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Old 15th March 2012, 05:48 AM   #111
richiem is offline richiem  United States
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Sorry, David -- I didn't mean to put words in your mouth, nor did I mean to leave any out; your observations just seemed especially germane to the discussion.
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Old 15th March 2012, 08:45 AM   #112
coluke is offline coluke  Italy
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Quote:
Originally Posted by davada View Post
Davada has observed the significant loss of open loop gain due to even moderate loading in an undamped state variable filter/ocillator from a small number of op amp specimens.
Of the same type (ie, 3 out of 10 OPAxxx tested in an undamped SVF ring showed a significant OLG loss when driving moderate loads), or of different types? Both interpretations could make sense, but they have completely different implications.

L.
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Old 15th March 2012, 02:16 PM   #113
davada is offline davada  Canada
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Quote:
Originally Posted by richiem View Post
Sorry, David -- I didn't mean to put words in your mouth, nor did I mean to leave any out; your observations just seemed especially germane to the discussion.
Not a problem Dick.

I thought it was important to point out the operating conditions. We are talking ppb distortion here.

Cheers,

David.
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Last edited by davada; 15th March 2012 at 02:29 PM.
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Old 15th March 2012, 02:28 PM   #114
davada is offline davada  Canada
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Quote:
Originally Posted by coluke View Post
Of the same type (ie, 3 out of 10 OPAxxx tested in an undamped SVF ring showed a significant OLG loss when driving moderate loads), or of different types? Both interpretations could make sense, but they have completely different implications.

L.
Hi Coluke,

Out of same type with consistent results. Of course each type has it own magnitude of distortion. Some types will do better than others.

Everything intensifies with loading. Distortion on the power rails, ground return currents and EM. It is difficult to isolate the source of distortion at these levels and at such high gain.

David.
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Old 15th March 2012, 11:37 PM   #115
coluke is offline coluke  Italy
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Quote:
Originally Posted by davada View Post
Hi Coluke,

Out of same type with consistent results. Of course each type has it own magnitude of distortion. Some types will do better than others.

Everything intensifies with loading. Distortion on the power rails, ground return currents and EM. It is difficult to isolate the source of distortion at these levels and at such high gain.

David.
Hi Davada,

I've just done a few quick tests on the SVF generator I did mention in one of my last messages to investigate the effects of heavy loading. The generator is based on a ring of 5534s equipped with a 4-phase peak detector and a pair of paralleled AD633 as level controlling element; feedback from LP out to HP virtual ground is through a 3k3 resistor, and the inverting stage operates at unity gain, so 2 out of 3 5534s works over 3k3 (or a little less) load. Generator output is at LP output, and THD @ 1kHz, 20Vpp over 10k or so is a bit lower than -126dBc, or 0.00005%, and this is the reason why - honestly speaking - lowering it anymore wasn't really a big issue - I was more interested in trying to lower THD+N.

Lowering the generator load down to 1.5 kohm * doubles * the output THD, but it seems to affect * only * the 2nd harmonic rising it to -120dBc, while the 3rd remains almost untouched at -130dBc - I can't see anything else down to -140dBc in the 4-20kHz range. Looking at the output of the multipliers confirms the 2nd harmonic rise, while both their output level and the oscillation frequnency * aren't affected at all * by the heavier load (and 1k5 @ 20Vpp is quite an heavy load for a poor old 5534...), which in turn means that the operating Q of the generator doesn't change (or changes by too a small amount to be detected).

Else I'm facing with some hard-to-find cancellation effects in my SVF notch (had no time to check THD rise with another notch based on a completely different topology), or test results are a little bit unclear, although they seem to indicate that the 5534 (which is indeed specified for 100V/mV OLG @ 20Vpp, RL>600 ohm) is a good performer on medium loads (or maybe a not-so-bad on heavy ones). Was the 5534 in the bunch of opamps you tested?

Here in the Bel Paese is late at night, so I'm not going to speculate at all on what I've found - I hope I'll have some more spare time during the weekend to recheck results and do some more testing, although I know that looking for something in the 1ppm range or below can be - and very often is - definitely frustrating...

Ciao,

L.
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Old 16th March 2012, 02:24 AM   #116
davada is offline davada  Canada
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Hi Coluke,

Yes the 5524 was in the group.

The 1.5k sounds about right for a 20Vpp signal. What I found is it depends on the operating level and the operating loop gain, or ring if you prefer, of the filter as to what load the op amp can tolerate. As you have found the maximum load is quite a bit higher than what the data sheets claim for unity gain at 100% feedback. Evey op amp is a bit different in the absolute effective value.

I also found with loads down to 1K ohms or so that I could not operate the oscillator much above 7Vrms but with out a external loading could get 9Vrms. Again this varies with op amp used.

It really comes down to what one wants to achieve. Therefore there will be trade-offs with operating level, SNR and distortion.

There is the possibility of paralleling op amps to improve the same but I have not tried this. Apparently, paralleling op amps also increases bandwidth by nearly a factor of 2.

How are you determining Q of a running oscillator? Are you monitoring both the output of the oscillator and output of the multiplier to determine ratio change?

"Else I'm facing with some hard-to-find cancellation effects in my SVF notch (had no time to check THD rise with another notch based on a completely different topology),".

I was playing around in spice with an elliptic response by combining the HP and LP outputs. A 1:4 ratio lands the notch on the second H. You can choose any harmonic with other ratios. I haven't tried this. As you can see the LP response is lost and the higher order harmonics won't be suppressed. I suspect the noise will be higher as well.
One could follow this with an additional LP function but that means another section to tune.

