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Old 20th January 2014, 11:52 PM   #3481
davada is offline davada  Canada
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Bob I have rather curious problem you might be able to help me with.
My SVO is tuned with current dacs, (Mdac). In each range I can tune the full range from 0 to
65535 on the dac (16 bit) 0 to 2mA at with 10V on the reference pin. At a 3.5Vp that would be 0 to 700uA. The dac's full on equivalent resistance is 5k. In the highest range the caps are 330pF. I can only get the dac out to a count of 20,000 (70kHz). After that the oscillator become unstable and goes to clipping. If I bypass the dacs with 5k resistors the oscillator is stable at 120kHz so I know there is no problem with the agc or other. It's very difficult to
trouble shoot. What's going on? Is the dac's capacitance getting in the way? Cout 200pF code dependent.

I wonder if adding a small series resistance on the output of the dac might help.
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Last edited by davada; 21st January 2014 at 12:05 AM.
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Old 21st January 2014, 02:41 AM   #3482
davada is offline davada  Canada
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Demian was asking if a SVO can run at sub audio frequencies.
I have this one running at 0.357Hz stable. So yes.
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Old 21st January 2014, 04:17 PM   #3483
1audio is offline 1audio  United States
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Quote:
Originally Posted by davada View Post
Demian was asking if a SVO can run at sub audio frequencies.
I have this one running at 0.357Hz stable. So yes.
?? I had a KH 4024 SVO (I think, it's been years) that would work to .001 Hz. Those also had essentially instant switching on ranges. It may have been someone else asking.

I'm impressed with your progress.
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Old 21st January 2014, 05:14 PM   #3484
davada is offline davada  Canada
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Quote:
Originally Posted by 1audio View Post
?? I had a KH 4024 SVO (I think, it's been years) that would work to .001 Hz. Those also had essentially instant switching on ranges. It may have been someone else asking.

I'm impressed with your progress.
Thanks Demian.

I thought it was you. Question answered to whoever asked then.
I think I can solve the Mdac tuning problem by using larger value tuning caps.
This will better swamp the dac's parasitic capacitance but it does mean having to use larger currents for tuning which in turn loads the op amps more. I guess it's a trade off of stability for distortion. Once this is done I'll look into a different tuning arrangement. I only need to switch resistor to ground using resistor T networks or R2R.

Cheers.
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Old 21st January 2014, 08:37 PM   #3485
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Quote:
Originally Posted by davada View Post
Bob I have rather curious problem you might be able to help me with.
My SVO is tuned with current dacs, (Mdac). In each range I can tune the full range from 0 to
65535 on the dac (16 bit) 0 to 2mA at with 10V on the reference pin. At a 3.5Vp that would be 0 to 700uA. The dac's full on equivalent resistance is 5k. In the highest range the caps are 330pF. I can only get the dac out to a count of 20,000 (70kHz). After that the oscillator become unstable and goes to clipping. If I bypass the dacs with 5k resistors the oscillator is stable at 120kHz so I know there is no problem with the agc or other. It's very difficult to
trouble shoot. What's going on? Is the dac's capacitance getting in the way? Cout 200pF code dependent.

I wonder if adding a small series resistance on the output of the dac might help.
Hi David,

I'm familiar with this issue. In my DIY THD analyzer (on my website), on the highest range the range capacitors are 200pF (200kHz range). I needed to put 68 ohms in series with them for proper operation. I believe this has to do with accumulating phase shift in the op amps (I used 5534s).

Hope this helps.

Cheers,
Bob
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Old 21st January 2014, 11:22 PM   #3486
davada is offline davada  Canada
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Quote:
Originally Posted by Bob Cordell View Post
Hi David,

I'm familiar with this issue. In my DIY THD analyzer (on my website), on the highest range the range capacitors are 200pF (200kHz range). I needed to put 68 ohms in series with them for proper operation. I believe this has to do with accumulating phase shift in the op amps (I used 5534s).

Hope this helps.

