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Heck of a way to run an oscillator?

Started by skeptical engineer May 13, 2014
>wrote in message news:vqh8n9p23ldhc76l7m1fd1fbpq0qrekdnn@4ax.com...
>On Tue, 13 May 2014 06:14:54 -0700 (PDT), skeptical engineer
<haiticare2011@gmail.com> wrote:
>>I was looking at the SAW oscillator circuits at a co. called >>Epcos, and they just take an op amp with the SAW filter in the feedback >>loop. They explain that the op amp amplifies noise, but preferentially >>at the filter allowed frequency. 2 components. > >>I love the simplicity, but does this approach have any drawbacks?
>When first switched on, the amplifier input related thermal noise is >amplified, Part of it is going to the frequency sensitive network, >which attenuates the lowest and highest frequencies, leaving a broad >peak around the resonant frequency.
If this were necessary, than Spice simulations would have problems as they don't have noise! This is not how an oscillator functions. An oscillator is not amplified noise. Oscillators will work without any noise at all. If the system is unstable, any initial DC condition that does not sit at a metastable/unstable equilibrium point will ensure that signals start changing in such a way as to head toward amplifier limiting. This may result in either steady state or chaotic oscillations. Usually a clean (power on) startup pulse will excite the tank, and it is this ringing that generates the oscillation build-up, just as it does in the real world. This bit about noise starting up the circuit is often quoted in the books, and er... ah...somewhat mindlessly repeated, but is very dubious. Theoretically numerical noise could start a simulation, and sometimes does, but it is not necessary. Kevin Aylward B.Sc. www.kevinaylward.co.uk www.anasoft.co.uk - SuperSpice
>"skeptical engineer" wrote in message >news:cfab6431-6d89-4c11-bd4a-5289d9ac8517@googlegroups.com...
>I was looking at the SAW oscillator circuits at a co. called >Epcos, and they just take an op amp with the SAW filter in the feedback >loop. They explain that the op amp amplifies noise, but preferentially >at the filter allowed frequency. 2 components.
The description of the oscillator being amplified noise is, essentially, twaddle. It is true that noise might start an oscillator, but so also will turning on the power supply, even if there were zero noise the circuit. A system linear system is unstable dependant on the presence of a plane in the right half plane. This means that there is a condition where an increasing AC waveform is generated, which, typically, limits at a power rail producing a steady state waveform. Even a DC initial condition that is not a DC solution to the circuit one will cause this waveform ramp start-up. Kevin Aylward B.Sc. www.kevinaylward.co.uk www.anasoft.co.uk - SuperSpice
On Fri, 16 May 2014 14:31:31 +0100, "Kevin Aylward"
<ExtractkevinRemove@kevinaylward.co.uk> wrote:

>"Jim Thompson" wrote in message >news:vbj9n9p3gebit51f22nagtmlfo3uaebe7p@4ax.com... > >On Thu, 15 May 2014 09:29:42 +1000, Clifford Heath ><no.spam@please.net> wrote: > >>>On 15/05/14 04:43, haiticare2011@gmail.com wrote: >>>> On Wednesday, May 14, 2014 10:46:54 AM UTC-4, Joerg wrote: >>>>> Cheapskates use logic parts for that because you get a six-pack for a >>>> OK thanks. - What logic part would produce a sine wave output? >> >>>One that has no extra gain than what is required to start, so it's not >>>constantly slamming against the amplitude limit. One that limits its >>>amplitude into a resistive load. Or one fitted with an AGC circuit. It >>>helps if the clipping is somewhat soft. Look at the 74HCU04 inverters >>>for a start. > >>"Clipping" is for digital guys. AGC is necessary for low distortion. > >Usually, however, clipping/clamping/limiting pretty much invariably produces >the lowest phase noise. Simple principle, is the more devices, the more >noise.
The best performance I've attained (at least distortion-wise) was a clipper whose value was adjusted by an "AGC" loop.
> >I have actually spent some time running sims on just about every reasonable >oscillator topology you can imagine for phase noise. One transistor wins. > > >Kevin Aylward B.Sc. >www.kevinaylward.co.uk >www.anasoft.co.uk - SuperSpice
...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.
