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Old 25th January 2012, 06:18 PM   #1
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Default Spectrum Analyzer: What to get ? What to look for in a spectrum analyzer ?

I am interested in getting a spectrum analyzer.
I work on mostly tube gears.

Could you guys tell me which one to get and what to look for in a spectrum analyzer ?
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Old 25th January 2012, 07:13 PM   #2
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if you're happy with a pc based solution try pete milletts:
Test & Measurement interface for Soundcard
i just built mine, but its not tested yet :-)
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Old 25th January 2012, 07:17 PM   #3
tomchr is offline tomchr  United States
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What do you intend to measure with the spec an?

A spec an is useful for figuring out the composition of a frequency spectrum. So, for example, if the distortion of an amp is mostly even order or odd order harmonics. It's also useful for debugging hum issues.
It can be used to measure THD, though, only the fancier spec an's have a THD function. For measuring THD, I find that a dedicated distortion analyzer works the best.

What to look for in a (used) spec an:
- Does it work?
- Is the screen bright and in focus?
- Frequency range
- Minimum and maximum frequency span
- Resolution bandwidth
- Noise floor
- Input voltage range(s)
- Input over-voltage protection

Note that an external computer sound card with a sound card interface (see the one made by Pete Millett as an example) can be a pretty darn decent audio spec an.

On my website, I have a write-up on distortion measurement equipment. You may want to take a look at it.

~Tom
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Old 25th January 2012, 07:52 PM   #4
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Well, the first use is to help tune my active tube crossover.
I don't know how to use a spectrum analyzer yet so I don't know what's all to measure with it. It seems like an essential equipment. I plan to use it to check the performance of amps, preamps, and etc.
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Old 25th January 2012, 07:58 PM   #5
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Could you guys tell me the pros and cons of the PC option and an actual spectrum analyzer ?

Which specific spectrum analyzer would you recommend ?
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Old 25th January 2012, 08:29 PM   #6
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The PC sound card based analyzers work quite well for audio analysis (especially 24 bit ones), but are typically limited to just the near audio band.
They are fast too and dual channel (useful for comparing input to output in real time). And storing spectrums are easy on the PC. You can do special tracking oscillator and analyser methods too for filter analysis.

A more conventional swept filter spectrum analyzer will typically be slower, but maybe higher resolution, and will provide wide band analysis (like say 5 MHz for a 7L5 Tek scope plug in). This is useful for finding HF resonances (NFDBK stability checking) or parasitic oscillations above the audio band. (although one would want more like several hundred MHz capability for that) Some units will have an optional or built in tracking oscillator for filter analysis. If you are using any kind of switchmode power supply electronics in your build or system, then a several hundred MHz unit will be needed to check for EMI compliance and interference effects.

With either kind, you need to be careful about overloading the input to the analyzer/sound card. Easy to blow them up, and costly.
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Old 25th January 2012, 08:42 PM   #7
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DL4YHF's Audio Spectrum Analyser

Get Speclab.

It's free. The finest piece of amature software I've ever seen. Absolutely superb!
Doc
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Old 25th January 2012, 10:14 PM   #8
tomchr is offline tomchr  United States
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Quote:
Originally Posted by manp111 View Post
I don't know how to use a spectrum analyzer yet so I don't know what's all to measure with it. It seems like an essential equipment. I plan to use it to check the performance of amps, preamps, and etc.
An audio spectrum analyzer is fairly expensive. I paid $800 for mine (early 1980'ies vintage). If you don't know what to do with it, I suggest using a computer sound card until a need for a dedicated instrument arises.

"Check performance" is not very specific. You can check performance with an oscilloscope and a voltmeter. Which parameters are you looking to measure with the spec an? Don't expect to be able to measure THD below 0.01 % on 1980'ies vintage spec an's. My HP 8903A distortion analyzer measures down to 0.003 %. If you want lower, you'll be looking at the $50k Audio Precision SYS-2700, AP SYS-1, or similar.

If you have money burning holes in your pockets, look at any of the Agilent audio spectrum analyzers. They're probably in the $30k~$50k range. If you don't have that much money burning holes in your pockets, I suggest looking at an HP 3562A Dynamic Signal Analyzer. It's a precision audio-range signal source plus an audio spectrum analyzer (FFT based). That'll set you back around $1k maybe less if you happen to find a good deal, maybe $2k~$3k for a calibrated unit. There is also a dedicated audio spec an in the HP 35xx-series. I forget its number. As you can see on my website (deep link in my previous post), there's also the HP 3581A which can be used with an XY plotter as a spec an. There's also the HP 3580 which is the same 1970'ies vintage as the 3581. Even older gear like HP 181 (I think it is) might work as well, though, I forget its lower frequency limit. That's in the $400-ish range.
Tektronix also has some offerings that went into the rack-based oscilloscope boxes.

Or you could spend $100~$200 on a solid external USB/FireWire sound card and get going that way. TrueRTA is a pretty good software package as far as I can tell. A soundcard-based solution does have some shortcomings but is pretty cost effective.

~Tom
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Last edited by tomchr; 25th January 2012 at 10:22 PM.
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Old 25th January 2012, 11:16 PM   #9
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Quote:
Originally Posted by tomchr View Post
There is also a dedicated audio spec an in the HP 35xx-series. I forget its number.
That would be the HP 3580A. I have used one of these for years and they are great for sweeping filters. They top out at 50KHz, but the adaptive sweep and 1Hz bandwidth along with the built in tracking generator fills the bill very nicely. Unfortunately none of the older HP analyzers mentioned are supported by mother. And they are not easy instruments to support without scrap units. Some of the parts inside are matched and selected by HP making upkeep difficult if they fail.
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Last edited by HollowState; 25th January 2012 at 11:18 PM.
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Old 25th January 2012, 11:30 PM   #10
chrish is offline chrish  Australia
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Ah, that looks like just the instrument required to work out the resonant frequency of your washing machine on spin cycle
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