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sivankumarpsp New Member Offline Posts: 6 India	how to measure PSR and PSRR of a voltage regulator Apr 14^th, 2009, 9:26pm Hi, I have designed a voltage regulator which gives the output of 1.5V. I need to measure the PSR of this regulator. This voltage is given as supply to a VCO. Some one told me that i need to measure PSR at the frequency of VCO. Is it right? Also please tell me the difference between PSR and PSRR? Sivan
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raja.cedt

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Re: how to measure PSR and PSRR of a voltage regulator
Reply #1 - Apr 14^th, 2009, 11:17pm

hi Sivan,
what is PSR,i know PSRR(power supply rejection ratio).I think there is no relevance of finding psrr at vco frequency because noise is coming from supply,so you have to check your supply noise profile and find psrr at those frequency.But finding supply noise is a big problem,so people used to give a step noise with relevant rise time at the supply and find psrr, this is good way to find psrr.but people will say find do ac analysis form supply noise to output and find gain, i think this is not so correct because ac analysis will use small signal parameters so u have no freedom to put supply noise correctly.

Thanks,
Rajasekhar

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sivankumarpsp

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Re: how to measure PSR and PSRR of a voltage regulator
Reply #2 - Apr 16^th, 2009, 9:20pm

Hi Rajasekar,

Thanks for your reply. Actually when we plot the PSRR plot we will see the response curve from DC to certain frequency. Normally the PSRR value will be much high at lower frequenices and then it decreases. Since this supply is connected to Ring oscillator, the noise at that frequency may be added with the phase noise of VCO. That is what i got the explanation from my colleque. Also PSR is just power supply rejection. I think there is some difference between PSR and PSRR.

Regards,
Sivan

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raja.cedt

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Re: how to measure PSR and PSRR of a voltage regulator
Reply #3 - Apr 17^th, 2009, 5:44am

hi sivan,
i thought you are connecting regulator output is to control voltage or supply voltage of ring oscillator,in this case global supply will not come into oscillator picture.And ring oscillator current has some component of the oscillating frequency,apart from do you think why operating frequency will come into picture..majority of dependency on supply noise spectrum rather than fosc.If you are using different architecture than the above explained please attach the schematic.Hope you g0t some clear idea now....

Thanks,
Rajasekhar.

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sivankumarpsp

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Re: how to measure PSR and PSRR of a voltage regulator
Reply #4 - Apr 18^th, 2009, 2:34am

Hi Rajasekar,

Thanks for your prompt reply. I think you already understood the architecture. The voltage regulator is just dropping the voltage from 1.8 V to 1.5V. This 1.5V is the supply for the ring oscillator. Ring oscillator is a general differential ring oscillator runs with 300MHz. One guy asked me to measure the PSRR at 300MHz frequency. When i did the PSRR simulation, the PSRR is very very low at 300MHz. It is around -70dB in the DC and around -40dB at 100KHz. Can you please tell me normally at what frequency this PSRR should be measured. Thanks a lot for all your answers.

Regards,
Sivan

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NiceFeng

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Re: how to measure PSR and PSRR of a voltage regulator
Reply #5 - Apr 18^th, 2009, 9:27am

Hi, Sivan,

The following paper might be helpful :

Systematic Design of Supply Regulated LC-Tank Voltage-Controlled Oscillators
Wang, X.; Bakkaloglu, B.;
Circuits and Systems I: Regular Papers, IEEE Transactions on
Volume 55, Issue 7, Aug. 2008 Page(s):1834 - 1844
Digital Object Identifier 10.1109/TCSI.2008.918004

Regards,
Feng

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HdrChopper

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Re: how to measure PSR and PSRR of a voltage regulator
Reply #6 - Apr 19^th, 2009, 4:49pm

sivankumarpsp wrote on Apr 18^th, 2009, 2:34am:

Can you please tell me normally at what frequency this PSRR should be �measured. Thanks a lot for all your answers.

Hi Sivan,

Usually PSRR is a concern within the operation bandwidth of your load circuit. For example, when driving modulators, PSRR at harmonically related frequecies should be a concern since either phase noise or down-mixing effect might arise.
In your case since you are driving an oscillator I would in particular pay attention to the PSRR @ harmonics of your oscillator frequencies, especially if the oscillator phase noise is a concern.

Regards
Tosei

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Keep it simple

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yvkrishna

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Re: how to measure PSR and PSRR of a voltage regulator
Reply #7 - Apr 25^th, 2009, 3:37am

Hi Sivan,

As u mentioned PSR means power supply rejection, people use PSRR/PSR interchangebly. Actually PSRR is a ratio which is gain from supply to gain from input. In regulators we need to call it as PSR where you check the bode plot from applying a small signal at the supply. It looks something like this..

From DC to certain freq its almost flat and then reaches a maximum,again falls down to another value.

So you need to ensure your PSRmax to be within the spec in the process of design, however actual noise response can only be checked in transient simulations only as suggested by rajasekhar.

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