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Old 9th July 2012, 10:59 PM   #31
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The AP2722 2nd harmonic is -129dB, and the 3rd -121dB --
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Old 9th July 2012, 10:59 PM   #32
1audio is offline 1audio  United States
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Quote:
Originally Posted by Pano View Post
Oddly, the 0dB & 20dB settings give higher THD+n reading than Automatic, 40, 60 - which all read the same. Gotta look into that.
Makes sense. The lower gain will leave the post notch amps noise floor as the limit. The ranging may not be able to make sense of that (or why have the extra gain?) With the extra gain the noise and distortion will be above the noise floor and the calculations will make sense.
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Old 9th July 2012, 11:12 PM   #33
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@ Loudthud -- well, just as you might expect, lowering the signal 20dB with the attenuator raises the level vs THD plot by 20dB. So the red curve isn't real helpful. I'm not familiar with the specs on the Sys 1 -- is it 50 ohm output Z? If it's not, I would avoid using 50 ohm attenuators or terminated cables....
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Old 9th July 2012, 11:38 PM   #34
klewis is offline klewis  United States
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Quote:
Originally Posted by Loudthud View Post
Interesting discussion. I'm not up to speed on the whole FFT thing, but I have an AP System 1. Below is a graph of distortion vs oscillator level with a cable from output back to input. Balanced I/O was used with adapters to a 50 ohm BNC coax cable. Three different frequencies are shown. The red graph is with a 10X 50 ohm attenuator inserted in the cable. Note that measurements were made with a VGA monitor sitting on top of the AP system. I'd be interested to see what newer AP systems look like.
Loudthud,

Try plotting the power spectrum with an input voltage of 1 Vrms at 1kHz over a range of 0 to 10kHz. It will show the immunity of the system to low frequency noise from a typical power supply. Then try a plot of 1 Vrms at 10kHz over a frequency of 0 to 100kHz. This will show immunity to switching power supplies used in computer monitors, computers, fluorscent lights, etc.

Ken
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Old 9th July 2012, 11:43 PM   #35
klewis is offline klewis  United States
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Quote:
Originally Posted by 1audio View Post
Makes sense. The lower gain will leave the post notch amps noise floor as the limit. The ranging may not be able to make sense of that (or why have the extra gain?) With the extra gain the noise and distortion will be above the noise floor and the calculations will make sense.
I tried it out myself last night, used the -40 setting on the 8903 and used the math function on the spectrum analyzer, just divided the spectrum by some power of 10 (I forgot which power ) of a known spectrum until I got a good match on the 2nd H from my other methodology. Worked well at 10kHz fundamental, but had a lot of noise at 1kHz fundamental.

Thanks for the tip!

Ken
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Old 10th July 2012, 03:19 AM   #36
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Originally Posted by klewis View Post
Loudthud,

Try plotting the power spectrum with an input voltage of 1 Vrms at 1kHz over a range of 0 to 10kHz. It will show the immunity of the system to low frequency noise from a typical power supply. Then try a plot of 1 Vrms at 10kHz over a frequency of 0 to 100kHz. This will show immunity to switching power supplies used in computer monitors, computers, fluorscent lights, etc.

Ken
The AP system one does not have an internal FFT. However connecting one to the monitor output works well. We used a Picoscope 8 bit unit and it worked very well the Picoscope software is as good as any digital scope.
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Old 10th July 2012, 04:22 PM   #37
klewis is offline klewis  United States
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Well, I'm so glad that I didn't spring for a System 1... I've gotten so much value out of my HP3562 along with Dick's Twin-T notch filter, they are my primary tools.

Ken
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Old 11th July 2012, 04:42 AM   #38
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"...The AP system one does not have an internal FFT..."
-------------------------------------------------------

What a System One has depends on the options it was ordered with.

The SYS-22 units are analog only with no FFT.

The SYS-222 units [System One + DSP] add generation and analysis of multitone signals, FFT spectrum analysis, and MLS quasi-anechoic acoustic testing.

SYS-322 units [Dual Domain] also have AES/EBU balanced and unbalanced digital inputs/outputs as well as consumer coaxial and optical I/O for digital domain generation and analysis.

In addition there are other useful options for generation of tone burst, square wave and noise signals, A-weighting, de-emphasis & low pass filters, wow & flutter analysis, and IMD.

If considering one of these units be informed!
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Old 16th July 2012, 05:57 PM   #39
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Default Low distortion oscillator board project

I was doing some research on the project, particularly on the AGC circuits and got sidetracked by my Fluke 510A. Here are some notes on my efforts: http://www.diyaudio.com/forums/blogs...reference.html I got it down to .0003% THD+N in a 30 KHz band.

In short, I thought I would look at the options and alternatives before I layout the one from the article. There are three typical oscillators circuits used, two wein bridge forms, one with a single amp and another with two (both inverting to make them more linear) and the state variable oscillator. There are several AGC control variations in use for this, the FET (used by Fluke above and KH in the 4400) the LDR in the article linked above, using an analog multiplier, and finally using a variable gain amplifier. Each has pluses and minuses. I wanted to do more research before committing.

Now that the Fluke distraction is back on the shelf (until I can get a cal done on my voltage reference) I'll get back to the project as time permits.
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Old 18th July 2012, 07:29 PM   #40
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The AP does have internal FFT to 24kHz on the SYS2722 -- but the FFT on the Stanford Research SR1 goes up to 200kHz.

Isn't it amazing how an elderly piece of equipment like the Fluke or Boonton takes over all your activities?
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