Cheers,

David.
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Old 16th March 2012, 10:04 PM   #117
coluke is offline coluke  Italy
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Hi Davada;

Quote:
Originally Posted by davada View Post
There is the possibility of paralleling op amps to improve the same but I have not tried this. Apparently, paralleling op amps also increases bandwidth by nearly a factor of 2.
I've been thinking about this possibility for quite a long time, but never tried - it should be effective in reducing somewhat output noise, too.

Quote:
Originally Posted by davada View Post
How are you determining Q of a running oscillator? Are you monitoring both the output of the oscillator and output of the multiplier to determine ratio change?
Yes - @ 1kHz, 20Vpp on 10k load, multiplier output sits at about 120mVpp: taking into account the 10:1 decoupling factor, the operating Q seems to be as high as about 65dB (with WIMA red PP timing capacitors). A fairly simple and quick way to check this estimate can be made by grounding the multiplier VC input (which definitely stops the generator), and then finding out which value of resistance between BP output and HP virtual ground just restarts oscillations - with positive quadrature feedback via a 5.1Meg resistor oscillations build up in a few seconds, so running Q is * at least * 5.1Meg/3.3k, or 64dB - this is fairly consistent with the previous estimate, and happily accounts for the pretty low THD values I see.

Ciao,

L.
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Old 17th March 2012, 12:53 AM   #118
davada is offline davada  Canada
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Hi Coluke,

There perhaps an easier and certainly more accurate way to measure the gain at Fr.

Disconnect the multiplier output from the input to the filter, either use the same value input resistor or my preferred way is to use a resistor the same value as the feedback resistor in the inverter HP section. This way the test is calibrated for unity. But if you choose the former then simply calculate the attenuation factor. Then either using another oscillator fitted with an attenuator on the output, apply and adjust the input signal until the output of the filter is at operating level. Measure the input level from the oscillator and divide this into the filter's output level. Repeat at different frequencies and ranges.

My preferred method is to use ARTA's frequency response test in single mode using PN (periodic pink noise). ARTA does the heavy lifting for you by automatically calibrating this test.Therefore you can set the ARTA PN generator to any input level and it is normalized to 0dBu (relative level). This way you can examine all of the filter's frequency response.
You can then sweep the filter through all your selected frequencies and ranges.

You may be surprised at what you find. But then again maybe not. You will however know exactly what you are dealing with.

If you do this test let me know what you find.

It also works with any network tuned within the audio range, like notch filters for example. It's best to use the highest resolution in ARTA as the lower resolutions may give errors in amplitude reading with high Q circuits because low resolutions step past the peak or notch.

I used this to log the amplitude errors with richiem's Twin T filter. This way the error can be added to the measurement for correction.

One idea I discussed with Richiem is to make up a calibration file to level the errors.
But this would involve a calibration file for each test frequency as the error increases with increased frequency. One could dump the results of the test to a file and run a normalization macro, say in excel, on the data and use this for a calibration file.

Cheers,

David.
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Last edited by davada; 17th March 2012 at 01:19 AM.
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Old 17th March 2012, 10:13 AM   #119
PChi is offline PChi  United Kingdom
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Hello,
After changing the OPA134s for LME49710s the total harmonic distortion (plus noise) at 25 kHz has dropped from 0.04 % to 0.02 % so a good result for me. No change at the lower frequencies. Interestingly the Texas Instruments Audio Guide (1Q 2012) doesn't include the OPA134 but I did buy them back in 1999.
I had some trouble with the physical layout of the components so I am not keen to make any more modifications to the Distortion Analyzer in it's current physical implementation (The air turned blue when wires fell off and dealing with a DIY 'manufactured' double sided PCB without plated through holes tested my patience).

Thanks to davada for attaching the pictures and I admire the thd results achieved by coluke.
I think that I need to modify the design and produce a new board for the Distortion Analyzer with PCB mounted switches so it's possible to make changes relatively easily.
I have attached a picture of what I have ended up with for the Distortion Analyzer. Some of the component choices were dictated what what bits I had so it's in no way optimised.
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File Type: png Ian Hickman measure thd below 0_001.png (76.3 KB, 480 views)
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Old 17th March 2012, 04:40 PM   #120
PChi is offline PChi  United Kingdom
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I believe that I have drifted some way from the original post:-
My specification and preliminary measurement results are:
12 Frequency ranges:-
10 - 20, 20 - 50, 50 - 100, 100 - 200, 200 - 500, 500 - 1k, 1k - 2k, 2k - 5k, 5k - 10k, 10k - 20k, 20k - 50k, 50k - 100k.
Level nominal 5 V rms
Amplitude flatness with frequency within +/- 0.5 dB
Distortion and noise, measurement bandwidth limited by the rms to DC converter (AD536A). Also limited by the notch filter.
100 to 250 Hz 0.002%
1 kHz to 2.5 kHz 0.003 %
10 kHz 0.01 %
25 kHz 0.02 %
So not great but at least all the components are on one PCB. I really need to produce a Rev 0.2 PCB to tidy up the changes and remove some of the excess features to make the PCB smaller.
I haven't mounted the stepped output attenuator, the amplitude control potentiometers or the output buffers and coupling capacitors.
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File Type: jpg Low Distortion Oscillator.jpg (98.0 KB, 465 views)
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