"Tim Wescott" <tim@seemywebsite.really> wrote in message news:CcqdnaNQnZ-n4-_OnZ2dnUVZ5qSdnZ2d@giganews.com...> It's field strength, not voltage, that would potentially (pun not > intended) rip apart the fabric of space-time. > > Assuming that you could achieve a high enough field strength without > ripping electrons off of your negative electrode, you would eventually > generate real positron/electron pairs from the quantum foam. I think -- > I'm not a quantum mechanic.Something like that. Or, to put it another way, the simultaneous up-conversion of, oh, a few billion photons, merged into assorted mass-energy. :) (The only way you can do that without electrodes, of course, is at the focus of a rather intense laser beam; photons are necessarily involved, without having to invoke the virtual kind.) I don't think the same is ever true of the magnetic field, so there's some symmetry breaking going on there. Though that's partly because you'd be counting a static field, which does nothing; dB/dt is where it's at. But, that's just E again. Tim -- Seven Transistor Labs Electrical Engineering Consultation Website: http://seventransistorlabs.com
Quadrature Oscillator
Started by ●May 13, 2014
Reply by ●May 13, 20142014-05-13
Reply by ●May 13, 20142014-05-13
In article <e2cb986d-f22f-45ef-bd4a-cc29648ff212@googlegroups.com>, gherold@teachspin.com says...> > I'd be tempted to integrate-integrate-invert. It would take three op- > > amps, but you may have a better chance to get each section working > > correctly. Of course you'd need the obligatory teeny bit of lag > > somewhere, to make the oscillation start. > > Hmm OK, I always figured it just needed the right bit of gain somewhere. > I've never had that much problem getting oscillators to start...They always seem to get going and bump into the rail right away. (I love the way LTspice goes up to several tera volts, before it pukes.... > Can you even make a tera volt without ripping apart the fabric of space-time?) >Ltspice do have parameters you can use to avoid that, it expects you to be a good little designer and not make such bad circuits! Lets put it this way, it's a simple way to test a theory and not be bothered by power & voltage limits! :) Jamie
Reply by ●May 13, 20142014-05-13
whit3rd <whit3rd@gmail.com> wrote:> On Tuesday, May 13, 2014 1:15:54 PM UTC-7, John Silverman wrote: > >> This is getting way too complicated. Analog circuits drift and can >> never guarantee quadrature. A simple 74HC74 will. See >> >> >> >> http://www.wb5rvz.com/sdr/ensemble_rx_ii_vhf/04_div.htm > > There's ways to make an analog signal lock-in to a digital > quadrature clock, of course. > > There's also a switched-capacitor solution, see figure 15 > > <http://www.ti.com/lit/an/snoa572c/snoa572c.pdf>The MF10 is intended for use with center frequencies up to 20 kHz. He wants 1MHz.> And, if one wishes to go old-school, an LC oscillator > puts out sin(wt) voltage on the capacitor, and cos(wt) > current through the inductor; you can make a sense > coil lightly coupled to the inductor to pick off the second > phase. If Q is high, it needs no correction; if Q is low, > you can make a weighted sum of the two signals that is > tunable to get closer to 90.00 degrees phase, and > a simple multiplicative mixer makes a DC signal of the > error, to guide the tuning.Too complicated, drifty. A single 74HC74 is all that is needed.
Reply by ●May 13, 20142014-05-13
Tim Wescott <tim@seemywebsite.really> wrote:> On Tue, 13 May 2014 20:15:54 +0000, John Silverman wrote:>> This is getting way too complicated. Analog circuits drift and can >> never guarantee quadrature. A simple 74HC74 will. See >> >> http://www.wb5rvz.com/sdr/ensemble_rx_ii_vhf/04_div.htm > > If George can deal with square waves, yes, that's going to work way > better than an analog solution. > > If he needs sine waves, by the time he's gotten the harmonics beaten > out of that he'll be back to depending on analog components for his > precision.I dunno why he needs quadrature signals for a lockin, but presumably they are for I-Q. Mixers work better with square waves. Also, the 74HC74 guarantees symmetry, which reduces the even harmonics. Analog methods can drift, so you never know what you have.
