol, Siglent, etc...)
Is it possible to extend the frequency range of those analyzers with simple mixers? (10Ghz? 20Ghz?)
I guess that it is not possible to design an infinite bandwidth perfect matching circuit between two active devices. If so how did spectrum analyzers manufacturers solve this problem? Or what happens if there is a mismatch?
Yes, it's possible to use a cheaper SA at higher Frequencies using with a Mixer but an accurate calibration must be done because Frequency Response of the Mixer is not flat at all.
Also, spurs and naturally created harmonics that will a obvious results of the Operation of Mixer may be troublesome and they have to be well filtered.One more, Input Sensitivity and Applicable Input Level of the Mixers cannot be as same as LNA.. take care..
spectrum analyzers use a complicated series of hardware and software means to "eliminate" spurious and image signals. This is handy when using it as a laboratory instrument, as you can be chasing ghost signals that are not really there otherwise.
But if you have a good signal source...yes a downconverting mixer in front of a spectrum analyzer will give a pretty good sample of the signal you want to see.
and you can do the math yourself sometimes to figure out ghost signals. Like change your local oscillator 100 Hz, and if the signal on your spectrum analyzer moves 300 Hz, you know it is a intermodulation product ghost.
They used to make "harmonic mixers" to do such a thing....you would input a low frequency (around 1 Ghz) to the mixer LO, it would use a comb generator technology to generate LO signals up to 10 or 20 GHz, spaced at that 1 GHz input signal spacing). this way you did not actually need a 1 to 20 GHz lo synthesizer.
typical harmonic mixer, from back in the day
https://www.ebay.com/itm/HP-Agilent-...-/232696971682
Yes, high frequency spectrum analyzers use harmonic mixers and tunable YIG filters (Yttrium Iron Garnet) in front of them to remove spurious.
Take a look to this video (from min. 43) and understand how this works:
https://www.youtube.com/watch?v=oi4ipg9qHR4
Although band selection filters are standard with modern spectrum analyzers, they are not absolutely necessary and can't be found with older instruments, e.g. Tek 491.
Go in the biggest chinese selling site and search for D6 spectrum analyzer, hope it will fit your request.
Mandi