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Old 8th June 2012, 08:15 AM   #1
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Default Square signal on ZEN V9

Hi all
I finished my V9 and am now testing it. It heats normally and seems to be very stable in that respect. However, I feel disappointed with the shape of output signal when I feed it with square signals. There is a big amount of 'overshoot' and it gets better only with a 100 pF feedback cap. The same if I reduce the Voltage gain of the V9.
Is this 'normal' ? I tried to tweak some resistors and it is the same. This overshoot which could be 2nd order harmonic, appears at low level as much as at high level. It gets somewhat worse around 20 khz, but I suppose it is because the wave length of this harmonic has then about the same duration as the input square.
I read that one can get as low as <1% total distortion, I am miles away ...
Your ideas ?
Thanks
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Old 8th June 2012, 11:51 AM   #2
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Did you have a load on the output? What exactly?
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Old 8th June 2012, 12:05 PM   #3
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Yes, the load is an 8 ohms power resistor. The same with or without it ...
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Old 8th June 2012, 12:28 PM   #4
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change mosfet gate resistor's value (in both cascode and Aleph CCS) , rather than massive change in lag compensation cap
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Old 8th June 2012, 01:04 PM   #5
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OK, I'll try. Currently is 220 ohms.
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Old 8th June 2012, 04:56 PM   #6
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What is causing the overshoot and how to fix it may be two different functions? Fixing it with some other method than correcting the cause may complicate things. Gain would be one thing to look at but, since that had no effect? Can you post the actuall circuit schematic you built? with actual values?
What is the output Z of the signal source?
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Old 9th June 2012, 07:39 AM   #7
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Old 9th June 2012, 07:43 AM   #8
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Hi flg
here it is, it is rather difficult to get it from Proteus, here it is anyway.
Thanks for your help
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File Type: pdf Zen V9.pdf (83.0 KB, 59 views)
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Old 9th June 2012, 07:46 AM   #9
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The output source is a BOZ, working nicely with sqaure input. NOt perfect, but I will improve it when the whole chain will work as well. It is not the BOZ which gives me trouble. By the wa, if I feed the V9 directly from the generator, same problem.
Thanks again.
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Old 9th June 2012, 05:39 PM   #10
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Well, it's kind of solved.
There where a number of small issues:
- indeed the Gate resistor of Q4 was 220, too high, brought it gradually down to 47. Improved the signal a lot. Howzit possible ...?
- I tried IRFP 044, IRF 140, and an ALFET. The best signal (at 20 khz full swing) was with the Alfet but it had the lowest bandwith (about 80 KHZ). The 044 had a good bandwith but the worse signal, the 140 is the best compromise. the bandwith is about 150 khz estimated. After 20 khz the square is slightly out of shape but it does not show too much on triangles.
- max output is 20 Vpeak to pk.
- now the strange things: the two channels running, exactly the same parts and PCBs and PSU. One needs 18pf feedback the other none at all ! how is that possible ? I tried among a batch of 20 LU1014 and did not see much difference. the famous R3 supposedly between 1.02 and 1.05 ohms : no apparent difference between 0.9 and 1.2 ohms...? my Vds is at 2.95 Volts.
another one: at 20 khz, the squares are square enough on the top but the bottom is somewhat not horizontal, something integrating ? what could it be ?

Any explanations about these ?

I should be connecting the bugger soon, after checking wether it needs a delayed output switch.

Regards
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