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 Forum index » DIY Hardware and Software
how to tune a bias/offset?
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isak



Joined: Dec 13, 2009
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PostPosted: Tue May 27, 2014 9:23 am    Post subject:  how to tune a bias/offset? Reply with quote  Mark this post and the followings unread

hi guys.

i have a newbi question.
as the headline says, how do i know if the bias of the vco or lfo is ok and tuned or centered?
i have a scope, beside see the wave shapes and tune the shape i dint really know how to use it to check bias or offset.
any help will be grateful.

cheers,
Isak E.

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gdavis



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PostPosted: Tue May 27, 2014 10:01 am    Post subject: Reply with quote  Mark this post and the followings unread

1. Adjust ground on the scope.
2. View wave form and adjust offset so that the upper and lower peaks are evenly spaced above and below the center voltage (e.g. ground).

There should be a button or setting on the scope that internally grounds the scope channel input. This gives you a straight line that you position with the vertical adjust so that you know where ground is. If your signal is going to be centered around ground, you'll usually adjust the line so that it's on the center division. If the signal is centered around some positive voltage, set ground on one of the divisions near the bottom.

Now that you know where ground is, you should be able to tell when the signal is properly centered while adjusting the offset (don't touch the scope channels' vertical adjust once it's been set).

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isak



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PostPosted: Tue May 27, 2014 10:06 am    Post subject: Reply with quote  Mark this post and the followings unread

thank you very much!
i'll check that and let yo know.

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isak



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PostPosted: Mon Jun 09, 2014 11:28 am    Post subject: Reply with quote  Mark this post and the followings unread

hi gdavis.

there is a ground push button like you said, next to each channel.
ones i press the ground the wave shape that was displayed is gone.
when push the ground button again the wave shape is back.
i suppose it should work like that, but how can i set the offset when i cant see the wave shape when ground is pressed?
should i do it with the channel 2 on the scope?
meaning channel 1 will be the centered ground and channel 2 will be the wave shape?

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gdavis



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PostPosted: Mon Jun 09, 2014 3:18 pm    Post subject: Reply with quote  Mark this post and the followings unread

No, the voltages of the two channels are completely independent, you need to do this on one channel.

Oh, and I forgot to mention that the channel input needs to be set to DC, not AC.

You just push the ground button to allow you to set the point on the display that will represent ground, then once that's set, press it again to get the signal back. You can now measure the signal with respect to your ground point.

With the ground button pressed, you should get a horizontal line that you can move up and down with the vertical position adjustment. If not, you might need to turn on auto trigger.

Set that line to where you want ground to be (i.e. center division). This is your "ground reference". Now turn off the ground setting to get your signal back and you can see it's offset from ground.

The idea is that the scope is a flexible instrument. There isn't a specific point on the screen that's always ground, you can set ground where ever you want it to be, then measure your signals with respect to that point.

If it's still not clear I'll try to do a quick video tonight showing how to do it.

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isak



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PostPosted: Mon Jun 09, 2014 8:31 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks.

Let me see if I understand correctly.
I get the line when pressing ground, I move it to the center as you suggested then press the ground again and then I see the offset of the signal, right?

just wanted to make sure I need to press ground again to see the signal, that's it.

ok, let's tune some offsets Cool

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gdavis



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PostPosted: Mon Jun 09, 2014 8:52 pm    Post subject: Reply with quote  Mark this post and the followings unread

Yes, once you've set the ground you need to press the button again to see the signal.
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isak



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PostPosted: Mon Jun 09, 2014 11:26 pm    Post subject: Reply with quote  Mark this post and the followings unread

so...i pressed the ground and see the line as expected, i move it down to the first line on the screen, see pic:

Posted Image, might have been reduced in size. Click Image to view fullscreen.

now, i pressed the ground again, the wave shape appear on the screen, see pic:

Posted Image, might have been reduced in size. Click Image to view fullscreen.

the wave shape is of the x0xb0x, which works from 0 to 5V and not (+)(-).
the wave shape is directly from the vco.
as i understand from your explanation the saw wave is way offset, right?
the wave should be from the bottom line (ground reference) up to the 5th line, right?

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gdavis



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PostPosted: Tue Jun 10, 2014 12:11 am    Post subject: Reply with quote  Mark this post and the followings unread

Yes, it's offset high. If I'm reading the pics right, you have your scope set for 2V/DIV (indicated by the "2V" in the lower left) which means 5V would span 2.5 divisions, not 5.

