opamp sine wave oscillator

[Jim Thompson]

Don't LDRs also have a nonlinear voltage effect? Light bulbs do at low
frequency. If you're hunting ppm distortion, may as well do it right.

Or use a 20-bit DAC with feedforward distortion tweaking.

John

Any digital AGC must, by definition, hunt.

And Ann Coulter has lost her sense of humor... thus, by definition,
has become a Democrat ;-)

...Jim Thompson

 

[John Larkin]

Any digital AGC must, by definition, hunt.

The dac don't need none!

And Ann Coulter has lost her sense of humor... thus, by definition,
has become a Democrat ;-)

Right, she's trying way too hard to be outrageous. The
Hillary-before-McCain thing is just plain idiotic.

 

[John Larkin]

On Feb 10, 2:54 pm, [email protected] (Don Klipstein) wrote:
[...]

I also suspect that dielectric constant varies a bit with electric field
due to electron orbits in the dielectric's molecules being squished out of
shape. I expect that effect to be mitigated by use of a thicker
dielectric (higher working voltage).

Some ceramic capacitors have dielectric constant so high that I would
not trust the dielectric constant to be all that constant. This is
probably worse with ones whose dielectric constant varies too much with
temperature anyway for the capacitor to be useful to make a good
oscilltor.

All of the high K ceramic materials are piezo electric and nonlinear.
The internal structure distorts under bias.

You can make a parametric amplifier using ceramic capacitors.

Or a high-voltage shock line.

John

 

[Jim Thompson]

The dac don't need none!

Show me... the gain can NEVER be exact.

Right, she's trying way too hard to be outrageous. The
Hillary-before-McCain thing is just plain idiotic.

Yep. For shame :-(

...Jim Thompson

 

[Tim Shoppa]

Don't LDRs also have a nonlinear voltage effect? Light bulbs do at low
frequency.

Heck, "regular" resistors also have nonlinear effects! Ohm's law is
more of a "serving suggestion" than a "law" if you are trying to do
things at the ppm level .

If you're hunting ppm distortion, may as well do it right.

But the $10-or-so analog multipliers I have worked with have a spec of
0.01% distortion in their most linear channel (the other is often
specced to around 1.0%). And those are factory-laser-trimmed parts.

The parts I'm familiar with are the MPY634 (0.01% linearity) for $15
or so, AD734 (0.025% linearity) for $20.

Is there a new class of analog multipliers with 0.001%, 0.0001%
linearities that won't break the bank? I admit that the parts I know
are as old as the hills!

Or use a 20-bit DAC with feedforward distortion tweaking.

Ah, at that point we just go DDS... consumer 20-bit DAC's are just a
dollar or two and pretty good through the whole audio range!

Tim.

 

[John Larkin]

Show me... the gain can NEVER be exact.

Just digitally generate some sinewave numbers and pump them into a 16
or 20-bit DAC. Follow with an undemanding lowpass filter.

Add a second DAC that's the distortion stomper. Sum it in, scaled down
by, say, 1000:1, and load it with distortion correction signals. Once
(somehow) you can measure the distortion of the main dac, it's simple
to program the stomper.

Amplitude is digitally adjustable, by programming the dac numbers, or
the dac reference, with the aid of downstream passive attenuators.

So now we have a digitally-programmable sinewave source with
ppm-accurate frequency and amplitude setability and ppm distortion,
down to milliHertz. May as well toss in other waveforms, and
programmable distortion, noise, and modulation, while you're in there.

Yep. For shame :-(

I'll email her and tell her we said so.

John

 

[Vladimir Vassilevsky]

Just digitally generate some sinewave numbers and pump them into a 16
or 20-bit DAC. Follow with an undemanding lowpass filter.

Add a second DAC that's the distortion stomper. Sum it in, scaled down
by, say, 1000:1, and load it with distortion correction signals.

It works only if the parameters of the first DAC are stable to the same
1000:1 precision.

Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

 

[John Larkin]

It works only if the parameters of the first DAC are stable to the same
1000:1 precision.

I'd imagine that harmonic distortion would be pretty stable over time
and reasonable temperature. THD would be very low from a good 20-bit
dac, so the stomper doesn't have to tweak things a lot.

At higher frequencies, the output amps become a serious problem.

John

 

[Don Klipstein]

A couple of others have been missed:

A resistively heated thermistor can be made to have a very long time
constant. It has the advantage over the light bulb that the
resistance value can be higher and the life expectancy very long.

I think that a thermistor heater would draw more power than a little
lamp that I don't notice any temperature rise of.

As for life expectancy - in Wien bridge oscillators, the lamp is usually
operated with the filament running a dim red-orange or close to that. I
consider such a dim brightness and color typical of a 750 or 1,000 hour
lamp being operated at 10% or less of rated voltage. (More than that only
for gas-filled lamps with design current around or less than twice the
minimum for gas to overall be of benefit.)
The usual "1-size-fits-most" rules say that life expectancy of an
incandescent is inversely proportional to voltage to the 12th power. I
would not get too worried about any inaccuracies here at this rate!

Although a thermistor with a heater sounds good for longer time
constant, I would go for the simplicity of using incandescent lamps since
they do have a good track record.

- Don Klipstein ([email protected])

 

[Michael A. Terrell]

You should have broken the thing up into multiple volumes, we'll all be
dead by 2010...

You might be. but the rest of us don't set a date to die by. You
give a whole new meaning to 'Expiration Date'! ;-)


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Tim Shoppa]

How about ppb instead of ppm:

Linear Technology Magazine, Feb 94 pp 26-28 - LTMag_V04N1_Feb94.pdf
athttp://www.linear.com/lt_magazines.jsp

"Using Super Op Amps to Push Technological Frontiers: an Ultra-Pure
Oscillator

An Ultra-Low-Distortion, 10kHz Sine-Wave Source for Calibration of
16-Bit or Higher Analog-to-Digital Converters

... The measurement of the harmonic distortion of this oscillator
defies all of our resources, but appears to be well into the
parts-per-billion range."

And only 11 Op amps .

Ahah - it uses the LT1228 as the variable-gain block. Remarkable part,
I forgot that I had used it several years ago. Thanks so much for
pointing that out! In a thread that started with a clipping oscillator
tweaked to not get more than a few percent distortion, we evolved to
the true-state-of-the-art ppb-level oscillator in a rather stellar
manner.

The LT1228 of course goes way higher than audio frequencies. Gives me
interesting ideas for low-distortion AGC of RF oscillators...

Tim.

 

[MooseFET]

Ahah - it uses the LT1228 as the variable-gain block. Remarkable part,
I forgot that I had used it several years ago. Thanks so much for
pointing that out! In a thread that started with a clipping oscillator
tweaked to not get more than a few percent distortion, we evolved to
the true-state-of-the-art ppb-level oscillator in a rather stellar
manner.

The LT1228 of course goes way higher than audio frequencies. Gives me
interesting ideas for low-distortion AGC of RF oscillators...

IIRC it works best with about a 30mV input swing. If you have a
widely varying signal, you can do something like:

----------
! !
Vin --------/\/\---+--!- \ !
! >---+--!+\
GND-------!+ / ! >-----
! --!-/
Iset !
!

The Iset is adjusted to always keep the input swing about the same.