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In Reply to: Re: Jitter at what point in the chain? posted by Ted Smith on March 4, 2007 at 12:42:49:
I read a very plausible explanation, but unfortunately can't find the link, otherwise I'll include it here.Basically, as we all know logic transitions in digital circuits are edge trigerred through level changes. Ie. the precise point in time at which a 1 becomes a 0 is approximately halfway between a voltage transition from say 5V to 0V.
Therefore the precise timing of logic transitions is heavily influenced by the stability of the voltage rails.
So the theory is anything that can cause micro voltage fluctuations can cause jitter. Even things like power cords, connecting cables from one device to another, variations in speed of a fan or motor, etc.
Anyway, the theory seems plausible to me. The link which I can't find at the moment provides some empirical data around typical voltage fluctuations caused by logic induced modulation, and corresponding impact on jitter.
Follow Ups:
Howdyhttp://members.chello.nl/~m.heijligers/DAChtml/PLL/PLL1.htm
http://www.xilinx.com/xlnx/xweb/xil_tx_display.jsp?sTechX_ID=al_vias
States the case reasonably well, without going into too much jargon. Jon Risch is a poster around here, perhaps he can chime in as well.
I am not an expert on switching power, but it just occured to me that there may be possible to design a switching supply that tries to minimise logic induced modulation and back EMF (for example, by altering the switching frequency).Any comments?
May help explain why some designers are using switching power supplies in high end players.
HowdyI'm no expert there either, but it doesn't seem likely to me. Good ground planes, trace routing, trace impedance management, etc. as well as local power supply filtering and appropriate bypass caps, etc. make all the difference. (Once again I'm talking as a software guy that spent too much time in the lab helping with system debugging, not as a hardware guy.)
Well, the reason I'm speculating is based on some comments from Bruce Candy regarding the design of the Halcro amps.Anyway, I'm just wondering whether it's possible to design a switching power supply that switches at a rate synchronized to the master clock, and draws power from different parts of the cycle for different parts of the circuit. Some of the more sophisticated PC power supplies already do similar things to stabilise the load to the CPU vs the graphics card.
I wouldn't like to be person doing such a design - I don't need the headache!
HowdyYep, syncing the switching supply freq to the important local clocks can be useful (for example we did it in a video monitor to hide the power supply noise in the retraces.)
But if the switching supply freq isn't perfectly synced with the clock(s) in question you have to worry about beating and sometimes it's hard to vary it as fast you might need to if the incoming clock rate is changing... Also you obviously need to handle the edge cases like making sure that unplugging the DAC input doesn't cause the power supply to shut down :)
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HowdyI found this doing a quick search, and tho I'm pretty sure it isn't the article you were talking about it has some practical jitter info. (Don't forget to go the the second page where it talks more about some grounding issues (including the ground bounce I alluded to in this thread.)
http://www.elecdesign.com/Articles/Index.cfm?AD=1&ArticleID=4476
That wasn't the article I was thinking about, but it's a good article nevertheless.The interesting thing about logic induced modulation is that it explains why there is jitter coming out of an optical drive even though as some people love to point out there's a RAM buffer in there.
First of all, the memory is probably clocked at the same rate as the chip controlling the motor and laser, which is usually not a multiple of the audio clock. The clock source itself is probably synthesized through PLL. It's also likely to share power with the mechanical components of the drive. Result: jitter.
It also explains why a PC is typically not a good place to do audio. It's possible to get good audio (and let's face most studios these days use PCs for DAWs) but some care needs to be taken.
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