On 05/16/2014 09:38 AM, Kevin Aylward wrote:
>> wrote in message news:vqh8n9p23ldhc76l7m1fd1fbpq0qrekdnn@4ax.com... > >> On Tue, 13 May 2014 06:14:54 -0700 (PDT), skeptical engineer > <haiticare2011@gmail.com> wrote: > >>> I was looking at the SAW oscillator circuits at a co. called >>> Epcos, and they just take an op amp with the SAW filter in the feedback >>> loop. They explain that the op amp amplifies noise, but preferentially >>> at the filter allowed frequency. 2 components. >> >>> I love the simplicity, but does this approach have any drawbacks? > >> When first switched on, the amplifier input related thermal noise is >> amplified, Part of it is going to the frequency sensitive network, >> which attenuates the lowest and highest frequencies, leaving a broad >> peak around the resonant frequency. > > If this were necessary, than Spice simulations would have problems as > they don't have noise!
They do, actually, just like all other numerical simulators. It comes from roundoff.
> > This is not how an oscillator functions. An oscillator is not amplified > noise.
Oscillators build up from noise, though, and they do amplify the close-in noise very strongly. See e.g. http://rfic.eecs.berkeley.edu/~niknejad/ee242/pdf/eecs242_lect22_phasenoise.pdf Oscillators will work without any noise at all. If the system is
> unstable, any initial DC condition that does not sit at a > metastable/unstable equilibrium point will ensure that signals start > changing in such a way as to head toward amplifier limiting. This may > result in either steady state or chaotic oscillations.
Not necessarily. There has to be some nonzero signal in the band where the oscillator is unstable.
> > Usually a clean (power on) startup pulse will excite the tank, and it is > this ringing that generates the oscillation build-up, just as it does in > the real world.
Sometimes. It's easily possible that the power-up transient is smooth enough that its spectrum dies away to well below device noise in the relevant band. Good oscillators are designed so that that is the case, because otherwise their supply rejection would stink.
> > This bit about noise starting up the circuit is often quoted in the > books, and er... ah...somewhat mindlessly repeated, but is very dubious. > > Theoretically numerical noise could start a simulation, and sometimes > does, but it is not necessary.
Arbitrarily sharp power-up steps kickstart the oscillation in simulation, maybe, but building up from noise will occur very often in real life, due to the aforementioned roll-off in the spectrum of the actual power-up transient in well-designed oscillators. Superregenerative receivers, for instance, are limited by the amplified thermal noise during the brief period when the quench waveform crosses the regeneration threshold, and the simple theory predicts their noise pretty accurately. If you can get a copy, "Superregenerative Receivers" by J. R. Whitehead is a really good read. 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
Kevin Aylward wrote:
> "Jim Thompson" wrote in message > news:vbj9n9p3gebit51f22nagtmlfo3uaebe7p@4ax.com... > > On Thu, 15 May 2014 09:29:42 +1000, Clifford Heath > <no.spam@please.net> wrote: > >>> On 15/05/14 04:43, haiticare2011@gmail.com wrote: >>>> On Wednesday, May 14, 2014 10:46:54 AM UTC-4, Joerg wrote: >>>>> Cheapskates use logic parts for that because you get a six-pack for a >>>> OK thanks. - What logic part would produce a sine wave output? >> >>> One that has no extra gain than what is required to start, so it's not >>> constantly slamming against the amplitude limit. One that limits its >>> amplitude into a resistive load. Or one fitted with an AGC circuit. It >>> helps if the clipping is somewhat soft. Look at the 74HCU04 inverters >>> for a start. > >> "Clipping" is for digital guys. AGC is necessary for low distortion. > > Usually, however, clipping/clamping/limiting pretty much invariably > produces the lowest phase noise. Simple principle, is the more devices, > the more noise. >
