Spectrum Analyzer opinions

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tortello

tortello

Member
2001-07-22 7:40 pm
Genoa (ITALY)
I'm thinking to buy a spectrum analyzer, above all for distortion measurement purposes, to place side by side to my HP 334 distortion analyzer.
I'm undecided whether to buy an HP3580 analogic s.a. ( for about $ 500) or a most modern FFT device, such as a Pico Technology ADC 212, with at least 12 bit of resolution.
Any piece of advice?
Thanks
 
A

alaskanaudio

Member
2001-09-12 3:18 pm
Fairbanks, Alaska
www.audioamps.com
Spectrum Analyzer versus FFT

tortello,

A spectrum analyzer for audio use is not a really not suitable device these days. The display update rates are just to slow with the low bandwidths and slow sweep speeds that are required.

The problem with a spectrum analyzer is that it only measures one frequency at a time, while a FFT unit measures the whole spectrum at once. Math takes care of the signal processing. Even a old computer running at 100 Mhz with a inexpensive 16 bit sound card and proper software will be a lot faster than a Spectrum analyzer.

Thus I think that a better way to go will be to get some FFT software to run on your computer along with a good sound card. You will be a lot happier with such a setup than the old HP instrument. I have considerable experience with both the spectrum analyzer and the FFT units. I have had lots of chances to buy the instrument you mentioned but doing so would be a great step backwards from my FFT setup.

With a inexpensive sound card you would be limited to around 22 Khz as the upper frequency limit. The accuracy of measurements will depend to a great degree on the quality of the sound card used. Your total investment for software and a suitable sound card would be about the same as the old HP unit if you already have a useable computer.

Another benefit with the computer setup is that you would also be able to generate low distortion test tones with 16 to 24 bit resolution depending on the software you use and the of course a suitable sound card. The distortion levels of my present sound card are rougly down -110 DB, while the noise floor over most of the frequency range is around -130 DB as I recall.

John Fassotte
Alaskan Audio
 
tortello

tortello

Member
2001-07-22 7:40 pm
Genoa (ITALY)
First of all, thanks for your reply, John.
I know that the 3580's "refresh" speed is very slow, to obtain the maximum resolution (1Hz) the sweep time is about 200 second/div., and we have to cover 10 divisions: 2000 seconds in effect are a lot of time.
I've tried - every time I re-format my HD ;-) - the Soundtechnology Spectralab trial version, very nice software, I think, even if a lot expensive: the limit there was my soundcard, a usual Soundblaster 128, the frequency response is not so flat (due to the awful anti-aliasing filter, I think), and 22KHz for the maximum frequency seems to be a little coercive, in my (outsider) opinion: it would be better to extend the measure beyond the audio band(?).
The phase plots are also disguised by the delay gap between the two channels, even if some informations can be anyway deduced, but the s.a. don't do this at all...
I've tried also with a Terratec EWX 24/96 soundcard, but was not compatible with the software, except the 44Ksa/s, 16bit method.
So, what is your Soundcard (maybe is avaible also in Europe)?
I'm not a skilled technician, forgive my simple-mindedness, but in THD and IMD measurement, that are static procedures, how acquisition speed is important?
How for my nervous system? :)
With regard to the wave generator, I've a low distortion unit ( about 0.003%, -90dB), it seems to me preferable to limit ground loop noise due to the PC > DUT > PC chain, but my experiments are not so comprehensive.
As you can see, I've a pea-soup mist in my mind...
Thanks for your (old & new) advices

Marcello :confused:
 
J

Joe Berry

Member
2001-03-15 6:15 pm
USA
Tortello,

For what it's worth, I have the PICO ADC-216 and am happy with it. I don't need the 212's bandwidth (I have a separate 100 MHz scope), but if I did, I might have opted for the 212 instead. I'm a bit skeptical of PC sound cards for measurements, due to the noisy environment they operate in, but having never tried one, I can't offer any comparison.
 
A

alaskanaudio

Member
2001-09-12 3:18 pm
Fairbanks, Alaska
www.audioamps.com
Sound card with balanced inputs and outputs

Marcello,

The sound card I use at the present time is the Card Deluxe in 24-bit mode. This card has balanced inputs and outputs. It also has digital inputs and outputs.

It can be found at www.digitalaudio.com

You should be able to use the card to around 48 Khz with the highest sampling rate that Spectra Lab allows.

You are correct in stating that having the ability to measure higher frequencies is beneficial. I have two HP 339 distortion analyzers that I use when I need to make such measurements. These also provide low distortion test signals when needed.

For general use I find that my computer driven FFT system is much more convenient than using the HP339 distortion analyzers since I can get a much better picture of the output harmonic structure of the item being tested. For general use low frequency test signals into the upper mid range or the low high frequency range are fine to get a real good idea of what the performance of the item being tested is. Thus the 48Khz upper limit of a good sound card and proper FFT software make such a system extremely useful.

John Fassotte
Alaskan Audio
 
A

alaskanaudio

Member
2001-09-12 3:18 pm
Fairbanks, Alaska
www.audioamps.com
More info

Marcello,

I would say that the flatness of frequency response over most of the useable frequency range is well within 1/10 of a decibel with the set up I use.

More info on the Card Deluxe can be found here at the link below. They used Spectra Lab for their tests and show results similar to what I see here.

http://www.pcavtech.com/soundcards/CardDDeluxe/index.htm

The least expensive FFT software (and it is truly good) is “HPW Works”. If you go to the list of customers on the web site listed below you can see that our name is mentioned as a user.

http://www.hpw-works.com


There is also some information available for the HPW Works software from the RME website. The link listed below shows some of HPW Works screens. Thus RME also uses this software for testing their digital products.

http://www.rme-audio.com/english/techinfo/adi1.htm

RME is another company that makes excellent sound cards.

Best regards,

John Fassotte
Alaskan Audio
 
J

Joe Berry

Member
2001-03-15 6:15 pm
USA
Marcello,

I've not actually measured the 216's response, but I don't notice any difference between it and my scope (TEK TDS220) when operated within its bandwidth. I will say that the 216 does not make a particularly good general-purpose scope substitute as its sampling rate is just too low for good HF waveform fidelity. If you were going to buy only one instrument of this type, I'd go with something like the 212.
 
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