The waveform in the pic spans 2.5 divisions so the amplitude looks correct (2.5DIV * 2V/DIV = 5V), but it's sitting just over 6V high (little more than 3 lines up from the bottom). Now you just need to trim the offset in the VCO until the bottom of the waveform is on the bottom line and you should be set.

The waveform should go from the bottom line up to half way between the second and third line above.

If you turn the VOLTS/DIV knob one step to the right you'll get 1V/DIV so 5V will be 5 lines. Then the waveform would go from the bottom to the 5th line up.

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isak



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PostPosted: Tue Jun 10, 2014 1:30 am    Post subject: Reply with quote  Mark this post and the followings unread

Your correct about the scope settings.
There is no trim for the x0x vco for offset, only for tuning it to scale (in tune).
I'm wandering if its suppose to be like that..mmm...
I know the x0x vca is offset by nature, but the signal you see is not going through the vca.
Nevermind, it's not the issue.
I think I understand how it suppose to work.
I'll gonna check this today with arp2600 vco and post the results.

Thanks man!

EDIT:

after checking the offset from the main output it set as it should.
i figure the vca will fix the offset.

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gdavis



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PostPosted: Tue Jun 10, 2014 11:30 am    Post subject: Reply with quote  Mark this post and the followings unread

Aha! I'm not familiar with the x0x but just did a little googling and it is indeed supposed to be offset just like you're seeing. The input to the VCF is AC coupled which removes the DC offset.
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isak



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PostPosted: Wed Jun 11, 2014 12:27 am    Post subject: Reply with quote  Mark this post and the followings unread

thank you gdavis for reading this!

i have another pro question.
is it possible to have a saw/square that runs from 0 to (+) and after a wave shape will be (-) and (+)?
meaning:
i build THeff's ARP2600 vco which sound very good and i love very much.
when checked the offset of the saw and square it looks on the screen that it works from 0 to (+), the tri is a wave shape from the saw, i checked the offset and it works from (-)(+), is it possible?
if its possible and i wanna tune the offset of all the wave shapes how its done?
my best guess will be the module after the vco, mixer, vcf etc...right?
if its ac coupled the dc offset will be gone in all the wave shapes assuming the vco is dc (how do i know the vco is dc?)

a lot of knewbi questions, i konw, still learning Smile

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gdavis



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PostPosted: Wed Jun 11, 2014 12:12 pm    Post subject: Reply with quote  Mark this post and the followings unread

isak wrote:
thank you gdavis for reading this!

i have another pro question.
is it possible to have a saw/square that runs from 0 to (+) and after a wave shape will be (-) and (+)?
meaning:
i build THeff's ARP2600 vco which sound very good and i love very much.
when checked the offset of the saw and square it looks on the screen that it works from 0 to (+), the tri is a wave shape from the saw, i checked the offset and it works from (-)(+), is it possible?


That's exactly how it's designed to work - saw and square are about 0-10V and tri and sine are about -5 - +5 V. If you search for the ARP 2600 service manual, it contains a pretty detailed description of how the thing works, including the VCO's and wave shapers. In fact, many old synths have detailed service manuals you can find online, good reading if you want to learn how they work.

Quote:

if its possible and i wanna tune the offset of all the wave shapes how its done?
my best guess will be the module after the vco, mixer, vcf etc...right?
if its ac coupled the dc offset will be gone in all the wave shapes assuming the vco is dc (how do i know the vco is dc?)

The first telltale sign that it's DC is that there is no series cap on the output. However, in general a circuit could still be trimmed so there's no DC offset. If there's no series cap and the schematic or documentation doesn't indicate the offset, you would just have to analyze the circuit.

How to adjust it depends on what you want to do. VCF inputs and/or outputs will often be AC coupled so you wouldn't need to do anything. VCA's might be AC couple but might not be, especially if they're meant to handle CV's. If they can handle 0-10V range the offset may not even be an issue, it's not audible as long as it doesn't cause anything to clip.

Using the VCO output as a CV might be a different issue depending how you want to modulate things with it. Some modules might have an offset adjustment on their CV input. If not, you could use a utility module that allows you to adjust things like offset.

Whole lot of possibilities depending on what you're doing.

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isak



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PostPosted: Wed Jun 11, 2014 11:24 pm    Post subject: Reply with quote  Mark this post and the followings unread

thank you for such a detailed answer.
i think i'll manage from here.

cheers Smile

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