Ultra low frequency VCO
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From: Phil Hobbs on 12 Jul 2010 23:29 JosephKK wrote: > On Mon, 12 Jul 2010 09:39:54 -0400, Phil Hobbs > <pcdhSpamMeSenseless(a)electrooptical.net> wrote: > >> JosephKK wrote: >>> On Fri, 09 Jul 2010 11:56:28 -0400, Phil Hobbs >>> <pcdhSpamMeSenseless(a)electrooptical.net> wrote: >>> >>>> On 7/9/2010 8:59 AM, JosephKK wrote: >>>>> On Thu, 08 Jul 2010 15:37:28 -0400, Phil Hobbs >>>>> <pcdhSpamMeSenseless(a)electrooptical.net> wrote: >>>>> >>>>>> Phil Hobbs wrote: >>>>>> >>>>>>> I don't know that -100 dBc/Hz is that hard at 60 Hz. I bet you could do >>>>>>> that by running a bog standard multivibrator at 1024*1024*60 Hz and >>>>>>> dividing down. You'd need a sine shaper, but the phase noise goes down >>>>>>> by N**2, so you'd get 100 dB improvement just from that. Alternatively, >>>>>>> you could make an LC VCO and divide that down. >>>>>> 120 dB. Can't count today. >>>>>> >>>>>> Cheers >>>>>> >>>>>> Phil Hobbs >>>>> Sure, you can mathematically "predict" it, but how do you measure it? >>>>> Or do you switch to another metric which can be both predicted and >>>>> measured? >>>> Let's keep the math bashing to the other thread, okay? >>>> >>>> Although it isn't highly relevant to the OP's problem, it wouldn't be >>>> very difficult to measure the residual FM--use MOSFET buffers to drive >>>> two divider strings running from independent power supplies, and >>>> cross-correlate their outputs, exchanging them periodically to get rid >>>> of the drift in the correlator. For the correlator design, see Hanbury >>>> Brown and Twiss, circa 1963--and they did it with discrete bipolars. >>>> >>>> There are hard measurements, but this isn't one of them. >>>> >>>> Cheers >>>> >>>> Phil Hobbs >>> My issue was not so much the direct difficulty of the measurement, there >>> are several fairly straight forward setups. But with the _time_ it would >>> take to make the measurement using many of those setups. The elapsed >>> time seriously aggravates other measurement issues, notably including >>> calibration. >> Modulation frequency isn't affected by heterodyning or frequency >> multiplication and division. If you take a 60 MHz sine wave and FM it >> at 1 Hz modulation frequency and 1 MHz peak frequency deviation (M=1E6), >> then divide it by a million, you get a 60-Hz sine wave modulated at 1 >> Hz with a 1-Hz peak frequency division (M=1). >> >> Cheers >> >> Phil Hobbs > > I am sorry. I think i am misreading your post, are you saying you can > get a 1 MHz deviation on a 60 Hz carrier? Naw, you must be trying to say > something else and i misunderstood. You can put a 1 MHz phase modulation on a 60 Hz carrier, but you sure don't wind up with anything pretty. For instance, you could put the 60 Hz on a varactor-loaded transmission line, and drive the varactors with 1.000000000 MHz. As long as the varactors were driven really differentially, you wouldn't get any 1.000000000 MHz on the line. That's way outside the quasistatic limit, of course, which is where we're all used to working. It would be a nasty splattery mess, but you'd get _something_. But that wasn't the point I was trying to make. ;) Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
From: Phil Hobbs on 12 Jul 2010 23:43 Tim Wescott wrote: > On 07/08/2010 12:29 PM, Phil Hobbs wrote: >> Paul Keinanen wrote: >>> On Tue, 06 Jul 2010 09:52:43 -0700, Tim Wescott <tim(a)seemywebsite.com> >>> wrote: >>> >>>> On 07/06/2010 09:10 AM, Daku wrote: >>>>> On Jul 5, 8:59 pm, Tim Wescott<t...(a)seemywebsite.com> wrote: >>>>>> I'd hardly call 60Hz "ultra low frequency". But it is pretty darned >>>>>> low. >>>>>> >>>>>> All the suggestions you've gotten so far are good as far as they go >>>>>> and >>>>>> may well be perfect -- but what are you trying to do? Do you need >>>>>> sine >>>>>> wave out or square? If sine wave, how pure? Do you have any >>>>>> specifications on jitter, phase noise, or frequency accuracy? >>>>> I am trying to design a PLL for very low frequencies, e.g., power line >>>>> grid. >>>>> I am concerned with the VCO as it is a crucial sub-circuit. I am >>>>> aiming for >>>>> a phase noise of approximately -100 dBc/Hz but not very sure of the >>>>> offset >>>>> frequency. Ideally, I would like to have frequency accuracy of 1 - 5% >>>>> at most. >>>>> Also, I am aware that S-parameter methods are not appropriate at these >>>>> low >>>>> frequencies. >>> >>> If you want to track the _actual_ mains frequency, just use a mains >>> driven synchronous motor. To get the noise sidebands down, use some >>> flywheels :-). >>> >>>> I think that those specs would be difficult to achieve with an >>>> all-analog oscillator running at 60Hz. Not impossible -- I could do >>>> it, and Joerg could do it in a fraction of the time I'd take. Using >>>> some sort of direct digital synthesis -- even if it's just a >>>> microprocessor -- running off of a crystal reference would be almost >>>> trivial in comparison and would probably take less board space and >>>> would be far more repeatable in manufacturing. >>>> >>>> If you just had to do this purely in the analog domain your best bet >>>> might be a pair of crystal oscillators, frequency steered with >>>> varactors, carefully built, and with their outputs mixed down to >>>> 60Hz. But that's a solution I would expect to see in a bit of kit >>>> from the 50's through the 80's -- anything later and I'd expect to >>>> see a DDS. >>> >>> Just a few minutes ago, the Nordel AC network (Danish isles, Finland, >>> Norway, Sweden) was running at 50.11 Hz or +2200 ppm above nominal in >>> order to allow the mains synchronized clocks to catch up. >>> A simple fundamental frequency VXCO can be pulled about +/-100 ppm >>> with the load capacitance. About 1000 ppm is the maximum with >>> adjustable serial inductance and adjustable parallel load capacitance >>> at the crystal. >>> >>> At 50/60 Hz, even a trivial processor can generate a variable >>> frequency sine wave using the NCO (Numerically Controlled Oscillator) >>> principle to generate a sine wave, which can be locked to the incoming >>> signal in some loop configuration. >>> >>> Even a trivial processor might be able to generate both sine and >>> cosine waveforms for 49.98, 50.00. 50.92 Hz etc. in parallel and >>> performing a phase comparison between all these in parallel to >>> determine the best match. >>> >> >> I don't know that -100 dBc/Hz is that hard at 60 Hz. I bet you could do >> that by running a bog standard multivibrator at 1024*1024*60 Hz and >> dividing down. You'd need a sine shaper, but the phase noise goes down >> by N**2, so you'd get 100 dB improvement just from that. Alternatively, >> you could make an LC VCO and divide that down. > > This actually kind of makes my point, which I didn't state clearly: if > you _don't_ use a divider it'll be hard. With a divider it gets easy, > as long as you ignore clock jitter in the divider (and clock jitter > probably isn't a big deal, given the output frequency). > >> You might even be able to do it with all analog--the OPA378 has 20 >> nV/sqrt(Hz) all the way down to DC. With a 5V sine wave at 60 Hz, that's >> something like 1800 V/s, so 20 nV gives you something like 10 >> picoseconds per root hertz. You probably lose a factor of sqrt(2) in >> there, but that ought to be good enough. Your ALC network would >> contribute more than that, almost for sure. > > Depending on how close to the carrier you want to get, you lose a factor > of up to infinity (if you get _really_ close to the carrier). > > The noise gain is something like 1/(s^2 + w0^2) -- it's an oscillator. > Worse, because it's an RC, the constant you're multiplying by is greater > than one -- I get Hn(s) ~ 15/(s^2 + w0^2). That's not taking the > current noise of the part into account (which, I admit, I haven't > checked on because I'm lazy). > > 1Hz away your noise gain is just about 200, for 4uV/sqrt(Hz). That's > doing OK, but at 0.1Hz away the noise gain is about 2000 -- all you have > to do is measure close enough to the carrier at a wide enough bandwidth > and your noise is too high (sure would be nice if the OP specified what > he wanted, but I think we lost him). > Sorry to be slow responding. That's an interesting point about the noise gain of the oscillator. I think we agree that the OPA378 is okay down to 1 Hz offset or thereabouts, anyway. (It's a very nice part btw--I'm using it in some millihertz things just now.) Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
