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Design >> RF Design >> Separate supplies for PLL blocks
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Message started by boa on Sep 24^th, 2009, 5:08am

Title: Separate supplies for PLL blocks
Post by boa on Sep 24^th, 2009, 5:08am

Hi all,

I am designing a fractional PLL and I have a question: what could be an optimal partition of PLL blocks between supply domains? Let's say I have VCO, Prescaler, PFD, CP and SDM and I have analog/digital power supplies from regulators. I would like to have a dedicated regulated power supply for VCO so that its ouput phase noise is not corrupted by anything else. SDM can be connected to general digital power supply (some other digital blocks over the chip are connected to it as well).

Is it OK to connect Prescaler, PFD and CP to the same power supply (e.g. I have 1 regulator for VCO, 1 - for everything else in PLL except SDM and SDM is connected to digital power supply)? Or could CP current pulses increase the jitter in Presclaler and PFD?
Or can I also connect PFD to a general digital power supply?

Thanks!

Title: Re: Separate supplies for PLL blocks
Post by loose-electron on Sep 29^th, 2009, 11:08am

Must have: VCO on its own supply.
After that its a little fuzzy - I would suggest that the switched current sources in the CP be filtered before they go thru the current steering process. The loop filter will LPF noise presented by the CP but I tend to keep this section quiet as a matter of good practice, and reduce the charge injection from the current steering switches as well.

Title: Re: Separate supplies for PLL blocks
Post by raja.cedt on Sep 29^th, 2009, 11:54pm

hi,
for VCO own supply means external? because typically people used to put high PSRR regulator in between filter and VCO rather than using separate supply from external. Correct me if am wrong?

Thanks,
Rajasekhar.

Title: Re: Separate supplies for PLL blocks
Post by loose-electron on Oct 2^nd, 2009, 9:21am

Not external - Typically an internal local regulator (LDO or an OTA type of structure.)

Intention is to provide isolation from the noise on the external power supply vs the internal VCO supply.

Oh, and if not known already - Never bring the loop filter outside the chip

Yes the filter is huge in size for the capacitor, but the noise on this node will lead to tons of jitter.

If you need to see what the voltage on the VCO input is doing, send it into an op-amp and bring the output of the opamp outside.

Title: Re: Separate supplies for PLL blocks
Post by boa on Oct 5^th, 2009, 2:16am

Hi,

Thanks for a reply. Could you please explain what do you mean about current sources filtering here: "switched current sources in the CP are filtered before they go thru the current steering process"? Do you mean decoupling caps between CP's VDD and GND or smth else?

About internal/external loop filter: some years I've seen in the lab Qualcomm's GSM (or WCDMA?) transceiver chip which had external loop filter - PLL's phase noise was very low. I am not saying that external filter is better, this is just a note that with a good design it was well in the spec. The other story is that everyone wants to have minimum number of external components to reduce area/cost...

Title: Re: Separate supplies for PLL blocks
Post by loose-electron on Oct 6^th, 2009, 11:06pm

0ther filter capacitors are what I am referring to on the charge pumps.

As for the Qcomm PLL, well I am not sure which particular design you are talking about (and I do design work for QC here in San Diego) but its a huge place with a lot going on.

Filters outside can be done, but as a general rule you pick up a lot of noise bringing that node outside the chip.

Title: Re: Separate supplies for PLL blocks
Post by cmos_cowboy on Oct 7^th, 2009, 4:45pm

loose-electron wrote on Oct 2^nd, 2009, 9:21am:

Oh, and if not known already - Never bring the loop filter outside the chip

Yes the filter is huge in size for the capacitor, but the noise on this node will lead to tons of jitter.

Hmmm. I would not say never. Depends on the PLL. Yes you need be very careful with the vtune node going off chip and coupling via the I/O ring etc. but I have worked on many low phase noise PLLs with external loop filters, most of which had high Kvco values.

Title: Re: Separate supplies for PLL blocks
Post by loose-electron on Oct 8^th, 2009, 9:31pm

I am pretty emphatic when it comes to known problems. A trip to the foundry can cost you as much as 2 million dollars (ebeam and masks for 32 and 28nm land these days) and the problems with taking this node outside are very well known. If you want to save 5 cents on die area and risk having to re-spin the chip and take a 2-3 month schedule hit, then go for it.

Title: Re: Separate supplies for PLL blocks
Post by cmos_cowboy on Oct 9^th, 2009, 2:14am

LE - I agree completely. Re-spins are expensive.

However the option to put the loop filter on or off chip depends on the application, process, cost etc. It is Horses for Courses.
e.g. for CMOS satelite tuners an external (3rd order) filter is sometimes the best option.

One thing an external filter obviously gives you is the ability to adjust the filter components without a re-spin.

I would always favor an on chip loop filter *if* the application allows.

There are a lot of very successful products out there with external loop filters.

Title: Re: Separate supplies for PLL blocks
Post by loose-electron on Oct 16^th, 2009, 11:06am

Agreed - even I have some devices out there with external filters, but I have also done things where the filter could be switched from external to internal under digital control, and for everything else the same the internal filters perform better, due to the reasons you stated.

Suggestion on internal filters - if you want a good amount of programmability on the filter parameters this can also be done under digital control

but then you get into - size vs. cost vs. performance vs. time to market.

Title: Re: Separate supplies for PLL blocks
Post by schen on Oct 16^th, 2009, 12:09pm

I did a PLL design about half year ago. CP is a tricky issue for lower noise and spurious issues. CP connection is not a trivier thing. Ideally if possible, you give CP power supply seperatedly from digital and VCO's. VCO power supply definitly needs to be regulated or well filtered. About internal filter or external filter usagew, My PLL also uses external filter. Of cause, using internal filter is better than external if area allows. But many times you just cannot use internal filter.

Title: Re: Separate supplies for PLL blocks
Post by Mayank on Nov 4^th, 2009, 4:13am

Hello Boa,
If Area is not a major concern, you should give your Oscillator a separate supply with an extra LDO maybe....If your area doesn't permit a separate LDO, atleast route the VCO supply separately & directly from PMU input. If spurs are not a major concern, then you can club together CP & PFD into same supply to avoid ESD concerns, level shifter noise etc., else club PFD and dividers together into digital supply.....On-Chip Loop Filter is preferred as it prevents extra transmission line delay+noise to get added to the circuit...To prevent re-spin as cmos_cowboy pointed out, introduce some programmability for Loop Filter...

--Mayank.