Cheers,
Bob
Thanks Bob.
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Old 28th January 2014, 09:00 AM   #3487
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Quote:
Originally Posted by davada View Post
My SVO is tuned with current dacs, (Mdac). In each range I can tune the full range from 0 to 65535 on the dac (16 bit) 0 to 2 mA at with 10 V on the reference pin. At a 3.5 Vp that would be 0 to 700 uA. The dac's full on equivalent resistance is 5k. In the highest range the caps are 330 pF. I can only get the dac out to a count of 20,000 (70 kHz). After that the oscillator become unstable and goes to clipping. If I bypass the dacs with 5k resistors the oscillator is stable at 120 kHz so I know there is no problem with the AGC or other. It's very difficult to trouble shoot. What's going on?
I have already discussed this with you back then when you were using the lamp multiplier. The issue is the "Q enhancement" effect of the state-variable topology, which drives the leveling loop out of its authority range at high frequencies.

The "Q enhancement" stems mostly from finite GBW of the opamps. Besides the series resistor for the integrator caps which Bob mentiones it is possible to adress this (IME more effectively) by the addition of a capacitor in the high-pass stage (C1512 in the SG505, C2503 in the System One). Select the capacitor value such that the output of the multiplier is as close to zero as possible acrosss all frequencies.

Doing this compensation is absolutely essential, as otherwise the required authority of the leveling loop becomes hopelessly large.

Samuel
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Old 28th January 2014, 01:37 PM   #3488
davada is offline davada  Canada
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Samuel as you recall I brought the Q enhancement issue and it's effects to you. You argued against using the compensation for weeks until I pointed out it was used in the sys1 then you took a closer look. It is in there. The problem was I had too much decoupling between the multiplier and osc to reach 100kHz. Not enough multiplier authority. The problem was solved a week ago.

Not wanting to give up the amount of decoupling used I found it sufficient to add a series RC in parallel with the decoupling resistor in addition to the 'Q' enhancement compensation cap as you described. This allows the use of greater decoupling at lower frequencies while satisfying the needed authority at frequencies above 15kHz.


Quote:
Originally Posted by Samuel Groner View Post
I have already discussed this with you back then when you were using the lamp multiplier. The issue is the "Q enhancement" effect of the state-variable topology, which drives the leveling loop out of its authority range at high frequencies.

The "Q enhancement" stems mostly from finite GBW of the opamps. Besides the series resistor for the integrator caps which Bob mentiones it is possible to adress this (IME more effectively) by the addition of a capacitor in the high-pass stage (C1512 in the SG505, C2503 in the System One). Select the capacitor value such that the output of the multiplier is as close to zero as possible acrosss all frequencies.

Doing this compensation is absolutely essential, as otherwise the required authority of the leveling loop becomes hopelessly large.

Samuel
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Old 28th January 2014, 02:10 PM   #3489
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Quote:
Originally Posted by davada View Post
This is with a sheet of grounded aluminum foil over the SVO board.
All else the same.

I guess I can live with this. I'd rather see the disco into the noise floor but it is 10kHz.

Rick I think I'm just going to build an enclosure out of FR4 for the time being. Since I'm not making boards anymore I have to do something with all the copper clad I have laying around.
Compared to the cost of aluminum enclosures these days the copper clad is probably cheaper.
If I had a source for aluminum mill stock here some aluminum bar with plate on the top and bottom would do. Just screw it all together. I'm getting tier of living in a small town. Cookie tins are starting to look appealing.

Cheer,
I like Par-Metal. Par-Metal
I think $100 or so is reasonable for a really good EMI and RFI resistant 1U box.
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Old 28th January 2014, 05:15 PM   #3490
davada is offline davada  Canada
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Originally Posted by dirkwright View Post
I like Par-Metal. Par-Metal
I think $100 or so is reasonable for a really good EMI and RFI resistant 1U box.
Thanks Dirkwright.

Are these all 19" wide? Do they do smaller enclosures?
No time look at the moment. Just home for lunch break.
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