Low-distortion Audio-range Oscillator - Page 4 - diyAudio
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Old 3rd February 2012, 01:19 AM   #31
mlloyd1 is offline mlloyd1  United States
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if you can convince yourself that you don't really need continuous frequency adjustment (hint, hint), use resistors and switches. that will match better than ANY dual pot you can reasonably get a hold of and not break the bank in the process.

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Old 3rd February 2012, 01:28 AM   #32
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Second line: Total res range +/-10%
Who will match them, how many $s are they and what is the abolute linearity of each pot? I think I will stay with a digital solution
Of course, one can leave all the digital salad out and hook up a stereo pot to the pcb. E
Or use mollyd1 suggestions!
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Old 3rd February 2012, 01:24 PM   #33
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Quote:
Originally Posted by mlloyd1 View Post
if you can convince yourself that you don't really need continuous frequency adjustment (hint, hint), use resistors and switches. that will match better than ANY dual pot you can reasonably get a hold of and not break the bank in the process.

mlloyd1
Yeah, that's a good idea. We can easily get resistors in 0.1% tolerance now.
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Old 3rd February 2012, 01:26 PM   #34
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Quote:
Originally Posted by mickeymoose View Post
Second line: Total res range +/-10%
Who will match them, how many $s are they and what is the abolute linearity of each pot? I think I will stay with a digital solution
Of course, one can leave all the digital salad out and hook up a stereo pot to the pcb. E
Or use mollyd1 suggestions!
Those Bourns precision pots have 0.1% (or was it 0.5%?) linearity, so if you get them to both "start" at the same resistive value, they should track within that window. They are about $70 each. I'm not sure how to gang them though.

I think he says in the application note that these pots should track within 0.1%. I haven't really found readily available ganged pots like that yet.

Last edited by dirkwright; 3rd February 2012 at 01:37 PM.
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Old 3rd February 2012, 02:06 PM   #35
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I sent an email to State Electronics asking for a suggestion on a dual gang pot where each section tracks the other better than 1%.
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Old 3rd February 2012, 02:42 PM   #36
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For another project, I've been working on a way to gang 2 single pots with tiny plastic sprockets and chain. I haven't built anything yet but I've got some of the parts.
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Old 3rd February 2012, 02:53 PM   #37
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Just to be clear, this is the schematic we are using, right? I believe that if C1 and C2 are switched, then the frequency range can be changed. So, we'd need matched pairs of 0.01, 0.1 and 1.0uF capacitors on a shorting type selector switch.
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Old 3rd February 2012, 03:28 PM   #38
PChi is offline PChi  United Kingdom
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It's easy to criticize but difficult to come up with a good working circuit.

I believe that even with a 1% matched dual gang potentiometer the Light Dependent resistor amplitude control will still have significant voltage across it in the Wien bridge Linear Technology circuit. I have used Silonex opto couplers in the past and they aren't all that linear. I believe also that Hewlett Packard and Sound Technology used multiple switched close tolerance components to get around this problem.
The beauty of the phase shift based Oscillator used in the first schematic of this topic that theoretically the loop gain doesn't depend on the matching of the potentiometer gangs.

I think that a continuously variable frequency oscillator is useful for general purpose but that a switched frequency is OK for distortion measurements.

National Semiconductor (now Texas Instruments) have an LME49990 that claims low distortion when used with a gain of 1 so implies that the common mode signal doesn't cause bad distortion.

I think that the way to go is a Phase Shift based circuit. Use a sample and hold for the amplitude control so theoretically zero control voltage ripple and use an analog multiplier gain control for low distortion compared to Light Dependent Resistors or FETs.

Another fixed frequency option is to base the design on Cyril Bateman's design that is available on the web.

I guess that for computer control of the frequency a couple of Multiplying DACs could be used instead of motorized potentiometers that sounds complicated.
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Old 3rd February 2012, 03:43 PM   #39
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I wasn't criticizing the LT circuit. I'm not sure how you came to that conclusion. I'm not sure also what kind of point you're making.

I'm not suggesting motorized potentiometers at all. That's your idea.

If you've got a variable audio oscillator that has lower distortion than 0.0003%, then please show it. I'd like to see the LT circuit available as a PCB at least, but if not, then I'm going to build it on perf board.
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Old 3rd February 2012, 03:58 PM   #40
LAJ is offline LAJ  United States
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try the XR2206 waveform generator chip. They will operate off one 9V battery but they usually like 12-18V range.
Also it gives triangle and square wave outputs.
You can usually find them on EBay.
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