Reply by ●May 13, 20142014-05-13
On Wed, 14 May 2014 01:05:59 GMT, John Silverman <Nospam@me.com> wrote:>whit3rd <whit3rd@gmail.com> wrote: > >> On Tuesday, May 13, 2014 1:15:54 PM UTC-7, John Silverman wrote: >> >>> This is getting way too complicated. Analog circuits drift and can >>> never guarantee quadrature. A simple 74HC74 will. See >>> >>> >>> >>> http://www.wb5rvz.com/sdr/ensemble_rx_ii_vhf/04_div.htm >> >> There's ways to make an analog signal lock-in to a digital >> quadrature clock, of course. >> >> There's also a switched-capacitor solution, see figure 15 >> >> <http://www.ti.com/lit/an/snoa572c/snoa572c.pdf> > >The MF10 is intended for use with center frequencies up to 20 kHz. He >wants 1MHz. > >> And, if one wishes to go old-school, an LC oscillator >> puts out sin(wt) voltage on the capacitor, and cos(wt) >> current through the inductor; you can make a sense >> coil lightly coupled to the inductor to pick off the second >> phase. If Q is high, it needs no correction; if Q is low, >> you can make a weighted sum of the two signals that is >> tunable to get closer to 90.00 degrees phase, and >> a simple multiplicative mixer makes a DC signal of the >> error, to guide the tuning. > >Too complicated, drifty. A single 74HC74 is all that is needed.While I agree with you that a quadrature counter is just ducky for synchronous demodulation when square wave will do, it's for crap if you want to use one phase as a sine wave excitation. "Drifty" has nothing to do with it when source and demod are "family" ;-) And it's downright trivial to make a 1% stable sine wave oscillator with a quadrature phase. ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
Reply by ●May 13, 20142014-05-13
On Wednesday, 14 May 2014 00:36:36 UTC+10, George Herold wrote:> Quadrature oscillator. >=20 >=20 >=20 > Hi all, As part of the CV measurements (from a previous post)=20 > I'm thinking about a 1 MHz lockin. > (Maybe a different thread to talk about how to do the switching.)=20 > I was wondering if I could make a quadrature oscillator from opamps.Here something that might be interesting http://www.janascard.cz/PDF/An%20ultra%20low%20distortion%20oscillator%20wi= th%20THD%20below%20-140%20dB.pdf The LME49710 is much too slow to be interesting at 1MHz - it's amazing dist= ortion performance stops being amazing above a few kHz - but the amplitude = control scheme might be worth looking at. I've played around (only in LTSpice) with using an AD734 to provide the var= iable part of the amplitude-controlling feedback for a 17kHz (100,000 radia= n per second) oscillator. At 1MHz you might look at an AD834. My amplitude control scheme looks a bit different - e-mail me if you want t= o look at it (since I'm thinking about publishing it if I ever get around t= o putting together and testing a real circuit). Using an AD825 or some other fast, low-distortion part to provide the bulk = of the gain (tweaked so that the AD834 is nominally doing nothing, and as = likely to provide negative feedback as positive as the component values dri= ft) is an attractive way to go.=20 Even "low distortion" four quadrant multipliers like the AD734 and AD834 ha= ve much higher levels of distortion than regular, low distortion op amps, b= ut if you only use them to tweak the loop gain the amount of extra distorti= on they feed in is small - verging on negligible if you spend heavily on st= able passive components. --=20 Bill Sloman, Sydney
Reply by ●May 14, 20142014-05-14
"Maynard A. Philbrook Jr." <jamie_ka1lpa@charter.net> wrote:> In article <e2cb986d-f22f-45ef-bd4a-cc29648ff212@googlegroups.com>, > gherold@teachspin.com says... >>> I'd be tempted to integrate-integrate-invert. It would take three op- >>> amps, but you may have a better chance to get each section working >>> correctly. Of course you'd need the obligatory teeny bit of lag >>> somewhere, to make the oscillation start. >> >> Hmm OK, I always figured it just needed the right bit of gain somewhere. >> I've never had that much problem getting oscillators to start...They >> always seem to get going and bump into the rail right away. (I love the >> way LTspice goes up to several tera volts, before it pukes.... >> Can you even make a tera volt without ripping apart the fabric of space-time?) >> > > Ltspice do have parameters you can use to avoid that, it expects > you to be a good little designer and not make such bad circuits! > > Lets put it this way, it's a simple way to test a theory and > not be bothered by power & voltage limits! :) > > > JamieI typically run LT Spice models at 500 kV. I'm too lazy to learn EMTP.
