Low-distortion Audio-range Oscillator - diyAudio
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Old 25th January 2012, 11:32 PM   #1
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Default Low-distortion Audio-range Oscillator

I am looking to add a low distortion audio sig gen to my bench. I have been watching eBay for good deals. Any worth bidding soon rise above my budget.

Anyway, I starting looking into a DIY Signal Generator. My research turned this up. Low-distortion Audio-range Oscillator

Anyone ever build this? Sounds good but is it? There is no build tips or PCB layout. Just a diagram and parts list.

This author has a book of all his circuit designs but comes at a steep price. However, my local library has a copy and I will pick it up sometime this week.
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Old 25th January 2012, 11:54 PM   #2
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Looks like a variation of the Wein Bridge oscillator. Look at the schematic of the HP 209A. There is much info here leading to even improving the 209. Also check out the oscillator in the HP339 or 8903 distortion analyzers.
But for low distortion, the Wein Bridge design is the way to go.
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Old 26th January 2012, 01:13 AM   #3
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For a reasonably well-documented DIY project, look at the oscillator section of Robert Cordell's THD Analayzer at < http://www.cordellaudio.com/papers/thd_analyzer.pdf >. Even after 30 years, the performance holds its own against the best commercial units. This design has been discussed in several threads here on the Forum, e.g. < My implementation of the Cordell Distortion Analyser >. For that matter, the general topic of low-distortion oscillators has been discussed in quite a few Forum threads, e.g., < Can we improve this low distortion sine oscillator ? >. Use the "SEARCH" feature to locate others.

A currently active DIY project using the same basic circuit architecture as Cordell's oscillator can be found at < http://www.users.on.net/~glenk/uldo/uldo.htm >.

For an introduction to the topic, look at < http://www.moorepage.net/RC.html >. Dick Moore's web pages (see < http://www.moorepage.net/index.html >) include several DIY efforts to upgrade or optimize the performance of several commercial test equipment models. A similar effort can be found at < http://www.tronola.com/html/ig-18_mods.html >.

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Old 26th January 2012, 09:24 PM   #4
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The lowest budget solution is to record test tones to a CD and use a CD player as your source.

But for everything you need to know about oscillators, Jim Williams did an application note way back in June 1990 about Bridge Circuits for Linear Technology AP #43. I don't have a current link. It had pages on great oscillators.
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Old 26th January 2012, 09:46 PM   #5
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Depends on what low distortion is...you can pick up a Heath IG18 (or the later IG5218) on eBay for under $50. Richiem on this forum has reported on upgrade to improve the performance substantially. Since the expensive parts are the case, multiple rotary selectors & decading components, this is a reasonable way to get there.
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Old 26th January 2012, 10:10 PM   #6
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Found the links to the Jim Williams AN 43.

http://cds.linear.com/docs/Application%20Note/an43f.pdf
Reading Jim Williams: App Note 43 part 1
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Old 26th January 2012, 10:35 PM   #7
coluke is offline coluke  Italy
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Quote:
Originally Posted by dennismiller55 View Post
Anyone ever build this? Sounds good but is it? There is no build tips or PCB layout. Just a diagram and parts list.
I've built it a few years ago, with a few modifications (a bit of filtering after the full wave rectifier): it's a very simple and clever design (the THD cancellation trick is lovely), and it is indeed capable of low settling time and very low THD - 0.001% or better mid-band:

APFGEN.png

but low settling time has a prize: amplitude flatness is far from optimum (with no integration in the level controlling loop the steady-state amplitude is a bit undeterminate...), and THD+N is somewhat high. If you are looking for something simple and fast to build it is certainly a good choice, but if you are managing to do some serious audio work then you should go for an SVF design - something like Bob Cordell's generator; or, maybe, improve a bit this generator with a better level controlling loop - never tried, but I think suprisingly good results are obtainable.

Ciao,

L.

ps - this is not a Wien bridge variation, but an all-pass filter; the differential output stage (IC2B) is designed to notch out almost exactly third harmonic and reduce somewhat higher order odd harmonics produced by the full-wave detector - that's why output THD is so low.
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Old 26th January 2012, 11:01 PM   #8
jcx is offline jcx  United States
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my $130 Juli@ does that good with much narrower noise skirt

here's one RMAA test: RightMark Audio Analyzer test : Juli@ loop
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Old 27th January 2012, 01:17 AM   #9
carpin is offline carpin  Netherlands
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ultra low THD sine oscillator based on the AN67 from Linear Technology.
THD performance : less than 1ppm@10kHz


DIY Analog-to-Digital Converter project.Audio measurements tool

[3rd round] AN67 Ultra-low THD 10kHz sine oscillator PCB Group buy. - diyAudio

Last edited by carpin; 27th January 2012 at 01:20 AM.
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Old 27th January 2012, 11:09 AM   #10
PChi is offline PChi  United Kingdom
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Another low cost variation that includes PCB layout is described at Low THD Audio Oscillator
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