From: JosephKK on 12 Jul 2010 23:47 On Mon, 12 Jul 2010 09:58:32 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon(a)On-My-Web-Site.com> wrote: >On Mon, 12 Jul 2010 08:33:56 -0700, John Larkin ><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >>On Mon, 12 Jul 2010 10:40:00 -0400, Phil Hobbs >><pcdhSpamMeSenseless(a)electrooptical.net> wrote: >> >>>Jim Thompson wrote: >>>> On Fri, 09 Jul 2010 14:08:28 -0400, Phil Hobbs >>>> <pcdhSpamMeSenseless(a)electrooptical.net> wrote: >>>> <snip> >> >>One interesting and often overlooked part is the coaxial ceramic >>resonator. It's essentially a shorted transmission line formed in a >>block or tube of hi-K ceramic, usually by silver or copper plating it. >>They are usually treated by the RF boys as resonators or inductors, >>but they really act like time-domain transmission lines. TCs are in >>the single-digit PPMs and Qs in the hundreds or thousands. Dielectric >>constants are in the hundreds or thousands, so they are very short for >>their delay/frequency. >> >>Remarkable parts. I use them to make instant-start/instant-stop >>oscillators in the 600 MHz range. As a VCO, they will have very low >>phase noise, somewhere between an LC and a quartz crystal. >> >>John > >I've been "using" them... designing them into GPS LO's since before >you were born ;-) > > ...Jim Thompson That is really good since GPS itself is not that old.
From: Jim Thompson on 13 Jul 2010 00:01 On Mon, 12 Jul 2010 20:47:12 -0700, "JosephKK"<quiettechblue(a)yahoo.com> wrote: >On Mon, 12 Jul 2010 09:58:32 -0700, Jim Thompson ><To-Email-Use-The-Envelope-Icon(a)On-My-Web-Site.com> wrote: > >>On Mon, 12 Jul 2010 08:33:56 -0700, John Larkin >><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >> >>>On Mon, 12 Jul 2010 10:40:00 -0400, Phil Hobbs >>><pcdhSpamMeSenseless(a)electrooptical.net> wrote: >>> >>>>Jim Thompson wrote: >>>>> On Fri, 09 Jul 2010 14:08:28 -0400, Phil Hobbs >>>>> <pcdhSpamMeSenseless(a)electrooptical.net> wrote: >>>>> ><snip> >>> >>>One interesting and often overlooked part is the coaxial ceramic >>>resonator. It's essentially a shorted transmission line formed in a >>>block or tube of hi-K ceramic, usually by silver or copper plating it. >>>They are usually treated by the RF boys as resonators or inductors, >>>but they really act like time-domain transmission lines. TCs are in >>>the single-digit PPMs and Qs in the hundreds or thousands. Dielectric >>>constants are in the hundreds or thousands, so they are very short for >>>their delay/frequency. >>> >>>Remarkable parts. I use them to make instant-start/instant-stop >>>oscillators in the 600 MHz range. As a VCO, they will have very low >>>phase noise, somewhere between an LC and a quartz crystal. >>> >>>John >> >>I've been "using" them... designing them into GPS LO's since before >>you were born ;-) >> >> ...Jim Thompson > >That is really good since GPS itself is not that old. I did my first Garmin chip more than 20 years ago. ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | Obama isn't going to raise your taxes...it's Bush' fault: Not re- newing the Bush tax cuts will increase the bottom tier rate by 50%
From: Paul Keinanen on 13 Jul 2010 01:31
On Mon, 12 Jul 2010 19:57:54 -0700, "JosephKK"<quiettechblue(a)yahoo.com> wrote: > >Now what is the equivalent bandwidth of -100 dBc for a 60 Hz carrier? >Since you said 20 log() basis 60 * 10^-5 is 600 microHz. That would have >to take some minutes, and if you wanted a proper 10 to 1 measurement >buffer, it takes ten times longer. Call it 10,000 seconds? A few hours. >And the reference stability etc., i remarked on is coming into play. To put this into perspective, national and continent wide mains networks are typically operated so that the 24 h average cycle count exactly match the nominal mains frequency, so that synchronous clocks would remain correct on average for long periods of time (days, months). However, the instantaneous frequency error can be several thousand ppms and while the time error is usually kept below +/-30 s so that clocks with hour and minute displays only would not be in error more than for the last digit, this corresponds to up to +/-650.000 degrees total phase error during the day. |