Reply by ●May 14, 20142014-05-14
On 05/13/2014 04:15 PM, John Silverman wrote:> Tim Wescott <tim@seemywebsite.really> wrote: > >> On Tue, 13 May 2014 13:03:28 -0400, Phil Hobbs wrote: >> >>> On 05/13/2014 12:03 PM, Tim Wescott wrote: >>>> On Tue, 13 May 2014 07:36:36 -0700, George Herold wrote: >>>> >>>>> Quadrature oscialltor. >>>>> >>>>> Hi all, As part of the CV measurements (from a previous post) >>>>> I'm thinking about a 1 MHz lockin. >>>>> (Maybe a different thread to talk about how to do the switching.) >>>>> I was wondering if I could make a quadrature oscillator from >>>>> opamps. (I've got some AD825's I thought I'd try.) >>>>> Anyway spice seems to think it's possbile. >>>>> Comments or ideas welcome. >>>>> (Oh I had to add the sine wave input to get the thing to start.) >>>>> You can play with C1 R7 C2 R8 to change the frequency. >>>>> >>>>> Here's a circuit scribble if you don't like the spice file >>>>> https://www.dropbox.com/s/sb5azyl69fy142t/DSCF0012.JPG >>>> >>>> How close to true quadrature and amplitude matching do you need? >>>> You'll be pretty dependent on both capacitor and op-amp >>>> characteristics for the match and phase shift between channels. >>>> >>>> If you really want precision, run a DDS at a high clock rate so the >>>> filtering can't add much phase shift or attenuation. >>>> >>>> >>> One method is to make one section a true integrator. That'll >>> guarantee quadrature. Then servo the frequency to make the >>> amplitudes equal. You'll need to measure the frequency to compute the >>> capacitance, but that's not so bad. >> >> That works if a "true" integrator is true enough at 1MHz, but leaves >> you with the problem of balancing the amplitudes. >> >> I'd be tempted to integrate-integrate-invert. It would take three op- >> amps, but you may have a better chance to get each section working >> correctly. Of course you'd need the obligatory teeny bit of lag >> somewhere, to make the oscillation start. >> > > This is getting way too complicated. Analog circuits drift and can never > guarantee quadrature.That isn't so. As long as slew limits are observed, an integrator will give a very accurate 90 degree phase shift. Servoing the frequency to make the amplitudes equal isn't very hard. That makes the component tolerances all come out as frequency shifts, and measuring the frequency accurately gets rid of all of it, for the purposes of capacitance measurement. A simple 74HC74 will. See> > http://www.wb5rvz.com/sdr/ensemble_rx_ii_vhf/04_div.htmJohnson counters are great in PLLs, where the square wave output is just what you want, but not much use for making quadrature sine oscillators (as others have noted). Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●May 14, 20142014-05-14
On Wednesday, 14 May 2014 23:47:02 UTC+10, Phil Hobbs wrote:> On 05/13/2014 04:15 PM, John Silverman wrote: > > Tim Wescott <tim@seemywebsite.really> wrote: > >> On Tue, 13 May 2014 13:03:28 -0400, Phil Hobbs wrote: > >>> On 05/13/2014 12:03 PM, Tim Wescott wrote: > >>>> On Tue, 13 May 2014 07:36:36 -0700, George Herold wrote:> That isn't so. As long as slew limits are observed, an integrator will > give a very accurate 90 degree phase shift. Servoing the frequency to > make the amplitudes equal isn't very hard.Servoing the amplitude isn't all that hard either. http://www.janascard.cz/PDF/An%20ultra%20low%20distortion%20oscillator%20with%20THD%20below%20-140%20dB.pdf shows one way of doing it without introducing much distortion. Analog multipliers are another (providing that you don't use them to provide the bulk of the gain).> That makes the component > tolerances all come out as frequency shifts, and measuring the frequency > accurately gets rid of all of it, for the purposes of capacitance > measurement.That's one way of doing it. Another is to use a parallel variable gain stage on feedback in quadrature, and control the frequency by a separate feedback loop.> Johnson counters are great in PLLs, where the square wave output is just > what you want, but not much use for making quadrature sine oscillators > (as others have noted).Running square waves through long shift registers, and hanging resistors on the taps to make a hard-wired FIR filters, can produce pretty good sine and cosine waves with a very well-defined phase relationship. IIRR the resistors need to be sinc-weighed, then tapered with Hamming window to avoid Gibbs oscillations (due to teh finite length of the shift register. You do need some low-pass filtering on each branch to smooth the treads out of the staircase approximation, but the harmonics involved can be pretty high and the delay through the filters correspondingly low and not all that hard to match accurately. -- Bill Sloman, Sydney
Reply by ●May 14, 20142014-05-14
On Tuesday, May 13, 2014 10:36:36 AM UTC-4, George Herold wrote:> Quadrature oscialltor. > > > > Hi all, As part of the CV measurements (from a previous post) > > I'm thinking about a 1 MHz lockin. > (Maybe a different thread to talk about how to do the switching.) >I was wondering if I could make a quadrature oscillator from opamps. > (I've got some AD825's I thought I'd try.) > Anyway spice seems to think it's possbile. > Comments or ideas welcome. > (Oh I had to add the sine wave input to get the thing to start.) > You can play with C1 R7 C2 R8 to change the frequency. > > Here's a circuit scribble if you don't like the spice file > https://www.dropbox.com/s/sb5azyl69fy142t/DSCF0012.JPG > >Hi guys, Well here's the data (opamp is an AD825). For R= 500/ C=1nF, Freq.= ~300kHz For R= 500/ C=120pF, Freq =~2.2MHz And for R=500 and C=39pF Freq.= 4.8 MHz. The X-Y plot for the last one is not very circle-ish, It looks almost like a pentagon... Maybe I can get some DOD funding :^) https://www.dropbox.com/sh/hv0sy2k7p9qjaoe/AACD18ouFQjv-4jFKYE1nJv4a sorry about the lines in some of the screen shots.. I don't know if that's my 'scope or the thumb drive. I labeled pics x-y300k, yT 2M.. etc. George H.
