Author Topic: Why are spectrum analyzers so expensive?  (Read 7838 times)

0 Members and 1 Guest are viewing this topic.

Offline codingwithethanolTopic starter

  • Regular Contributor
  • *
  • Posts: 73
  • Country: us
  • always tilted
Re: Why are spectrum analyzers so expensive?
« Reply #25 on: June 10, 2019, 12:04:25 am »
@T3sl4co1l So yttrium is used to create a magnet controlled bandpass?
@ebastler mostly low freq stuff like audio filters but I did want to experiment with AM/FM so yeah up to around 100mhz
http://prntscr.com/nzpvbc
@RoGeorge yeh im gonna need a few more fundamental fridays to understand that shit
its your boi, dj ethanol
 

Offline T3sl4co1l

  • Super Contributor
  • ***
  • Posts: 22436
  • Country: us
  • Expert, Analog Electronics, PCB Layout, EMC
    • Seven Transistor Labs
Re: Why are spectrum analyzers so expensive?
« Reply #26 on: June 10, 2019, 01:47:38 am »
I think the role of yttrium specifically is either, it opposes the magnetic field less than other options, or it doesn't interact and just acts as a dilutant; and in either case, it happens to be the right size atom to make a garnet crystal, when combined with iron.  And the garnet is important because of where in the crystal the iron atoms sit, which determines how they interact and therefore the magnetism.

In any case the important part is not yttrium, it's that it's a garnet made with iron, and yttrium happens to be necessary to make that happen.  And then that the result is magnetically tuned, yes. :-+

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 

Offline RoGeorge

  • Super Contributor
  • ***
  • Posts: 7012
  • Country: ro
Re: Why are spectrum analyzers so expensive?
« Reply #27 on: June 10, 2019, 02:31:35 am »
gonna need a few more fundamental fridays to understand

The first couple of pages from here might help:  http://www.sjsu.edu/people/raymond.kwok/docs/project172/EE172_YIG_oscillator.pdf

About the spectrum analyzer, for audio range it wont help you much.  Many of those very expensive SA doesn't even cover such low frequencies.  They are mostly for the RF, where MHz or GHz range is needed.  kHz range might be useful for switching power supply noises and such, but not much for audio.  There are dedicated SA for the audio range, thought.

For frequency up to 100MHz, you can sometimes get away without a proper SA by improvising with just an oscilloscope and a sweeping generator, depending on what you want to measure.

For audio only, a sound card and a FFT program will be enough in most cases.  In fact, a good sound card would be hard to beat even by those very expensive specialized audio spectrum analyzers.  IMO for learning purposes, a sound card is more than enough.

Offline David Hess

  • Super Contributor
  • ***
  • Posts: 17427
  • Country: us
  • DavidH
Re: Why are spectrum analyzers so expensive?
« Reply #28 on: June 10, 2019, 03:01:06 am »
Also, speaking of plugins, I don't have a 7000 series tek to plug that into, but i was also wondering why there isn't just a black box with some BNC ports you can connect to your oscilloscope to have SA functionality on a budget?

Tektronix made one like that which could be used with any oscilloscope.

http://w140.com/tekwiki/wiki/1401
 

Offline ejeffrey

  • Super Contributor
  • ***
  • Posts: 4034
  • Country: us
Re: Why are spectrum analyzers so expensive?
« Reply #29 on: June 10, 2019, 04:05:26 am »
As for why YIG spheres are used: it basically boils down to "inductors are garbage".  The phase noise of an oscillator is controlled by the Q factor of the resonance you use to set the frequency.  Same with the out of band rejection of a filter.  Resonators can be made from ordinary inductors and capacitors.  Capacitors are not too bad: vacuum or low k dielectrics are nearly lossless.  But inductors tend to be quite lossy at high frequency unless you make them out of superconductors which isn't usually convenient.

The normal solution is to use quartz crystals.  The Q of their electro-mechanical modes are incredibly high which is why they make such great oscillators and filters.

The one problem with quartz crystal filters is that you can't easily tune them.  You can slightly pull the frequency with an external capacitor, but you can't easily change the motional LC circuit parameters.

YIG spheres are basically the solution to that problem: a high-Q resonance that is broad-band electronically tunable.
 

Offline hagster

  • Frequent Contributor
  • **
  • Posts: 394
Re: Why are spectrum analyzers so expensive?
« Reply #30 on: June 10, 2019, 05:09:15 am »
The Airspy mini is an om option as an SDR based spectrum analyser. SDR# comes with an SA application that sweeps pretty quick and has good dynamic range and low noise floor. Much better than the hackRF within the frequency limitation of the Airspy. Only 70USD. https://airspy.com/airspy-mini

Its not really the same as a proper SpecAn and you won't really be able to make any kind of calibrated measurements with it though.
 

