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From: email@example.com (Dmitri Katchalov)
Subject: Re: FPGA+DAC: decoupling and impedance matching
Date: 16 Sep 2002 23:45:55 -0700
References: <firstname.lastname@example.org> <3D8692EB.683FEEB6@webaccess.net>
NNTP-Posting-Date: 17 Sep 2002 06:45:55 GMT
Chuck Simmons wrote in message news:<3D8692EB.683FEEB6@webaccess.net>...
> Dmitri Katchalov wrote:
> > I'm trying to get TV output from FPGA dev.
> > There are 3 problems with the picture:
> > 1) There is a fine moire pattern. As a result black is not
> > quite black and white is not quite white. I understand this
> > is most likely digital noise getting into analog output through
> > power supply rails.
> > 2) There are strong overshots on the borders of high contrast.
> > Eg. white areas have bright-white edges. I assume this has to
> > do with impedance matching. I wonder why, the datasheet seems
> > to indicate I can connect 75ohm output directly.
> > 3) There are horisontal stripes (about 20 scanlines each)
> > of varying intensity. These stripes run up or down the screen
> > for some time then they seem to get phase-locked and the
> > pattern stays still for some time then starts rolling again.
> > I don't have any idea about the source of this interference.
> > I'm really looking for a simple solution that works.
> > I don't need broadcast quality but I want to display text
> > as clearly as possible.
> > Thanks,
> > Dmitri
> 1) I suspect that the DAC is not perfectly glitchless. Filtering the
> video might help but see 2) below.
Well, this pattern fills entire screen, including all-black and all-white
areas so I don't thing it is a DAC glitch. In fact I also have areas
of ramp-up and ramp-down intensity in my test pattern and they look
fairly linear to me.
> 2) This is likely impedance mismatch. Is the TV monitor really 75 ohm?
> Is your cable 75 ohm? Change the cable length and see if there is some
> change in this effect.
I assumed they are 75 ohms but really I've no idea. It's an ordinary
TV with ordinary cable. It works just fine with other signal sources.
> 3) this would seem to be interference by some nearby device. For
> example, a close by computer monitor can cause this (I see the problem
> occasionally at work when we put a TV monitor too near a computer
> monitor). Move the TV to a new location and see if the problem changes.
I tried it on 2 TVs in different rooms with exactly the same results.
I also tried disconnecting the computer and switching it off. I blame
it on my switch-mode power supply, what else is there that can
phase-lock with my signal?
What I really want to know is:
* Is there any rule-of-thumb that I have overlooked?
* How do I make clean analog +5V from noisy digital +5V? The absolute
maximum difference between VCCA and VCCD must not exceed 0.5V
* How do I match impedances? The datasheet
seem to indicate I don't need to do anything, is it right?
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