Ultra low frequency VCO
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From: Phil Hobbs on 8 Jul 2010 15:29 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. 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. 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 8 Jul 2010 15:37 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 -- 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 9 Jul 2010 08:59 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?
From: Phil Hobbs on 9 Jul 2010 11:56 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 -- 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: whit3rd on 9 Jul 2010 13:22
On Jul 8, 12:29 pm, Phil Hobbs <pcdhSpamMeSensel...(a)electrooptical.net> 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 Eh? I'd think it's N**0.5 (the multivibrator has cumulative but random errors). |