Offline chrisc

  • Supporter
  • ****
  • Posts: 49
  • Country: au
Re: Why are spectrum analyzers so expensive?
« Reply #31 on: June 10, 2019, 05:26:19 am »
Which, on that note -- a DSO with FFT functionality isn't much different.  It won't have the spectrum-related features that a DSA does, but if all you want is to see some peaks, it's a good deal.

Yeah they're not too terrible at that actually. Not a bad solution.

On that topic, ElectroBOOM gives an example of using FFT in a scope while debunking yet another “free energy” claim:



(Skip to 3:40; it looks like the auto-embedding strips the time offset).

« Last Edit: June 10, 2019, 05:29:13 am by chrisc »
 

Offline T3sl4co1l

  • Super Contributor
  • ***
  • Posts: 22436
  • Country: us
  • Expert, Analog Electronics, PCB Layout, EMC
    • Seven Transistor Labs
Re: Why are spectrum analyzers so expensive?
« Reply #32 on: June 10, 2019, 06:14:01 am »
As for why YIG spheres are used: it basically boils down to "inductors are garbage".  The phase noise of an oscillator is controlled by the Q factor of the resonance you use to set the frequency.  Same with the out of band rejection of a filter.  Resonators can be made from ordinary inductors and capacitors.  Capacitors are not too bad: vacuum or low k dielectrics are nearly lossless.  But inductors tend to be quite lossy at high frequency unless you make them out of superconductors which isn't usually convenient.

The normal solution is to use quartz crystals.  The Q of their electro-mechanical modes are incredibly high which is why they make such great oscillators and filters.

The one problem with quartz crystal filters is that you can't easily tune them.  You can slightly pull the frequency with an external capacitor, but you can't easily change the motional LC circuit parameters.

YIG spheres are basically the solution to that problem: a high-Q resonance that is broad-band electronically tunable.

One final touch to this -- you can still make a very nice LC oscillator (if still maybe not as low noise as you'd like, for the above reasons), that's voltage controlled using varactors, but it won't be very linear.  The neat thing about YIG tuning is, it's proportional to magnetization because physics -- so the VCO linearity is very good indeed, and little compensation or calibration is needed!

On a related note, I wonder if anyone ever tried making, say, a tunable quartz or mechanical filter, that uses a stretchy or bendy substrate as resonator, so that the dominant mode can be varied over a modest range.

In a completely different direction, there are cavity resonators with free electrons -- that's right, vacuum tubes, specifically, reflex klystrons.  Apparently these are quite low noise, and are electrically variable over a modest range.  The internal function is something like: an electron beam shoots towards a negatively charged reflector electrode, inside a resonator cavity.  The field inside the resonator controls electron motion, transforming electron energy into resonant energy.  The reflector electrode controls how soon this happens, tweaking the frequency within the bandpass range of the resonator.  IIRC, a typical range is maybe 100MHz out of a 5GHz center frequency -- good in its day (a reminder that the widest bandwidth all-[conventional] tube system was probably the Tektronix 585, topping out at 100MHz baseband).  Downsides include, well, all the vacuum tube stuff you need (heater power, high voltages), FM-to-AM conversion, and relatively low power output.

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 

Offline bd139

  • Super Contributor
  • ***
  • Posts: 23099
  • Country: gb
Re: Why are spectrum analyzers so expensive?
« Reply #33 on: June 10, 2019, 07:05:08 am »
I’m using the aforementioned LC + varactor oscillator as the first LO on my toy spectrum analyser. This uses an SA612 as the first mixer and VCO. I can get 180-305MHz out of that. As for linearity, yep it’s awful. I’m playing with a linearisation network using diode clamps on the sweep input. This is as far as I have got though.
 

Offline lordvader88

  • Frequent Contributor
  • **
  • !
  • Posts: 935
  • Country: ca
Re: Why are spectrum analyzers so expensive?
« Reply #34 on: June 10, 2019, 07:07:25 am »
I for 1 can't wait to make a 1970/80s era kit style SA. They look so cool with all the copper clad shielding and parts.
 

