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Old 19th August 2011, 07:01 PM   #31
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Quote:
Originally Posted by benb View Post
I want to mention something I haven't seen much if anything about in these threads using LED/CdS Photocell (LDR) devices. The light output of LEDs drops over time. Of course all components age, but LEDs appear to change their characteristics more drastically than most others. I recall a figure of about 50 percent reduction in light output at full current over six months. This oscillator may not be in operation over that time (unless someone leaves it powered up on the bench continuously), but I think it's something to be aware of. The adjustment of the 5V value across the 470 ohm resistor in series with the LED would have to be increased as the LED ages to maintain the original spec. I'm on a seismometer list where the idea of a LED and phototransistor are used with a vane in between to detect the position of a seismometer arm, and a small tungsten-filament light bulb is suggested instead of an LED as a long-term stable light source.

It might also be "important" in the threads on LED/LDR attentuators used for volume control, especially as these LEDs are run for long periods of time listening, but that solution is simply to change the knob setting.
Of course a better method for a seismometer is to have a photo receptor monitor the led and keep the output constant.

The led is barely on for this circuit and it has it's level constantly adjusted by the output level. So it is in a feedback circuit.
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Old 19th August 2011, 07:24 PM   #32
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@dgta -- I think that the oscillator in the AP is perhaps not as good as the spectrum analyzer section due to circuit constraints in the DAC system, and so for testing purposes, where a single frequency will provide the data needed, they chose to make a good analog one.
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Old 19th August 2011, 07:43 PM   #33
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Originally Posted by EUVL View Post
The LDR is actually cheaper at RS than Farnell.
But it is very easy to get and the price difference is not really relevant, compared to the rest.

So I am still looking for a since oscillator at below -140dB.
The one from Audio Precision System One is also not much better than -110dB.

Maybe Scott would kindly drop us some hints ??


Patrick
Samuel Groner covers a lot here, Low Ripple/Fast Settling AGC For Oscillator.

He is worried a lot about fast settling, if you just want low distortion Jim's circuit built with composite super op-amps or discrete ones with better than -140dB THD and VERY carefully chosen passives (ask Ed ) should be able to do it. The key is to make the controlling non-linear element have as little as possible to do.

BTW I don't think this is really the article promised, since the output amp could never do -160dB (or any that I could imagine).

I would give this a shot. Take a very good sound card and sum the channels with one attenuated by 1000. Program your fundamental into one channel and subtractive harmonics in the other until you get nulls at the harmonics. Verifying with a very good twin-tee (AP has a white paper on that).

Tedious yes, but if you just want to test something at a fundamental and pick out the first few harmonics this should work. Remember there is no magic trick to make an ordinary IC op-amp suddenly have -160dB THD.
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Last edited by scott wurcer; 19th August 2011 at 07:49 PM.
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Old 19th August 2011, 09:55 PM   #34
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I will mention a few of the interesting design items as I see it.

The resistors are a low distortion type I have not used. In the past JW has used vishays and other "Magic" parts.

He is quite specific about the capacitor types and I think the actual ones used are very important.

The output buffer has feedback to roll it off around 2K as does the passive filter on the output.

The Photocell is running around 80 ohms in use. This means any distortion contribution will be reduced by 50 db by the fixed resistors. It also means the gain trim is extremely tight.

Interestingly I think the noise floor might be lowered a bit by using a better filter on the 5 Volt offset reference, but I am not familiar with the particular part.

I would have to run distortion measurements on the Photocell to see what the actual limits of the circuit could be. That would be my guess as to the most critical part.

It seems to be chosen not just for response linearity but also low noise. I suspect a VCA across a resistor might yield better results.

Last edited by simon7000; 19th August 2011 at 09:57 PM.
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Old 20th August 2011, 12:57 AM   #35
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Originally Posted by simon7000 View Post
The resistors are a low distortion type . . .

He is quite specific about the capacitor types . . .
Reading between-the-lines in the schematic "Notes", I see the message "Attention to detail is important here.".

Quote:
. . . I think the noise floor might be lowered a bit by using a better filter on the 5 Volt offset reference . . .
When I saw that I wondered if the Jung/Didden/Salas work on voltage regulators might point to a lower-noise design for the voltage reference.

Quote:
The Photocell is running around 80 ohms in use. . . .

I would have to run distortion measurements on the Photocell to see what the actual limits of the circuit could be . . .

It seems to be chosen not just for response linearity but also low noise . . .
If the moderators don't rap my knuckles for transplanting my comments from the thread, "R2R Attenuator for Volume Control without Transition Contacts in the Signal Path":
The atch image is Fig 7 (THD+Noise vs. signal level for a NSL-32SR2 configured as a 3 db shunt attenuator with a 2 KOhm source resistance) from the Silonex page at < Silonex Inc.: Products: Audiohm Optocouplers: Audio Characteristics >.

From the description on the Silonex web site, it sounds like the 32SR2's photocell was connected as the shunt element in a simple voltage divider (L-pad) with a 2 kilohm series element. Then the LED current was adjusted until the voltage divider produced a 3-dB loss. (I think this means the photocell resistance was around 4.8 kilohms.)

Quickly grabbing two data points, I see 0.010% (-80 dB) THD with -6 dBu input level; and 0.10% (-60 dB) THD with +4.5 dBu input. That's a 20 dB increase in THD for a 10.5 dB increase in input level. Thinking makes my brain hurt, so I will approximate it as a 2:1 ratio. If I recall correctly, that points to a predominantly cubic (third-order) nonlinearity in the photoresistor's V-I curve at this particular illumination level.

If I re-learned MATLAB I could probably determine the coefficients for a polynomial V-I curve, but maybe somebody who already knows MATLAB will do it.

There is more information about using the Silonex optocouplers as audio attenuators on the Silonex page called "Audio level control with resistive optocouplers". Scroll down to the discussion on the "Shunt Attenuator".

They say that a 3-dB shunt attenuator is a worst-case situation, distortion-wise. I don't know if they REALLY mean "a 3-dB attenuator will give the worst THD, regardless of the series resistance in the attenuator"; or if they mean "the optocoupler has its worst distortion when when the photocell is near the upper end of its usable resistance range - which is where it's operated in this example of a 3-dB attenuator". I'd like to see curves for THD versus attenuation for various values of series resistance - e.g., 2K ohms, 600 ohms, 200 ohms, etc.

Dale
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Old 20th August 2011, 01:09 AM   #36
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Originally Posted by simon7000 View Post
The led is barely on for this circuit and it has it's level constantly adjusted by the output level. So it is in a feedback circuit.
Yes, from the perspective of distortion the important parameter is the operating point of the CdS photoresistive element in the optocoupler. As the LED ages, the integrator in the ALC control loop (A7, an LT1006) will need to slew to a greater voltage to establish the steady-state operating point. This will affect settling time from a cold-start, but I don't think it'll change the distortion performance after the output level is in regulation.

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Old 20th August 2011, 06:09 PM   #37
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Thanks for the photocell distortion data. It looks like that part can do -130db distortion.

But you do need to brush up on your shunt regulator pop culture history.

The power supplies are not shown but do need to be decent. For -140 or better then they need to be great.
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Old 20th August 2011, 08:52 PM   #38
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You guys don't get it the distortion of the gain control element has nothing to do with the ultimate THD achieved.
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Old 20th August 2011, 09:11 PM   #39
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Scott,

The signal passes through the gain control element in the Wien bridge version used here. It is followed by an inverting amplifier and then another with a 2K LPF to the output filter.

It is not just the precision of the unity gain that adjusts the distortion it is also affected by the parts choice. That is why the only common 1% MF resistors are used in the level detector path.

The gain adjustment range is .3%! That is why the resistors inside the loop must be .1%.
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Old 20th August 2011, 10:45 PM   #40
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Originally Posted by scott wurcer View Post
I would give this a shot. Take a very good sound card and sum the channels with one attenuated by 1000. Program your fundamental into one channel and subtractive harmonics in the other until you get nulls at the harmonics. Verifying with a very good twin-tee (AP has a white paper on that).
Tedious yes, but if you just want to test something at a fundamental and pick out the first few harmonics this should work. Remember there is no magic trick to make an ordinary IC op-amp suddenly have -160dB THD.
l like this idea very much. Thanks.
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