From: Fred Bloggs
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Subject: Re: low distortion oscillator
Date: Sat, 09 Nov 2002 17:30:01 GMT
NNTP-Posting-Date: Sat, 09 Nov 2002 09:30:01 PST
Organization: EarthLink Inc. -- http://www.EarthLink.net
Winfield Hill wrote:
> Martin Griffith wrote...
>>MIKE CONNELL scribbled:
>>>There is a schematic for a cheap low distortion oscillator based on a
>>>Wein bridge, page 30 figure 40 in http://www.linear.com/pdf/an43.pdf
>>>The distortion is below .003% ...
>>go for wire wound 10 turn pots, as Phil said, also look at this Walter
>>Jung circuit, http://martingriffith.users.btopenworld.com/LDO.HTM
> Good advice, Martin. Walter's approach, using inverting amplifiers
> rather than the more common non-inverting form, is preferred for low
> distortion, because it eliminates opamp input common-mode voltages.
> He also uses a linearized FET level regulator (see AoE page 139).
> For even lower distortion composite opamp stages can be used, e.g.
> see Linear Technology Magazine, Feb 1994 for Dale Eagar's amazing
> 1ppb oscillator. Of course we don't all need 0.0000001% distortion
> levels, achieved with his triple-opamp "Super opamp," but a simple
> dual-opamp composite can perform wonders. Here's one that should
> work well. (Note the first opamp's inverted polarity.)
> . 3.3pF 10k
> . __ ,--||--/\/\--,
> . ---|+ \ 10k | __ |
> . | >--/\/\--+--|- \ |
> . ,--|-_/ | >-----+----
> . | gnd --|+_/
> . gnd OPA627 OPA134
> First, the loop gain increased, especially useful at high oscillator
> frequencies, where the opamp's GBW normally limits loop gain. The
> composite stage above has 118dB of gain at 10kHz.
> Second, the first opamp's input stage has lower distortion because its
> output stage is driving a higher load impedance. This is an important
> issue for input-stages at low frequencies, where distortion can arise
> as a small thermally-induced change in offset voltage each cycle.
> To design a composite amplifier suitable for use as an integrator, as
> is needed with a Wien-bridge oscillator, simply choose the 2nd opamp's
> gain-setting components to insure the composite has a 6dB/octave slope
> at unity gain. For example, the 1st-stage OPA627 has a 16MHz GBW, so
> I chose a 5MHz integrator bandwidth for the 2nd-stage OPA134. That
> means the gain slope is -6dB/octave above 5MHz for good phase margin,
> but increases to -12dB/octave below 5MHz for high loop gain at the
> frequencies of interest.
> - Win
Any tips on how one goes about organizing what must be equivalent to
several billion cubic meters of documentation, or is this just a