How Distortion Free are the Distortion Measurers?

[Dirk95100]

[klewis]

My experience matches Dick's, a spectrum analyzer is far more valuable than a the meter. I see a lot of junk on the spectrum between 40 and 50khz that is from my computer and computer monitor power supply. My hp8903b does not decriminate these extra spikes from harmonic spikes when it calcs thd + N. For a recent 10kHz measurement of a buffer with signal supplied by a KB 4402, I calculated the thd at 0.0009% but the HP 8903 said 0.013%.
)
Ken

[Pano]

Good point. Below is a HP 8903B output vs M-Audio soundcard output, both going into the unbalanced input of the soundcard. M-Audio is green, HP 8903B is orange. IIRC, this was 1V RMS.

The 8903B has a monitor out that is the signal after the notch filter, and it can be scaled. It's handy for seeing what's being measured as THD+n. I could post a graph of that on Monday, if anyone is interested.

[klewis]

Interesting idea, I haven't tried hooking the monitor out up to the spectrum analyzer.

What most here on the site have said is that the Thd value without the spectrum doesn't have much value. I've come to understand why. The spectrum can tell you a lot; bad power supply, weak psr, bad grounding, pour shielding, where the harmonics are... Good stuff!

[Conrad Hoffman]

I don't know about AP, but if I don't turn off the CFL on my test bench when using the 8903B or the Cordell analyzer, all bets are off.

[richiem]

Hi Ken -- RE hooking up the monitor output to the spectrum analyzer -- The self-distortion of the 8903, as least for the 8903E I used to own, makes it fairly useless in further investigation. I was able to see the somewhat more than 115dB of fundamental null, but as to anything else, no so much.

I also had the problem that the 8903 monitor output gain varied in some way with analyzer gain so, unlike the 339A, in which the monitor output 1VRMS full scale is always referred to the fundamental full-scale of 1VRMS, thanks to the auto set-level, the 8903's output has a hard-to-determine full-scale referent.

There probably is some work-around that I couldn't find in the short time I owned the 8903E -- maybe you can count the clacking of the various relays and figure out the final monitor gain setting from that.

[RNMarsh]

Originally Posted by klewis

Interesting idea, I haven't tried hooking the monitor out up to the spectrum analyzer.

It works very well.... I used an HP spectrum analyzer with my Sound Tech THD/IM instruments output a couple decades ago. Then I got the AP and sold everything else. Now I am at the limits of the AP. Now what? Loooking for a killer ADC now to use with software/PC.

[richiem]

@ RNMarsh -- so am I, and everyone else too. I want 26 bits or more and 768kHz Fs. But right now, I'd settle for 20 bits and 1MHz Fs in a USB box so I can see the products form a 100kHz oscillator. Let us know if you find anything or design something...

[1audio]

1) analyzer sensitivity to external stuff. if properly boxed up (no covers off) most analyzers (all the ones I have tried) are pretty well shielded and don't pick up noise or hum from the transformers. That won't hold true when you connect to an external device. getting the two boxes to play well is very difficult. When making low noise measurements I shut off the lights on the bench. I don't have a computer near the bench (no room) which helps.

The Boonton 1120 I use has the monitor output, which I connect to both a scope and an FFT box. I also hacked in an input monitor that follows the input signal so I can see what is coming in. Saves lots of time when things are not right. The gain changes so you can't be sure of the levels you see, but the relative content is obvious, which is what you want to know.

Also remember the measurement is THD+N so everything in the band, including harmonics and noise is added. And the Mfr's specs are a clue as to where they stopped fighting with the low level stuff. If its spec's at -95 dB then you may find the input circuit will have a noise level at just below -100 dB. There would be no path to -110 dB in that system.

I have a super oscillator (-120 dB THD). Its a great sanity check. There are several published designs that get to the -120 dB mark. I think a battery powered fixed frequency one with a good attenuator on its output would be a great tool.

[jan.didden]

Originally Posted by richiem

@ RNMarsh -- so am I, and everyone else too. I want 26 bits or more and 768kHz Fs. But right now, I'd settle for 20 bits and 1MHz Fs in a USB box so I can see the products form a 100kHz oscillator. Let us know if you find anything or design something...

Audio Precision has a box that does 24bit/1Mhz FFT.
Maybe they take your car as downpayment

jan