Regulator noise

Aug 07, 2005 3 Replies
M

I'm wanting to put together an xmtr and rcvr pair using some Linx modules (ES series 916mhz) and it's a very simple straightforward process to get it done but....



I'd like to learn some more about the power supply requirements and the issue of regulator noise.



I see alot of suggestions for using a LDO regulator like a Micrel part and it seems that it's not just the fact that's low drop out but also has a good "noise figure". (The spec sheet says the module needs a supply with noise and hash below 20 mv).



I'd like to use one of my LM317 parts if possible (because I have a ton of them).



I know any circuit generates noise but where are the concerns with power supply regulators? What is going on in the regulator and what is the freq spectrum of particular concern?



Sure, I know all about fighting 60hz ripple...but I'd like to hear about the higher freq components that are in the mix.



I suppose the fact that the 317 requires a larger input voltage (compared to a LDO) is a factor in noise generation as well?



THANKS for help.


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M

Which is something I've always done......OK, thanks. I just looked at the datasheet on the 317 and see they show it as a percentage (.003) from 10hz to 10khz.

(15mv for a 5v output?)

If it's audio be choosy and use good design but if it's RF again, use good design but... the internal noise isn't a factor?

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A

The LM317 is perfectly adequate for your application. The datasheet quotes internally generated regulator noise as a percentage of output voltage. The frequency spectrum of concern is low to audio frequency.

Higher frequencies are dealt with using passive power supply filtering and decoupling e.g. using 100nF decoupling capacitors, and - if necessary - RF chokes.

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A

.003% of 5V is 0.15mV

Low frequency noise on the power supply can be a problem in any application.

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