Ultra low frequency VCO
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From: Daku on 6 Jul 2010 12:10 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. > > You could digitally synthesize a 60Hz sine wave with a smallish > processor -- I believe there are some TMS430 parts that could do it all > in one package with a PWM output to be filtered by a simple RC. > > But "best" depends heavily on what you want. > > -- > > Tim Wescott > Wescott Design Serviceshttp://www.wescottdesign.com > > Do you need to implement control loops in software? > "Applied Control Theory for Embedded Systems" was written for you. > See details athttp://www.wescottdesign.com/actfes/actfes.html
From: Tim Wescott on 6 Jul 2010 12:52 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. 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. I think in another response I mentioned a TMS430 -- use one of those (or a PIC on an AVR or a Stellaris, etc.) with the right ADC, and you can build the whole PLL application into the software, and probably whatever measuring you're planning on doing as well. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html
From: George Herold on 6 Jul 2010 14:10 On Jul 6, 12:10 pm, Daku <dakup...(a)gmail.com> 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. Have you thought about a current charging and discharging a cap? This gives a nice triangle wave with the frequency proportional to the current. Just make sure the voltage that defines the 'trip' points is clean. (A mistake I made.) Or do you need a sine wave? George H. > > > > > > > You could digitally synthesize a 60Hz sine wave with a smallish > > processor -- I believe there are some TMS430 parts that could do it all > > in one package with a PWM output to be filtered by a simple RC. > > > But "best" depends heavily on what you want. > > > -- > > > Tim Wescott > > Wescott Design Serviceshttp://www.wescottdesign.com > > > Do you need to implement control loops in software? > > "Applied Control Theory for Embedded Systems" was written for you. > > See details athttp://www.wescottdesign.com/actfes/actfes.html- Hide quoted text - > > - Show quoted text -
From: Paul Hovnanian P.E. on 6 Jul 2010 14:56 Daku wrote: [snip] > Also, I am aware that S-parameter methods are not appropriate at these > low frequencies. Why not? Actual L/C component values might be a bit larger than most people here deal with, but the math is still the same. -- Paul Hovnanian paul(a)hovnanian.com ---------------------------------------------------------------------- Have gnu, will travel.
From: Paul Keinanen on 6 Jul 2010 16:16
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. |