[time-nuts] Low phase noise VCO

Nick Foster bistromat at hotmail.com
Wed Feb 10 05:36:09 UTC 2010


You're right, it's for a USRP. I just got annoyed with the constant frequency offset, so I'm rolling my own. Turns out there isn't much available for good off-the-shelf 64MHz VCXOs. The USRP2 has built-in support for 10MHz sync, but not having one, I'm left to what I do have. Can't injection lock the oscillator on board, as it's a self-contained square-wave clock. So it looks like I'm going to try my hand at a Butler VCXO.

That said, I know that for PLLs, the maximum control loop bandwidth you can use is limited by the pullability of your oscillator: if you use a VCXO with very low Kv, you might end up with a maximum useful loop bandwidth of 10Hz. No sense in using a 10544A to tune that! The phase noise performance would be pretty awful, since you can't tightly lock the reference oscillator to it. So does it make more sense to use a high-Q LC oscillator instead, with a much higher Kv? I'd be able to phase-lock it much more tightly to the reference oscillator. Plus, a 64MHz LC oscillator is pretty easy to build.


--n

> Date: Tue, 9 Feb 2010 21:50:57 -0700
> From: djl at montana.com
> To: time-nuts at febo.com
> Subject: Re: [time-nuts] Low phase noise VCO
> 
> Hi Bob and all:
> This is interesting, because I suspect this frequency source is for an
> Ettus Research USRP. A little further downtimeline I will be faced with
> this problem as well. The SDR is designed already and requires a 64 MHz
> clock, especially as this clock is used for the microwave transmit and
> receive front ends. I had planned a synthesizer filtered...
> The oscillator that is in the USRP that I have is good only to about 1
> part in 10^8 or so for accuracy, better in stability. maybe the existing
> osc. can be injection locked, or temp controlled in place.
> Don
> 
> Bob Camp
> > Hi
> >
> > With most SDR's a spur on the clock creates a spur in the radio. No matter
> > how you do your multiply, you will wind up with some sub-harmonics running
> > around. Much better / easier / quicker to start at 64 or 65 MHz.
> >
> > Bob
> >
> >
> > On Feb 9, 2010, at 8:32 PM, Bruce Griffiths wrote:
> >
> >> Thats not very useful when you want the 4th harmonic as its amplitude is
> >> zero fro a 25% duty cycle.
> >> Using a duty cycle of 1/8, 3/8 or 5/8 will maximise the amplitude of the
> >> 4th harmonic.
> >>
> >> see:
> >> http://www.wenzel.com/pdffiles1/pdfs/choose.pdf
> >>
> >> Bruce
> >>
> >> Max Robinson wrote:
> >>> If you start with a square wave odd order is all you can get but if you
> >>> start with a pulse with a 25% duty cycle you can get even order.  It's
> >>> best to optimize the pulse width for the harmonic you want.
> >>>
> >>> Regards.
> >>>
> >>> Max.  K 4 O D S.
> >>>
> >>> Email: max at maxsmusicplace.com
> >>>
> >>> Transistor site http://www.funwithtransistors.net
> >>> Vacuum tube site: http://www.funwithtubes.net
> >>> Music site: http://www.maxsmusicplace.com
> >>>
> >>> To subscribe to the fun with transistors group send an email to.
> >>> funwithtransistors-subscribe at yahoogroups.com
> >>>
> >>> To subscribe to the fun with tubes group send an email to,
> >>> funwithtubes-subscribe at yahoogroups.com
> >>>
> >>> ----- Original Message ----- From: "Nick Foster"
> >>> <bistromat at hotmail.com>
> >>> To: <time-nuts at febo.com>
> >>> Sent: Tuesday, February 09, 2010 6:35 PM
> >>> Subject: Re: [time-nuts] Low phase noise VCO
> >>>
> >>>
> >>>>
> >>>>> From: bill at iaxs.net
> >>>>> To: time-nuts at febo.com
> >>>>> Date: Tue, 9 Feb 2010 18:24:39 -0600
> >>>>> Subject: Re: [time-nuts] Low phase noise VCO
> >>>>>
> >>>>> Which leads me to ask a novice question:
> >>>>>
> >>>>> Why not pull a 16 MHz crystal and multiply to 64 MHz?
> >>>>>
> >>>>> If you count down from 64 to 10 MHz, isn't the multiplication inside
> >>>>> the
> >>>>> PLL?
> >>>>>
> >>>>> Perhaps the noise is multiplied by 4, but would it work for the
> >>>>> intended
> >>>>> purpose?
> >>>>>
> >>>>> Bill Hawkins
> >>>>
> >>>> Can you do x4 multipliers? I thought odd-order harmonics were usually
> >>>> used for multipliers. I'd be happy to be wrong!
> >>>>
> >>>> Nick
> >>>>
> >>
> >>
> >>
> >> _______________________________________________
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> >>
> >
> >
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> 
> -- 
> Dr. Don Latham AJ7LL
> Six Mile Systems LLP
> 17850 Six Mile Road
> POB 134
> Huson, MT, 59846
> VOX 406-626-4304
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> www.sixmilesystems.com
> 
> 
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