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PLL ( Offset Loop ) configuration - RF Cafe Forums

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mac
Post subject: PLL ( Offset Loop ) configuration Posted: Tue Dec 09, 2008 3:22 am

Captain


Joined: Fri Nov 02, 2007 3:45 am
Posts: 8
Location: germany
Hi all,
I am in a PLL design process,
trying to tame a VCO 1.5-3.6GHz with a PLL.
Main goal is low phase noise -

cause the frequency range is that wide i thougt about dividing vco signal down first by variable N (maybe N= 6-16 to get to Frequency Range 250MHz - 292MHz) to narrow frequency range and mixing it down another some MHz afterwards ( e.g LO Frequ = 300MHz ), all in the same feedback path supplying phase detector (with this IF 8MHZ- 50MHz ).

I have often seen it the other way round, mixer at first then dividing down and feeding pd - but then i get another wide frequency range after the mixing process

anyone done this configuration before, or seen it ?
or is it out of question - worrying about spurs especially from divider harmonics feeding the mixer -

mny. thanks for comments

mac

_________________
regards
mac


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markapexwireless
Post subject: Posted: Wed Dec 10, 2008 3:38 pm

Captain

Joined: Tue Jul 08, 2008 7:15 pm
Posts: 11
Location: Boulder Colorado
Mac,

Most up mixing PLL synthesizers were used to generate small frequency increments at the output, which is no longer much of a problem with Fractional N synthesizers and DDS driven PLLs.

If your VCO must tune over an octave, you will probably be stuck with fairly poor phase noise outside the PLL bandwidth, which practically is over 200KHz or so from the carrier. A fundamental problem with VCOs is the wider they tune the higher the phase noise. If you mix a poor phase noise VCO with a clean one, you will get a poor phase noise VCO.

If you do not need to sweep entire band quickly, you could switch in multiple narrow band VCOs, each with better phase noise performance than one that tunes an entire octave. If you can design VCO, you could switch in multiple resonators essentially giving you lower noise sub-bands.

Good luck on your project.

- Mark -


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mac
Post subject: Posted: Thu Dec 11, 2008 3:22 am

Captain


Joined: Fri Nov 02, 2007 3:45 am
Posts: 8
Location: germany
Dear Mark,
thanks for your reply -

You're right, if i use a wideband vco phase noise gets worse cause of high
Kv from the vco etc. - i build some systems with switched vcos or resonators in the past but that was not what i am for with my question.
Maybe i missed to say that tuning speed is important, too and there is the old problem - Phase noise / Loop bandwith / switching speed all relates to each other.
I use this wideband vco to design a wideband synthesizer dividing down for the lower frequencies, with low noise and fast tuning capability and knowingly choose a wide bandwith pll.
I ve seen what all the other guys do from agilent/ rohde schwartz etc
with their wideband synthesizers using a mix of DDS/ Frac -N and convetional PLL Techniques and they use wideband vcos, yigs etc aswell

in past times as fast dividers were not available it was common to design plls with a down conversion mixer to improve phase noise because of running the phase detector on a lower frequency or using harmonic samplers combining downconversion mixer and phase detector


ref signal -- pd --- filter --- vco
\divider -- mixer /
\lo signal ( or harmonics, with vco presteering )

but now fast dividers are available and i wondered if i cant interchange divider and mixer in the feedback path or if i will get problems with this configuration

ref signal - pd - filter - vco
\ mixer - divider /


regardless of answers think i have to set this up and
evaluate it in the lab

mny thanks

mac

_________________
regards
mac


Top

markapexwireless
Post subject: Posted: Fri Dec 12, 2008 1:06 pm

Captain

Joined: Tue Jul 08, 2008 7:15 pm
Posts: 11
Location: Boulder Colorado
Mac,

Please post your results. We are always looking for techniques to reduce phase noise in fast tuning wideband synthesizers.

Thanks,

- Mark -








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