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Old 17th September 2004, 10:48 PM   #1
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Default Low Distortion Signal Generator

(Mods...had no idea where to put this...seemed more related to testing of the SS gear I fool with, so here it is. Move if you feel appropriate)

For years I've had a very old Waveforms tube sine-wave generator, but lately it has developed some problems so I'm looking for a more modern replacement.

I'm using a distortion analyzer to check out my work, and I'd like distortion of 0.01% or less. I have a opportunity to grab a used Leader LAG-126, which is supposed to be capable of less than 0.005% thd. Attractive specs, but it will cost around $200 US. Seems high for a used piece nearly 10 years old.

What other ultra-low distortion generators are out there? What do you all use? I'm really too busy to build something, and the DIY circuits I've seen are not capable of distortion readings as low as the Leader I found.
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Old 17th September 2004, 11:15 PM   #2
SY is offline SY  United States
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I stuck it in "Everything Else" for lack of a better place to move it.

In any case, what frequency range do you need? If you don't need really high frequencies, two really cheap and easy solutions are using a soundcard and software or a CD player with a volume control. The distortions are surprisingly low- with the soundcard that came stock with my POS HP, I get 0.01% THD at 1 kHz. S/N ratios can be in the -90 to -100 dB range.
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Old 17th September 2004, 11:45 PM   #3
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What worries me is that digitally based oscillator equivalents might have a lot of higher order distortion. This is bad, because it obscures what you are measuring for.
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Old 17th September 2004, 11:45 PM   #4
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I also considered the soundcard option, but I'd really like to be able to go to 100KHz, which a soundcard ain't gonna do. Still, for distortion measurements, the standard audio frequencies would be OK.

I have a computer near my work area with an Aureal soundcard in it. Using software from Daisyweb Labs. It's actually a pretty decent soundcard, but with this software the best it can manage is about 0.2%. Better software maybe? What did you use?

Also, the concerns that John voices are also on my mind. Plus, a computer is pretty cumbersome commpared to a dedicated piece of gear.
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Old 17th September 2004, 11:50 PM   #5
SY is offline SY  United States
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I used DACtester, with the level set up as high as I could go. If you don't use all the range of the DAC, distortion will be higher. I've had good results with the Daisyweb signal gen, but haven't measured it in detail to see how it compares- the user interface is clusier than I'd like.

John, there may be higher order stuff there, but I couldn't see it with the HP3581A you loaned me. If I were worried about it, I'd just throw together a simple lowpass filter. But if it's good enough for HP...
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Old 17th September 2004, 11:53 PM   #6
paulb is offline paulb  Canada
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Check out Jim William's app note (about halfway through):
http://www.linear-tech.com/pdf/an43.pdf
Also the ESP site has a couple of projects:
www.sound.au.com
There was also an article in AudioXpress a while ago.
Should be plenty more circuits around.
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Old 18th September 2004, 12:37 AM   #7
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SY...any links to this DACtester?

paulb...good reading, but I really don't have the time to construct something, although I was pretty amazed by the results.

Any thoughts as to whether $200 is too much for the Leader generator?
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Old 18th September 2004, 01:25 AM   #8
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DACtester was some freeware I downloaded a long time ago. Don't know the URL, but it should be easy to find.
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Old 18th September 2004, 03:25 AM   #9
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I remember seeing this on my ebay browse yesterday. Still have a mind like a steel trap! It's a good old HP 200CD wide range tubed CW audio generator. I used to have one. The price is right!

http://cgi.ebay.com/ws/eBayISAPI.dll...sPageName=WDVW
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Old 18th September 2004, 08:12 AM   #10
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I use a Wien-bridge oscilator that I made myself. To get the lowest THD I use a 220Volt/3Watt lamp for amplitude stabilisation and it works fine .. < 0.001% THD.. opamp was a either a opa627 or NE5532, made no difference for THD.. all in all is is an enormously little simple thing that has very low THD... I think because I used a lamp with the biggest thermal timebase I could find..
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