Offline bd139

  • Super Contributor
  • ***
  • Posts: 23099
  • Country: gb
 

Offline JohnPen

  • Regular Contributor
  • *
  • Posts: 240
  • Country: gb
Re: Why are spectrum analyzers so expensive?
« Reply #36 on: June 10, 2019, 07:54:51 am »
For those of an adventurist nature you could build a Scotties Spectrum analyser. However it is not that cheap, because of component costs, and takes quite a while to build.  Also sourcing suitable VCO chips has become more complicated as Minicircuit's no longer make the VCO chips.  It is still cheaper than purchasing a Rigol or Siglent and you learn a lot but it is a somewhat long term construction project.  :)  However reading through Scottie's site provides a lot of useful information on understanding and building your own Spectrum analyser.   There is also a dedicated Yahoo group, see below, which provides great support for those who bite the bullet.

http://scottyspectrumanalyzer.us/

https://groups.yahoo.com/neo/groups/spectrumanalyzer/conversations/messages
 

Offline RoGeorge

  • Super Contributor
  • ***
  • Posts: 7012
  • Country: ro
Re: Why are spectrum analyzers so expensive?
« Reply #37 on: June 10, 2019, 08:28:49 am »
On a related note, I wonder if anyone ever tried making, say, a tunable quartz or mechanical filter, that uses a stretchy or bendy substrate as resonator, so that the dominant mode can be varied over a modest range.

That was exactly my thought, too.  To experiment with a decapped Quartz by pressing it in a vice with rubber jaws, and see how much the resonant frequency can be shifted without snapping the crystal.  If it would be enough, the next step would be to replace the vice with some piezo material in order to electrically control the mechanical pressure, and thus the resonant frequency.   ;D

Most probably this idea was already tested before, and the results were nothing spectacular, so we don't have today such a device commercially available, but still, it will be an interesting experiment to tinker with.   :-DMM

Online ebastler

  • Super Contributor
  • ***
  • Posts: 7375
  • Country: de
Re: Why are spectrum analyzers so expensive?
« Reply #38 on: June 10, 2019, 09:19:44 am »
On a related note, I wonder if anyone ever tried making, say, a tunable quartz or mechanical filter, that uses a stretchy or bendy substrate as resonator, so that the dominant mode can be varied over a modest range.

That was exactly my thought, too.  To experiment with a decapped Quartz by pressing it in a vice with rubber jaws, and see how much the resonant frequency can be shifted without snapping the crystal.  If it would be enough, the next step would be to replace the vice with some piezo material in order to electrically control the mechanical pressure, and thus the resonant frequency.   ;D

Most probably this idea was already tested before, and the results were nothing spectacular, so we don't have today such a device commercially available, but still, it will be an interesting experiment to tinker with.   :-DMM

My gut feeling is that this would provide a small tuning range and bad frequency stabilty (since the inherent stability of the quartz crystal is compromised by external, macroscopic mechanisms which will be affected by temeperature drift, vibration etc.). The worst of both worlds.  ;)
 

Offline IDEngineer

  • Super Contributor
  • ***
  • Posts: 1950
  • Country: us
Re: Why are spectrum analyzers so expensive?
« Reply #39 on: June 10, 2019, 05:37:01 pm »
In any case the important part is not yttrium, it's that it's a garnet made with iron, and yttrium happens to be necessary to make that happen.  And then that the result is magnetically tuned, yes.
I've studied YIG's for quite a while as a sort of side interest. Like the other respondent(s), it still feels like the stuff of science-fiction. Kinda like "red matter" in Star Trek or that beryllium time-travel device in Galaxy Quest. "Captain, we can't increase the field through the YIG sphere! We're already past the redline! Nobody's ever even tried to go this far before!"

IIRC, the magnetically tunable behavior is believed to result from the asymmetric location of two of the Fe atoms in the crystalline matrix. It took me a while to learn that. For a long time I thought it was due to the yttrium content, because in many applications yttrium behaves a lot like a rare earth - which have consistent outer shells and differ by how their 4f shells are filled, giving rise to their similar chemical behavior yet differing magnetic behavior. However, in the case of YIG, your comments appear spot-on... what's needed is a crystalline matrix that asymmetrically positions the Fe atoms and Y+I does that.

The one thing I haven't been able to discover is who the heck originally figured this out - or even conceived of the concept that properly, asymmetrically positioned Fe atoms would yield magnetically tunable resonance in a physical object, with linear response to boot. That's a person or team I'd like to know more about. What the heck else did they do that doesn't get enough publicity?
 

Offline ejeffrey

  • Super Contributor
  • ***
  • Posts: 4034
  • Country: us
Re: Why are spectrum analyzers so expensive?
« Reply #40 on: June 10, 2019, 06:18:53 pm »
I haven't looked at the behavior specifically of YIG spheres, but in other non-linear optics (like an EOM) the normal requirement is that you need a crystal that does not have inversion symmetry in order to get a second order non-linearity.  In that case your output field is proportional to (input field x control field) (aka a bilinear response -- linear in two different fields).  This has a straightforward analogy in non-linear circuits: if you have an analog circuit with symmetric clipping diodes you get only third order non-linearity (and higher odd orders).  You need an asymmetric circuit to get even order distortion.

If you have an inversion symmetric crystal you only get 3rd order non-linearity which usually means adjustment is quadratic in the applied field, which tends to require much larger tuning fields.
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf