Low-distortion Audio-range Oscillator - Page 18 - diyAudio
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Old 17th May 2012, 07:25 AM   #171
Frex is offline Frex  France
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Hi Terry,

The diode D2 allow IC4 to be closed looped when the oscillator is not settle because it can diverge. So, you can replace the BAV99 by any single signal diode.

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Old 17th May 2012, 08:09 AM   #172
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Thanks Frex

It was the other diode beside D2 that puzzled me. I hadn't realised that the BAV99 is in fact a pair of diodes, and that leaves one half left over.

So that then raises the question, why choose a BAV99 if you only need a single diode? What characteristics of the BAV99 recommend it for use here?

Terry
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Old 17th May 2012, 08:38 AM   #173
Frex is offline Frex  France
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Only because the bav99 is already use in the rectifier section, so using same diode avoid multiplying parts reference.

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Old 3rd August 2012, 04:47 AM   #174
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Just completed a build of the oscillator in post #169. Note there are a few errors in the schematic in case you are interested in building- there is a node on either side of the photoresistor; it's not clear from the drawing. Also, the R-C values are for a 200Hz frequency, instead of 2kHz. I changed the caps to 10nF to compensate. Also pulling a little extra current through the TL431 for better compensation against leakage in the integrator feedback path. Based on an email from the author, I decreased the 47uF cap in the IC4 feedback path to 10uF. Using 15V series regulators in place of the zeners.

Anyways, my FFT is attached, and it is disappointing. Hopefully someone can help with the clear and obvious amplitude modulation. I was very careful to hand select resistors and capacitors for a nearly perfect (as accurate as I can measure) frequency matching between R-C-1 and R-C-2. I can even see the AM with the Fluke multimeter on the output.

Distortion and sidebands as shown in the original article are extremely low, so either the article is misleading or I am doing something very wrong. I have played around with the R and C values in the integrator, with essentially little effect. I know my soundcard is capable of much more, as the output of my DCX is much better than what is shown here.

Any gurus in PID control systems? As I read through this thread it seems the integrator is the weak link in the overall THD result.
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Old 3rd August 2012, 05:18 AM   #175
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What do you see when you look at TP1 with a CRO? I imagine it will confirm that the negative feedback loop is hunting. We may learn something from the waveform there.

Also what's the oscillator's output swing (Vpk to pk)?

If you laid out a PCB, let us have a look at the layout. It's easier for other eyes to spot the obvious mistake than your own!

Terry
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Old 3rd August 2012, 06:01 AM   #176
richiem is offline richiem  United States
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@Terry -- I was thinking the same thing; maybe the LED driver is partially out of linear range...
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Old 3rd August 2012, 07:21 AM   #177
1audio is offline 1audio  United States
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First: How are you measuring the THD? -100 dB is pretty good for most audio cards. Without a twin tee or the like few audio cards will see the distortion floor of an oscillator like this.

Second: I spent many days trying to duplicate Jim Williams low distortion oscillator he claimed to get running in an afternoon. I finally gave up. I could not get anything like his performance. These things often look too simple and are much more tweaky that they look.

Third: If you are getting amplitude modulation the AGC may have too much gain. Its oscillating at a low frequency. The LDR is a non linear device with a long, nonlinear time constant. It turns on much faster than it turns off. First, try to keep the voltage across it low. The distortion increases quickly as the voltage across it increases. That means the led is on the very on. Second you want to reduce the gain on the opamp comparing the reference voltage to the rectified peaks.

Starting with adjustment there are two pots, the one in series with the LDR sets the drive point of the ldr to meet the conditions of oscillation. If the system is too prone to oscillate then the ldr will be almost dark and most likely to hunt. Start with the pot at max and the LED drive will be highest to get oscillation going. If it doesn't oscillate and the led drive is max reduce the resistor until it does oscillate. The use the level adjust to set the output level.

The integrator is always a difficult problem. The commercial ones often use sample and hold systems to capture the peak voltage on every cycle. Cordell used a great trick to double the number of peaks per cycle that speeds settling time and does not require a sample and hold. The ripple on the integrator can feedback and add second harmonic (and sometimes reduce it).
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Old 3rd August 2012, 12:04 PM   #178
PChi is offline PChi  United Kingdom
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Thanks for building the circuit and posting the test results.
I am not a guru in PID systems unfortunately and have tried to analyze the loop stability of an Oscillator but my differential equation skills aren't good enough. I used a sample and hold circuit to try to meet the conflicting requirements of no delay in the control circuit for stability along with low ripple.

The FFT plot shows mainly even hamonics which could be due to the photcell as well as the integrator.
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Old 3rd August 2012, 01:40 PM   #179
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Wow, thanks for the replies. I have a little work to do in order to answer some of your questions, so give me a little time. A few basic answers for now-

Pot is adjusted to provide for 10mA LED current, per the author. I did adjust this from 5mA to 15mA, with no discernible difference in modulation.

Output swing is adjustable from 2.2 to 4V RMS. Again, no difference in modulation with output swing.

Photoresistor has around 15mV across it, as it is in parallel with a 15 ohm resistor. Sounds fairly low to me.

Attached is what I am trying to improve on- output of my DCX2496 with digital in, transformer coupled out. My sound card is quite repeatable with these measurements, so at this point I trust it.
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Old 3rd August 2012, 04:36 PM   #180
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PCB. Don't laugh
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