Hi A good 64 MHz VCXO should have *better* phase noise at 100 Hz than the multiplied 10 MHz out of a GPSDO. Even if you don't hit "state of the art" at 100 Hz, it will be better past a few hundred Hz.
A narrow loop is just fine. If the bandwidth gets to wide, you have to start worrying about the noise floor of the dividers and the phase detectors. With a narrow loop, a $1 or $2 all in one PLL / divider chip will do just fine. If you try to lock up something that's got -160 dbc/hz on a 10 MHz reference, that's not going to be the case. If you are building a VCXO from scratch, build more than one. Set up a simple quadrature / mixer / sound card phase noise tester. If you already have a good sound card, the rest of it is a sub $100 investment. Then you will know what you have. Bob On Feb 10, 2010, at 12:36 AM, Nick Foster wrote: > > 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: [email protected] >> To: [email protected] >> 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: [email protected] >>>>> >>>>> 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. >>>>> [email protected] >>>>> >>>>> To subscribe to the fun with tubes group send an email to, >>>>> [email protected] >>>>> >>>>> ----- Original Message ----- From: "Nick Foster" >>>>> <[email protected]> >>>>> To: <[email protected]> >>>>> Sent: Tuesday, February 09, 2010 6:35 PM >>>>> Subject: Re: [time-nuts] Low phase noise VCO >>>>> >>>>> >>>>>> >>>>>>> From: [email protected] >>>>>>> To: [email protected] >>>>>>> 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 >>>>>> >>>> >>>> >>>> >>>> _______________________________________________ >>>> time-nuts mailing list -- [email protected] >>>> To unsubscribe, go to >>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>> and follow the instructions there. >>>> >>> >>> >>> _______________________________________________ >>> time-nuts mailing list -- [email protected] >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >>> >> >> >> -- >> Dr. Don Latham AJ7LL >> Six Mile Systems LLP >> 17850 Six Mile Road >> POB 134 >> Huson, MT, 59846 >> VOX 406-626-4304 >> www.lightningforensics.com >> www.sixmilesystems.com >> >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. > > _________________________________________________________________ > Hotmail: Free, trusted and rich email service. > http://clk.atdmt.com/GBL/go/201469228/direct/01/ > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
