CGS48 VCO Problems

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Post by guest » Tue Mar 29, 2016 2:46 pm

yan6 wrote:Im headed home in a few minutes if you need any scope readings from anywhere just let me know
if its not too much trouble, could you double check that the 68pF is 68pF? i noticed from your photos that it has a black dot on the top, just like the 18pF across the way.
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Post by guest » Tue Mar 29, 2016 2:49 pm

srfnsmy wrote: Ok I took out the 68pF and put in a 47pF in (smallest i had). Its a little less round on the top! Its not that I want a perfect saw, I just assumed this would be the same problem that I'm having with the Sine and a couple other features of the board (Sub & Staircase).
did you try it without any capacitor? what did the sine look like without a capacitor? i think this is the issue with the sine, as the sine is a fullwave rectifier, so when you flip the rounded off waveform around its midpoint, you get double humps at the peaks.
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Post by yan6 » Tue Mar 29, 2016 3:04 pm

I pulled it out of the rack, opened it up and confirmed that it is a 68pf and what looked like a jumper is a 100k resistor rnmf series, so those little ones

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Post by srfnsmy » Tue Mar 29, 2016 3:09 pm

guest wrote:did you try it without any capacitor? what did the sine look like without a capacitor? i think this is the issue with the sine, as the sine is a full wave rectifier, so when you flip the rounded off waveform around its midpoint, you get double humps at the peaks.
I didn't try it without a cap in there - i was in a hurry before work and just wanted to see if there would be any results with a different cap in there, which there was a small change as far as i could tell..

will have to stop for 10pF caps on the way home.. and a 220pF ceramic,,

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Post by guest » Tue Mar 29, 2016 3:41 pm

srfnsmy wrote: will have to stop for 10pF caps on the way home.. and a 220pF ceramic,,
the ceramic versus poly doenst make much difference for that.
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Post by guest » Tue Mar 29, 2016 3:45 pm

yan6 wrote:I pulled it out of the rack, opened it up and confirmed that it is a 68pf and what looked like a jumper is a 100k resistor rnmf series, so those little ones
now im really curious. does the saw out still look like a saw out? you dont have to post pictures or anything, but a double check would be interesting. and is the 68pF actually in-circuit (ie, no cracked traces or cold solder joints)? im trying to figure out why it isnt filtering off the peaks like the one on my breadboard.
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Post by BugBrand » Tue Mar 29, 2016 3:56 pm

One further thought (and, yes, it does all seem strange)
- that first scope shot suggests you had it running there around 6kHz. Does the same behaviour occur at lower freqs? I'd tend to be checking more around sub 1kHz initially.

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Post by guest » Tue Mar 29, 2016 4:02 pm

BugBrand wrote:One further thought (and, yes, it does all seem strange)
- that first scope shot suggests you had it running there around 6kHz. Does the same behaviour occur at lower freqs? I'd tend to be checking more around sub 1kHz initially.
thats a good point. yan6 had a 200Hz signal on the scope, so maybe the rounded section isnt visible as a result.
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Post by yan6 » Tue Mar 29, 2016 4:14 pm

I just took these photos of the various outputs at ~1.5kHz

Image

Image

Image

Image

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Post by yan6 » Tue Mar 29, 2016 4:16 pm

I did notice that the distinct distortion is missing from the OP's sine wave

EDIT: forgot the sine wave pic :razz:

Image

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Post by guest » Tue Mar 29, 2016 4:30 pm

ha! thats totally it. this can be added to the debugging facepalm thread. well, if anyone wants the dip to go away in the sinewave, or to have a sharper triangle, getting rid of that capacitor will do the trick.
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Post by yan6 » Tue Mar 29, 2016 4:46 pm

:doh: that's funny

Good eye BugBrand :tu:

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Post by tojpeters » Tue Mar 29, 2016 5:00 pm

Oscillator running at 100Hz. Taken from output jacks. Sorry they all are sideways. Module running on 12v

Saw DC
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Saw AC
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Square
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STOS,DC, shape knob full CCW
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STOS,AC,CCW
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STOS,DC, shape knob full CW

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STOS,aC, shape knob full CW

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Last edited by tojpeters on Tue Mar 29, 2016 5:18 pm, edited 1 time in total.

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Post by srfnsmy » Tue Mar 29, 2016 5:05 pm

guest wrote:ha! thats totally it. this can be added to the debugging facepalm thread. well, if anyone wants the dip to go away in the sine wave, or to have a sharper triangle, getting rid of that capacitor will do the trick.
Wow! Great work guys! Cant wait to get home and check this out..

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Post by srfnsmy » Wed Mar 30, 2016 11:49 am

Ive removed the 68pF cap and put the 240k resistor back in.

But now the mutating waveform is back! :zombie:

it did improve the wave shapes though..

here is the sine
Image

saw ac
Image

saw dc
Image

its got to be a bad connection somewhere :deadbanana:

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Post by BugBrand » Wed Mar 30, 2016 11:56 am

Why did you change back to 240K?!!
That was key to all downwind shapers - changing it brought the saw to be correctly centred around 0V.

So change it back to whatever (400k?) you'd figured before.

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Post by yan6 » Wed Mar 30, 2016 11:58 am

Put all components back as per the build instructions, lower the frequemcy to 1k and send some scope shots. It looks like your still sitting at about 4khz with your sine wave

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Post by srfnsmy » Wed Mar 30, 2016 12:09 pm

BugBrand wrote:Why did you change back to 240K?!!
That was key to all downwind shapers - changing it brought the saw to be correctly centred around 0V.

So change it back to whatever (400k?) you'd figured before.
The Saw is centered around 0V is it not?

These pics i just uploaded are with the 240K installed.

I figured since yan6's build included it that mine should too?

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Post by srfnsmy » Wed Mar 30, 2016 12:11 pm

although the sine is pretty hot by the looks of that photo.. :hmm:

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Post by BugBrand » Wed Mar 30, 2016 12:21 pm

BugBrand wrote:That was key to all downwind shapers - changing it brought the saw to be correctly centred around 0V.
Actually I may well be wrong about the sine & TSO sections needing the saw to be balanced around 0V - they have input caps and use the LM3900 (which I never got my head around)

I still think you should have the 240k changed to bring the DC saw down to centre around 0V.

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Post by guest » Wed Mar 30, 2016 1:22 pm

actually, this is all looking pretty good, but i would reccomend testing a bit lower, so you can compare to the screen shots from the other builders.

is the sine adjusted there? the sine shaper does 2 things: 1. it does a fullwave rectification, and 2. it changes the slope at a certain breakpoint to round off the top. it is doing both of these things, but the right now the breakpoint is too low, so you have that funky looking wave. you should be able to adjust the input amplitude with the trimmer to get the breakpoint to happen lower.
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Post by srfnsmy » Wed Mar 30, 2016 3:11 pm

these should be better:
Image
Image
Image
Image
Image

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Post by guest » Wed Mar 30, 2016 5:22 pm

does the trim pot do anything? it should be able to get a good sinewave at this point if you turn the amplitude down. does turning the trimpot all the way down make the sinewave disappear?
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Post by tojpeters » Wed Mar 30, 2016 5:39 pm

Are you using a good quality name brand trimmer?

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Post by guest » Wed Mar 30, 2016 6:26 pm

BugBrand wrote:they have input caps and use the LM3900 (which I never got my head around)
i found this really helpful in trying to understand the LM3900:
http://users.ece.gatech.edu/~lanterma/s ... new_an.pdf

although i dont really see any great advantage to them outside of the fullwave rectifier example (although 2 opamps do it better anyways).
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