Which audio analyzer?

[slow_rider]

I decided it's time to upgrade my rig with an audio analyzer. I do need one! Simple and quick to setup THD measurements alone justify the purchase.

I'm looking at two possible options:

1. HP 8903B
2. Keithley 2015-P

The price both both of these units, used is about the same ($650 - $750). It seems like the Keithley is a bit easier to use to generate plots because of it's more modern communication protocol support.

I need an analyzer to create plots of THD / THD+N / SINAD (freq. sweeps) also would not hurt to have a high quality DMM as my old Fluke is starting to act out (it's mechanical issues but take too much time to sort out). So which one would you take? HP - Agilent quality or Keithley which is kind of Tektronix?

[max-bit]

Hi
Is it better to buy a sound card? eg SB E-Mu? 0204 or 0404? or any similar?

[slow_rider]

Do any of these cards have software which enables them to run the same type of tests the above instruments can?

[max-bit]

The web is a bit of free software.
But even if you buy any software paid., It will be cheaper ...

[edavid]

https://www.eevblog.com/forum/testgear/distortion-analyzer-suggestions/

[IvoS]

I use Asus Xonar STX sound card with Arta soft. Here is the test signal generated using the card in loopback.

[macboy]

The 8903B has a fully analog signal generator and analyzer. On mine, the residual (output loopback to input) is about 0.0020% or less (80 kHz low pass). I believe that the Keithley 2015 is digital/DSP based. Its residual distortion is quite a bit higher, but it does other interesting things like showing you the amplitude of individual harmonics. The 8903B won't do that, but you can connect its notch filter output to a scope to visualize the distortion signal and to run a FFT on it.

A limitation of a digital based system, including a soundcard + software, is the upper frequency limit. You can easily do accurate distortion measurements at 1 kHz, since the harmonics will be in the audio band. But what about distortion at a 20 kHz fundamental? With a 192 kHz soundcard, the bandwidth is 90 kHz or less, so you will only get three harmonics (40, 60, 80 kHz) in your measurements. The 8903B has a measurement bandwidth of 750 kHz, and can generate and analyze with a fundamental up to 100 kHz.

[slow_rider]

So this gets a bit complicated since the devices are too different to be easily compared. I did not think about a sound card based solution mainly because of the BW limit however another limitation of sound cards that I don't want in my rig is the low voltage swing, both inputs and outputs. Usually these are about 2Vrms max. In almost all the tests I am performing the return signal is over 2V, so this calls for an attenuator which is another components in the test rig and signal chain...

I'm sorry if it gives the impression I am lazy, it's just bad experience from the last time I tried a software based solutions. I bought a dedicated external sound card to use with Visual Analyser software (no typo). I ended up spending a couple of weeks debugging the electronics in the dedicated hardware which prevented any useful measurements at 1KHz (posted fix on DIYaudio) after that I tried using the software ZRLC metering and got poor results even after calibrating the damn thing with Kryptonite resistors... Long story short. I ended up buying an LCR meter and wasted two weeks instead of getting anything done. The real cost of this "savings" solution was probably enough to get me a proper HP spectrum analyzer. So I'm a bit doubtful when it comes to easy software based solution even thought ARTA seems very well written.

The HP is an HP, it gets the job done and you can count on it. I worked with a lot of HP / Agilent gear at one of my previous jobs and I've asked my boss why are we getting everything Agilent they are kind of expensive and we get better quotes from others. He told me that the amount of downtime we save by having their gear almost never fail on us is worth the extra and a lot more. Today I understand what he is talking about with some 20 and 30 year old gear I have around that still works wonderfully.

Back to the analyzers though... Since I am mainly doing AF stuff, I need freq. up to 20KHz fund. for THD type measurements. The HP is an overkill - which is a good thing! The Keithley is a killer desktop DMM - no doubt about it. The noise floor on the Keithley seems kind of low if I remember the datasheet. It does cool things like the harmonics amplitude, but having that features means also having less accurate readings from the instrument.

[edavid]

Did you read the other thread?   It covered exactly the same ground.

[bobwidlar]

I have same kind of question, I didn't want to open a new one or revive that old topic.

Well, I need to test a pretty linear ADC. I mean, we're talking about -110dB THD and -120dB SNR (20Hz - 20kHz,a not A-rated)  here. So, finding a proper analog signal source to satisfy this linearity and noise floor needs is one problem. The second one is to have a reference ADC or audio analyzer to at least test the test setup itself.

I'm probably going to make my own signal source with a top-notch DAC and some LC lp filtering. But, with which instruments should I measure this kind of low level THD? NI has a audio analyzer card http://sine.ni.com/nips/cds/view/p/lang/nl/nid/13634 which has -107db THD contribution by itself, and it's kind of expensive.

Is there anybody who can think of an alternative? I've read all the posts on both topics and the best THD measurement limit I've seen was around -96dB. Should I just use a lower spec instrument like a DSA or that cheap keithley DMM+THD meters but make my own notch-filter + amplification?

[edavid]

Your user name is somewhere between macabre and offensive...

[TMM]

Soundcard + arta + holmimpulse

[max-bit]

Each solution has its pros and cons.
Paying $ 1000 for old hardware (in addition large).
For amateur solutions I think that a better solution is to card + software (ARTA etc).
If you want to Profesionalne systems are still AUDIO PRECISION, Brüel & Kjaer and others already listed companies. However, the purchase price for the amateur are prohibitive prices

[bobwidlar]

Your user name is somewhere between macabre and offensive...

Oh, why?