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Old 8th July 2012, 05:10 PM   #21
Pano is offline Pano  United States
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Originally Posted by richiem View Post
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.
How about the PicoScope 4262? Real 16 bit, 20 bit "enhanced", 5 MHz. And won't break the bank like the AP.
I've used the older Picoscope stuff (10 MHz, 8 bit) and found it very handy indeed. It's worth a look.
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Old 8th July 2012, 05:18 PM   #22
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Originally Posted by 1audio View Post
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.
Can you point to any designs? I'd love to have one of these to as a reference.
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Old 8th July 2012, 05:37 PM   #23
1audio is online now 1audio  United States
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Low distortion oscillator tests measurement circuits - 2012-05-03 04:33:52 | Test & Measurement World

I was thinking of making an expresspcb layout for it in my copious free time. It does use readily available parts and the distortion spectrum shows the differences in parts used for the filters. I like the LDR for the gain adjust. However it would be slow to settle and needs to be used very carefully since LDR's are not very linear on this scale.

The one I use is a variation on this: http://www.shibasoku.com/download/avc/ag15c_e.pdf. They are rare so not really practical to find. Its about 10 dB better than the KH4400. And 4X the size so not too convenient.
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Old 8th July 2012, 06:45 PM   #24
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Thanks. That looks like the beginning of a great project. If you do manage to set aside some of your copious free time to to do the PCB layout, I'd be interested. A polystyrene cap version, of course!
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Old 8th July 2012, 07:04 PM   #25
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super low distortion oscillators aren't the only path to high resolution distortion measurement

I keep pointing to indirect multitone IMD testing as allowing relatively low quality oscillators - or DAC outputs to give very high resolution of DUT nonlinearity

basic premise - create different frequency test sines with separate osc or DAC - they only have each fundamental and its harmonic distortions (and possibly spurs)
then combine in a low distortion summing circuit, apply to amp/test circuit/device, look for IMD sum and difference distortion products in output
filters can knock down test signal amplitude, boost specific IMD frequencies - then even cheap PC soundcard ADC/DAC can manage 160 dB distortion measurement floors with averaging
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Old 9th July 2012, 01:30 AM   #26
klewis is online now klewis  United States
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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.
Click the image to open in full size.
Yes, I would be interested. Just hooked my monitor out up to my spectrum Analyzer. I would be interested in understanding how you scale the monitor output.

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Old 9th July 2012, 01:46 AM   #27
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OK, I'll take a shot tomorrow and post it. The monitor output scale is under the "Special" functions. You type in a code then hit the Special key. I'll report more on it tomorrow.
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Old 9th July 2012, 03:53 AM   #28
richiem is offline richiem  United States
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I'm with jcx -- a very interesting road to travel when the goal is superior performance of audio amps.

I've also pointed out my (ahem) improved build of Bob Cordell's SV oscillator in a Heath IG-18 with 1kHz THD (not THD+n, of course) much less than 1ppm (< -120dBu), and 10kHz THD only somewhat higher. These are pretty dang good results.

Simple magnetic shielding of the toroid power transformer has reduced all line-related noise peaks to less than -120dBu as well.

If you want to build this as a single frequency oscillator, you can get much higher performance (under 0.00003% or -130dBu) by tweaking the AGC JFET's operating point:
IG-18 #2, the BIG-18
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Old 9th July 2012, 02:26 PM   #29
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Below is a shot of the HP 8903B monitor out set at 0dB & 20dB Post Notch Gain.
The generator is set to 441Hz, 2V RMS which was adjusted to read -6dBFS on the spectrum analyzer.
Default Post Notch Gain seems to be set to Automatic, but you can change that. Just type in these number on the keypad and then hit the SPCL (special) button
  • 3.0 Automatic
  • 3.1 0dB gain
  • 3.2 20dB gain
  • 3.3 40dB gain
  • 3.4 60dB gain
Oddly, the 0dB & 20dB settings give higher THD+n reading than Automatic, 40, 60 - which all read the same. Gotta look into that.
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Old 9th July 2012, 09:36 PM   #30
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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.
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