Heck of a way to run an oscillator?

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.

The best performance I've attained (at least distortion-wise) was a clipper whose value was adjusted by an "AGC" loop.

...Jim Thompson

They do, actually, just like all other numerical simulators. It comes from roundoff.

Oscillators build up from noise, though, and they do amplify the close-in noise very strongly. See e.g.

Oscillators will work without any noise at all. If the system is

Not necessarily. There has to be some nonzero signal in the band where the oscillator is unstable.

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.

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

Audio guys massively parallel device to push noise down.

And/or low current operation _and_ low VCE ...Jim Thompson

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.

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

Well, of course it would; a non-ideal offset represents a step change at t=0, and therefore contains all harmonics. :)

Tim

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

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

How does that give better drift than the crystal in the frequency counter?

Tim

produces

reasonable

wins.

Thank you. Though somehow it did not really surprise me.

?-)

IIRC, Wes Hayward's book "Introduction to Radio Frequency Design" contained a design around a long-tail pair that with suitable component selection, the transistors wouldn't saturate.

Unfortunately I have misplaced my copy, so I can't check it.

There is one sort of an exception, but more than likely not physically doable, and

A cross coupled oscillator can, in principal, be configured to null out 1/f up conversion. Whether or not the null point stays over time is another matter.

x-coupled LC oscillators still won't beat single transistor xtal ones though.

Kevin Aylward B.Sc.

Sure, but something has to limit in an oscillator otherwise the output will get to trillions of volts and keep on increasing for ever.

Oscillators can be designed to be current or voltage limited, but it needs one to actually work!

Kevin Aylward B.Sc.

This was of course a problems when using computers initially designed in the 1950's to 1970s that did not support denormalized floating point values. With the IEEE floating point standard, anything designed after about after 1980 supports denorms.

Even with IEEE floats/doubles, even adding a billion times a very small value to a huge value does not change the result a bit. Thus, it is still critical how the equations are evaluated. A mathematical statement that looks very sensible, might not work properly on a computer, so an alternative mathematical formula may have to be used to work around the computer limitations.

You missed my bit about "numerical noise" then ?

They do, but it is not necessary. A pulse exciting the tank will start the oscillation, independent of noise.

A pendulum clock does not start because of noise.

Well....

Although the paper correctly points out non-linear capacitors, implying non linear time constants, as a cause of up conversion of low frequency noise, it incorrectly states/implies that "pumping" action of the oscillator also generates up conversion noise. Simple mixing does not produce phase noise, which is what usually matters, despite the creation of up converted amplitude noise.

Unfortunately that paper perpetuates the myth as to the value of the Hajimiri- Lee ISF method. The HL method is wrong, and is completely useless with regard to up conversion, producing errors easily at the 50 dBc level. Its 1/f up conversion formula is wrong. Period.

This is all shown here:

In particular:

Shows why the HL ISF theory is wrong in a manner much more accessible than A.Dimre's highly mathematical paper I reference.

Directly demonstrates the invalidity of orthogonal perturbation (hitting the tank at peaks and zero x-ings for those more mathematically challenged) parroted in the lec 22 paper.

Shows in more detail why HL is wrong, and what is the correct mechanism for up-conversion of phase noise.

A. Dimir trashed the HL paper pretty much immediately it came out. Its pretty stunning why such a flawed, unusable approach is still being taught in universities.

I summarise:

"The HL-LTV method can be shown to be valid for spherical chickens in a vacuum"

Ho...mm humm... ok to be precise, if the initial DC condition is sitting in the already mentioned unstable oscillator system at such a point that it can transverses to the designated limit cycle. Sure, the system might be unstable for various DC points, but actually sit at a point that is locally stable.

"Band" implies frequency. Frequency is irrelevant. Instability has nothing to do with frequency. A chaotic oscillator has no fixed frequency. Only in the pathological sense that an impulse may be analysed as a continuous spectrum is it arguable that there is a frequency in the band where the oscillator is unstable. Even then, this is still dubious for chaotic oscillators.

Possible, but clutching at straws me thinks.

Sure, precision oscillators need need very good PSRR. Mine have 100db @

You walked into that one...

Note, a one transistor oscillator has 10,000 supporting transistors...LDO, LNBG, Function Generators, Constant power circuits, limiters, LN dividers etc....

Good oscillators are also designed to start as fast as possible.

This missies the point being made. An unstable system is an unstable system. Noise has nothing to do with it. An oscillator in not amplified noise despite that fact that it may well amplifier noise.

Kevin Aylward B.Sc.

SPICE was a glimmer in some student's eye back in the 50s; it has more to do with the RELTOL parameter than the numerical stability of the underlying datatypes. By about twelve orders of magnitude.

Was SPICE ever even developed on a platform that didn't support IEEE? I don't even know what they used. Something PDP?

Tim

If you buy a pendulum clock and the transport company very hardly slams it on your floor (power on transient) it might quite well start ticking.

Use a resistor (noise source) followed by 10-100 sections of amplifiers and band pass filters to simulate an oscillator startup. Probing through these stages and you get a view how the oscillator starts.

This can not be done on regular Spice. Resisters only generate noise in AC simulations. There are some Spices with Transient noise extensions, but this still wont allow you to see what might happen in the real world regarding noise.

The issue is that Spice is temperamental as to when it will start on its own due to numerical noise or needs a kick start. The Colpitts oscillator in my SuperSpice examples, just starts on its own. In contrast, its often been impossible getting Cadence PSS to start an oscillator when it has specific functions in it to do so. It doesn't even allow you to have any time changing sources on the schematic in PSS runs to start it.

So, there would probably be no realistic way to isolate start-up due to transient noise and start-up due to, that's what it does without noise.

Noise is in the nV region. I wager in the majority of cases, moving bias conditions on power up, overwhelmingly start up a real oscillator. In fact, some oscillators won't start up at all on power on. Those ones also need a lax removal man. Even with a slow PS, a real system has so many bits and bobs like PORs and devices that there are going to be transient mV/ma floating about that will break the symmetry from a 0=0 loop response.

Kevin Aylward B.Sc.