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Isolation with a switch mode regulator - RF Cafe Forums

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darcyrandall2004

Post subject: Isolation with a switch mode regulator Posted: Wed Aug 08, 2007 8:11 am

Colonel

Joined: Tue Feb 27, 2007 6:16 am

Posts: 46

Hello.

It is true that the switch mode regulator produces alot of RF noise due to the switching action

Does the switching action of the regulator isolate its output from noise on the supply input? If at no time the output is directly connected to the input, then surely this is the case.

My latest transmitter project is suffering from noise creaping in through the supply. I thought perhaps I could remove the noise creeping in by use of a switch mode regulator and then deal with the switch mode regs noise.

What do you think?

Cheers

Darcy Randall, Perth Western Australia

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IR

Post subject: Posted: Fri Aug 10, 2007 5:56 pm

Site Admin

Joined: Mon Jun 27, 2005 2:02 pm

Posts: 373

Location: Germany

Hello,

I think that this is a bad idea to provide the direct power supply to almost any RF circuit from a SMPS.

The best practice is to use an SMPS followed by an LDO. By this you provide both efficiency and long battery life and a clean, noise-free power supply to your RF circuitry.

Quote:

Does the switching action of the regulator isolate its output from noise on the supply input?

As far as I know, the answer to that is no.

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darcyrandall2004

Post subject: Posted: Tue Aug 14, 2007 1:18 am

Colonel

Joined: Tue Feb 27, 2007 6:16 am

Posts: 46

Hello IR.

A 12V switch mode reg supplies a number of linear regulators that supply my RF circuits.

I will replace the Linear regs with LDO's and hopefullly this will solve my problems.

Cheers, you information has been of great value.

Darcy Randall, Perth, Western Australia

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IR

Post subject: Posted: Tue Aug 14, 2007 2:28 am

Site Admin

Joined: Mon Jun 27, 2005 2:02 pm

Posts: 373

Location: Germany

You are welcome.

If:

Quote:

A 12V switch mode reg supplies a number of linear regulators that supply my RF circuits.

Then I think you should add some more decoupling capacitors and/or beads to the supply rails and also check the PCB layout.

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darcyrandall2004

Post subject: Posted: Tue Aug 14, 2007 6:44 am

Colonel

Joined: Tue Feb 27, 2007 6:16 am

Posts: 46

Hello IR.

Do LDO's typically have superior noise performance compared to the standard typical linear voltage regulators?

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IR

Post subject: Posted: Tue Aug 14, 2007 10:27 am

Site Admin

Joined: Mon Jun 27, 2005 2:02 pm

Posts: 373

Location: Germany

From my experience they do have better noise performance. There are also many LDO's available (e.g. from Micrel: www.micrel.com) that have a dedicated pin used for extendedl noise suppression by connecting an external capacitor.

Please check Micrel's website for more information.

Good luck!! If you need more help, please let me know.

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