Audio guys massively parallel device to push noise down. [...] -- Regards, Joerg http://www.analogconsultants.com/
On Fri, 16 May 2014 09:46:10 -0700, Joerg <invalid@invalid.invalid>
wrote:

>Kevin Aylward wrote: >> "Jim Thompson" wrote in message >> news:vbj9n9p3gebit51f22nagtmlfo3uaebe7p@4ax.com... >> >> On Thu, 15 May 2014 09:29:42 +1000, Clifford Heath >> <no.spam@please.net> wrote: >> >>>> On 15/05/14 04:43, haiticare2011@gmail.com wrote: >>>>> On Wednesday, May 14, 2014 10:46:54 AM UTC-4, Joerg wrote: >>>>>> Cheapskates use logic parts for that because you get a six-pack for a >>>>> OK thanks. - What logic part would produce a sine wave output? >>> >>>> One that has no extra gain than what is required to start, so it's not >>>> constantly slamming against the amplitude limit. One that limits its >>>> amplitude into a resistive load. Or one fitted with an AGC circuit. It >>>> helps if the clipping is somewhat soft. Look at the 74HCU04 inverters >>>> for a start. >> >>> "Clipping" is for digital guys. AGC is necessary for low distortion. >> >> Usually, however, clipping/clamping/limiting pretty much invariably >> produces the lowest phase noise. Simple principle, is the more devices, >> the more noise. >> > >Audio guys massively parallel device to push noise down. > >[...]
And/or low current operation _and_ low VCE ...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.
"Joerg"  wrote in message news:btmtmnFk22sU1@mid.individual.net...

Kevin Aylward wrote:
> "Jim Thompson" wrote in message > news:vbj9n9p3gebit51f22nagtmlfo3uaebe7p@4ax.com... > > On Thu, 15 May 2014 09:29:42 +1000, Clifford Heath > <no.spam@please.net> wrote: > >>> On 15/05/14 04:43, haiticare2011@gmail.com wrote: >>>> On Wednesday, May 14, 2014 10:46:54 AM UTC-4, Joerg wrote: >>>>> Cheapskates use logic parts for that because you get a six-pack for a >>>> OK thanks. - What logic part would produce a sine wave output? >> >>> One that has no extra gain than what is required to start, so it's not >>> constantly slamming against the amplitude limit. One that limits its >>> amplitude into a resistive load. Or one fitted with an AGC circuit. It >>> helps if the clipping is somewhat soft. Look at the 74HCU04 inverters >>> for a start. > >>> "Clipping" is for digital guys. AGC is necessary for low distortion. > >> Usually, however, clipping/clamping/limiting pretty much invariably >> produces the lowest phase noise. Simple principle, is the more devices, >> the more noise. >
>Audio guys massively parallel device to push noise down.
Sure, some use 2N3055 power jobs as an input device, however, that only works for base resistance (rbb') thermal noise. For up conversion and flat-band noise in RF oscillators, rbb' doesn't usually come into the picture. Base current noise is a problem, typically dropped across the capacitance of the load capacitance in parallel with bias resistors. Bias resistors want to be high to maximise Q, hence a problem. Thermal noise of the bias resistors is also a problem. It takes a lot to get down to -170dBc :-) Kevin Aylward B.Sc. www.kevinaylward.co.uk www.anasoft.co.uk - SuperSpice
"Phil Hobbs" <pcdhSpamMeSenseless@electrooptical.net> wrote in message 
news:7tydnQENnKRyquvOnZ2dnUVZ_rWdnZ2d@supernews.com...
>> If this were necessary, than Spice simulations would have problems as >> they don't have noise! > > They do, actually, just like all other numerical simulators. It comes > from roundoff.
It is necessary, and a problem, from time to time. But this is also *because of* roundoff. If the derivatives are close enough to zero, they'll be approximated *to zero*, the timestep cranks up to max and the simulation skims along, blissfully unaware of its own instability. Tim -- Seven Transistor Labs Electrical Engineering Consultation Website: http://seventransistorlabs.com
"Kevin Aylward" <ExtractkevinRemove@kevinaylward.co.uk> wrote in message 
news:3vmdnUugq9OoiOvOnZ2dnUVZ8tSdnZ2d@bt.com...
> Even a DC initial condition that is > not a DC solution to the circuit one will cause this waveform ramp > start-up.
Well, of course it would; a non-ideal offset represents a step change at t=0, and therefore contains all harmonics. :) Tim -- Seven Transistor Labs Electrical Engineering Consultation Website: http://seventransistorlabs.com
On Friday, May 16, 2014 12:08:15 PM UTC-4, Phil Hobbs wrote:
> On 05/16/2014 09:38 AM, Kevin Aylward wrote: > > >> wrote in message news:vqh8n9p23ldhc76l7m1fd1fbpq0qrekdnn@4ax.com... > > > > > >> On Tue, 13 May 2014 06:14:54 -0700 (PDT), skeptical engineer > > > <ihaiticare011@gmail.com> wrote: > > > > > >>> I was looking at the SAW oscillator circuits at a co. called > > >>> Epcos, and they just take an op amp with the SAW filter in the feedback > > >>> loop. They explain that the op amp amplifies noise, but preferentially > > >>> at the filter allowed frequency. 2 components. > > >> > > >>> I love the simplicity, but does this approach have any drawbacks? > > > > > >> When first switched on, the amplifier input related thermal noise is > > >> amplified, Part of it is going to the frequency sensitive network, > > >> which attenuates the lowest and highest frequencies, leaving a broad > > >> peak around the resonant frequency. > > > > > > If this were necessary, than Spice simulations would have problems as > > > they don't have noise! > > > > They do, actually, just like all other numerical simulators. It comes > > from roundoff. > > > > > > > > This is not how an oscillator functions. An oscillator is not amplified > > > noise. > > > > Oscillators build up from noise, though, and they do amplify the > > close-in noise very strongly. See e.g. > > > > http://rfic.eecs.berkeley.edu/~niknejad/ee242/pdf/eecs242_lect22_phasenoise.pdf > > > > Oscillators will work without any noise at all. If the system is > > > unstable, any initial DC condition that does not sit at a > > > metastable/unstable equilibrium point will ensure that signals start > > > changing in such a way as to head toward amplifier limiting. This may > > > result in either steady state or chaotic oscillations. > > > > Not necessarily. There has to be some nonzero signal in the band where > > the oscillator is unstable. > > > > > > > > Usually a clean (power on) startup pulse will excite the tank, and it is > > > this ringing that generates the oscillation build-up, just as it does in > > > the real world. > > > > Sometimes. It's easily possible that the power-up transient is smooth > > enough that its spectrum dies away to well below device noise in the > > relevant band. Good oscillators are designed so that that is the case, > > because otherwise their supply rejection would stink. > > > > > > > > This bit about noise starting up the circuit is often quoted in the > > > books, and er... ah...somewhat mindlessly repeated, but is very dubious. > > > > > > Theoretically numerical noise could start a simulation, and sometimes > > > does, but it is not necessary. > > > > Arbitrarily sharp power-up steps kickstart the oscillation in > > simulation, maybe, but building up from noise will occur very often in > > real life, due to the aforementioned roll-off in the spectrum of the > > actual power-up transient in well-designed oscillators. > > > > Superregenerative receivers, for instance, are limited by the amplified > > thermal noise during the brief period when the quench waveform crosses > > the regeneration threshold, and the simple theory predicts their noise > > pretty accurately. If you can get a copy, "Superregenerative Receivers" > > by J. R. Whitehead is a really good read. > > > > 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
Thank you all for the thoughts. OK. The circuits using a logic inverter have a feedback resistor which forces the inverter to operate in its transition region - the Pierce circuit. I then googled in google patents "SAW oscillator circuit" and got 540,000 hits! A good paper is http://www.ieee-uffc.org/main/awards/outpapers/t8830342.pdf I haven't read it yet, but it's got me wondering if a counter on the oscillator could provide a feedback loop to tune the saw via a heater. Frequency counters are easy to make. I just wonder whether a temperature sensor could adjust the oscillaor, or perhaps more realistically, provide a lookup table to compensate for the SAW temperature variation. My app is to sense the air, so an enclosed oven not possible, perhaps a counter- current heat exchanger could be speculated about. jb