Electronics > Beginners

Why are spectrum analyzers so expensive?

<< < (5/9) > >>

bd139:
Might have a play with that. The software FFT on the Ds1054Z is a bit poo for some things to the point for low frequencies I use a sound card and Spectogram for it instead.

T3sl4co1l:

--- Quote from: codingwithethanol on June 09, 2019, 10:58:53 am ---Why is Yttrium required???? I had a hunch these things ran on fairy dust but HUH?

--- End quote ---

"Physics.  It works, bitches." :-DD

That was an incomplete description, but yttrium is indeed involved -- the module is a YTO (YIG Tuned Oscillator).  The active ingredient is a YIG (yttrium iron garnet) sphere.  YIG is a ferrite (i.e., a compound of some metal oxide with Fe2O3, or to put it another way, iron(III) ferrate), specifically one taking the garnet crystal structure.  (Whereas most ferrites are in the spinel group*.)

*Chemistry aside:
The mineral spinel has the formula MgO.Al2O3, but the Al can be substituted with Fe giving a ferrite, and the Mg with a variety of metals (Fe, Mn, Zn, Ni, etc.), making this a huge family of continuous mixtures of similar atoms.  Soft ferrites are (Mn,Zn) and (Ni,Zn) ferrite.  The mineral magnetite is actually iron ferrite, FeO.Fe2O3!

In terms of electrical properties, YIG has lower saturation flux density (because there's less active magnetic iron, and more diluting or opposing atoms), lower relative permeability, higher resistivity, lower losses, and notable paramagnetism.

It's the last property we're interested in.  When subject to a static magnetic field, unpaired electrons (the part responsible for bulk magnetism in the first place) precess at a frequency proportional to that field.  (The physics is similar to the precession of a spinning top, where the precession rate depends on the force trying to tip it over, i.e., gravity in that case, ambient magnetic field in this case.)  The precession in turn manifests as electromagnetic energy (photons; this is a quantum process too, after all), which is to say, light waves.  Or at typical field strengths, radio waves more specifically.

So the really interesting property is this: when an inductive loop is coupled to a YIG crystal, and the crystal is placed in a magnetic field, there is a blip in the frequency response of that loop, which varies with magnetic field intensity.  The blip looks much like the impedance response of a quartz crystal, having a series and a parallel resonance, and you can tune in either one.  Or, as far as I know, it's usually done with two perpendicular inductive loops, so that they are only coupled through the YIG crystal's resonance (getting a bandpass characteristic), and serve as the input and output terminals of a microwave-frequency resonator.  Add an amplifier and buffer, and you've got a YTO module!

The tunable range doesn't go all the way to zero, of course; that would be kind of horrible to design (those inductive loops won't work very well at DC!).  Usually it's, like, say, 3-6GHz or such.  Which is perfectly fine because as mentioned before, LO1 will be somewhere at high frequency

A few other practical considerations include magnetic design (the module housing is usually steel, cut in a "pot core" sort of shape to give good shielding to external fields, and better field intensity (and uniformity) around the crystal itself), thermal design (the electromagnets are normally wired differentially, so as to maintain the same total heat dissipation while varying the total field), and so on.

So, because the YTO is tuned by magnetic field, it can be tuned electronically.  Fantastic, huh? :)

Tim

ebastler:

--- Quote from: Theboel on June 09, 2019, 12:14:53 pm ---I think You don't give information about how high You want to go 100MHz, 2GHz ?

--- End quote ---

Yep, without any information about the required frequency range, this discussion and any recomendations are flapping in the breeze.
@codingwithethanol: What signals do you want to look at?

* In the audio range, a decent sound card and free FFT software are a good bet. You can even get a combined sweep generator and Bode plotter for your sound card.
* In the 100 MHz range (order of magnitude), the FFT function on entry-level DSOs may do what you need -- or, again, the Bode plot function some scopes offer.
* If you want to look at GHz frequencies, things get expensive...

RoGeorge:

--- Quote from: codingwithethanol on June 09, 2019, 10:58:53 am ---Why is Yttrium required???? I had a hunch these things ran on fairy dust but HUH?

--- End quote ---

If you want to know more, take a look inside a YIG oscillator (starting from minute 33:28):

Bicurico:
Thanks T3sl4co1l and RoGeorge:

YIG/YTO are magic, no matter how great the explanation on how they work!

:)

Regards,
Vitor

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod