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Old 2nd November 2012, 07:39 AM   #821
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Quote:
Originally Posted by dennismiller55 View Post
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 2nd November 2012, 02:40 PM   #822
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No filtering at all in the level detector !??!
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Old 2nd November 2012, 03:14 PM   #823
vicnic is offline vicnic  Latvia
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Originally Posted by scott wurcer View Post
No filtering at all in the level detector !??!
Yes , it is near of a truth. I am also thinking about this. Properly level of unfiltered full wave detector signal, when injected in JFET gate, may help to reduce 3rd.

Victor
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Old 2nd November 2012, 04:26 PM   #824
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No filtering at all in the level detector!??!
The time constant of the LDR provides a bit of filtering, and (if I did look at it correctly in the few seconds) the filter topology has an elliptic response, which will notch out the dominant harmonic (probably the 3rd).

Samuel
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Old 2nd November 2012, 04:42 PM   #825
EssB is offline EssB  United Kingdom
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You do realise you're commenting on the very first post of this thread ?
'Mr Ohm 1970' just quoted it without comment.
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Old 2nd November 2012, 04:50 PM   #826
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Sorry I missed that, but now more to think about.
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Old 2nd November 2012, 08:52 PM   #827
coluke is offline coluke  Italy
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Quote:
Originally Posted by Samuel Groner View Post
The time constant of the LDR provides a bit of filtering, and (if I did look at it correctly in the few seconds) the filter topology has an elliptic response, which will notch out the dominant harmonic (probably the 3rd).
You're right (as usual ) - with the 3rd notched out by the output buffer, adding a simmetry trim to the detector can help to reduce quite a bit the 2nd (which is probably mostly due to ripple injection). This way you can get very fast settling and reasonably low THD (but poor flatness and long term stability). All in all, it's a very simple and clever topology.

L.
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Old 3rd November 2012, 05:22 PM   #828
PChi is offline PChi  United Kingdom
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The Roger Rosens phase shifting Oscillator is clever but I am unsure how accurate the potentiometer matching needs to be for the distortion cancellation to help. The original Wireless World article states that the distortion was measured using 1 % tolerance resistors.
Another variation on the Wien bridge is described at Injection-lock a Wien-bridge oscillator | EDN
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Old 5th November 2012, 02:30 PM   #829
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The Roger Rosens phase shifting Oscillator is clever but I am unsure how accurate the potentiometer matching needs to be for the distortion cancellation to help. The original Wireless World article states that the distortion was measured using 1 % tolerance resistors.
Another variation on the Wien bridge is described at Injection-lock a Wien-bridge oscillator | EDN
Interesting he quotes basicly amplifier limited THD (<120dB) with an ordinary one amplifier circuit and Christmas tree lamp.

I have never been able to observe this but I will try. I wonder if the color matters
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Old 5th November 2012, 03:34 PM   #830
1audio is offline 1audio  United States
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Maybe the LT1468 has exceptional common mode rejection?
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