So you think Skipp Isaham's oscillator has feedback pulses at the peak of the sinewave? Interesting.
Jerry On Mon, Jul 27, 2015 at 8:58 PM, Bill Byrom <[email protected]> wrote: > Bob, I think you meant to write: "Hook a 100 pf cap from emitter > to ground." > > Also remember that the feedback loop can use multiple active devices. > You can design a crystal oscillator which has only linear loading on the > crystal but which has nonlinear characteristics somewhere in the > feedback loop. Some oscillators use a filter to insure that only a > linear signal is fed back from a very nonlinear amplifier in the loop. > The output can be taken from various places in the feedback loop, and > the harmonic content varies depending on that location. > > One idea is to only add energy to the crystal at the peak of the > sinewave voltage, when the derivative of the voltage is zero. If a short > pulse is added at the peak the amplitude is increased without > significantly affecting the phase. But if the pulse is added at the > middle "zero crossing" of the sinewave (with resepect to the peak-to- > peak swing) where the derivative is large, the phase can be affected by > the feedback. Ideally you want to add just enough energy each cycle to > overcome the energy lost in the crystal due to limited Q. > > If the feedback loop is linear, then the noise generated by the active > and passive devices is continuously fed back to the crystal, generating > amplitude noise. Due to the angle of the feedback (described above) and > AM to PM conversion due to device characteristics (junction capacitance > changes with voltage), some of the amplitude noise gets converted to > phase noise. The total resulting noise sidebands (made up of both > ampitude and phase noise) is what everyone wants to minimize. > > You could also use a feedback loop which was very nonlinear by using an > active switch with a low ON resistance (so hopefully a low noise figure > when closed) to send spikes back to the crystal resonator at the peak of > the voltage waveform. If you could arrange the circuit so that the duty > cycle of this switch was low, the residual noise would only be amplified > and fed back to the crystal resonator over a small portion of each > cycle, reducing the average noise figure of the oscillator. If the > feedback circuit is highly nonlinear the amplitude noise might be > clipped and reduced. Of course, the jitter in the feedback circuit is > important. > > The active devices need to be well bypassed at audio frequencies so that > low frequency shot noise isn't amplified with high efficiency. Since the > junction capacitance (and other AM to PM conversion sources) in > downstream amplifiers can be affected by audio frequency noise, I think > that it's very important that such noise isn't allowed to phase modulate > the desired RF signal. > > -- > Bill Byrom N5BB > > > > On Mon, Jul 27, 2015, at 05:39 PM, Bob Camp wrote: > > Hi > > > > > >> On Jul 27, 2015, at 11:52 AM, jerry shirᴀr <[email protected]> > wrote: > >> > >> Here's the rub Bob. I have been trying to find a way or have you explain > >> how a high harmonic oscillator stage > > > > You are confusing the current through the crystal with the current in the > > oscillator transistor. > > So: > > > > Connect a 2N918 with the collector to +12V through a 50 ohm resistor. AC > > couple that resistor to your spectrum analyzer. > > Connect the base of the transistor with a 10K to +12 and a 10K to ground > > Connect a 1K ohm resistor from the emitter of the transistor to ground > > Hook a 100 pf cap from base to emitter > > Hook a 100 pf cap from emitter to base > > Hook a 10.0 MHz fundamental crystal with a resistance of < 10 ohms to > > the base of the transistor. > > Hook a 32 pf cap from the other side of the crystal to ground > > > > I **hope** that’s specific enough for you. > > > > That circuit **will** oscillate. > > > > Look at the current on the 50 ohm resistor. It’s got plenty of harmonics. > > > > With me so far or is this still to theoretical? > > > > Now, this **does** get a bit exciting, but it’s the way this circuit has > > been analyzed since the 1930’s (when it used a tube): > > > > You shift the ground to the emitter for the purposes of seeing what’s > > going on. You now have an “input side” and an “output side” to > > the active stage. This lets you break the loop for analysis. > > > > In this format, the current in the collector is more clearly flowing > > through the 1K resistor and one of the 100 pf caps. > > The current that passes through the crystal flows through the other 100 > > pf cap (and the base) to ground. > > > > The current in the oscillator stage is every bit as nonlinear as you saw > > before. > > > > Since this is an oscillator, the current flows in a loop. There is no > > independent current in any one leg. They all are related. > > If you want to see this, hook up an oscilloscope to the collector > > resistor and apply power to the oscillator. The output does not > > go instantly to a full output. It slowly builds up to the full value. The > > current circulates around the loop **many** times as the > > stage oscillates. > > > > Now: > > > > Break the circuit (AC) at any of the connection points. It stops > > oscillating. (A DC break also does the same thing, but that’s cheating). > > Without everything hooked in a loop, you do not have oscillation. > > > > Next: > > > > Charles posted a long list of interesting transistors a few messages > > back. Try them one at a time and look at phase noise at 20 KHz offset. > > You will find that some are better than others. Take a look at the > > harmonics in the collector. They don’t correlate with the phase noise… > > > > So, unless you are looking at the crystal as being the oscillator (which > > it is not), there’s not much way to say that there are no harmonics > > running around in this circuit. > > > > Is that simple enough? > > > >> is even possible and zip. You don't > >> know and I certainly don't know. So there's that. > > > > **IF** your desire is for an explanation, offensive comments probably are > > not a good idea…. > > > > Bob > > > >> > >> Jerry > >> On Jul 27, 2015 9:33 AM, "Bob Camp" <[email protected]> wrote: > >> > >>> Hi > >>> > >>> Here’s the basic point: > >>> > >>> What is **required** for low phase noise? > >>> > >>> If you can build **one** oscillator that violates a “law” then that > “law” is > >>> not > >>> valid. In tis case the question is “do you **need** low harmonics in > the > >>> oscillator > >>> stage to get low phase noise?” > >>> > >>> Here on the list, we get obsessed about all sorts of stuff. That’s > fine. > >>> It’s fun. > >>> We learn things taking stuff past “the limit”. The gotcha is that can > make > >>> it > >>> hard to keep track of “what is necessary ”. > >>> > >>>> On Jul 27, 2015, at 12:47 AM, jerry shirᴀr <[email protected]> > wrote: > >>>> > >>>> Thanks Tim. I love reading these papers. However my copy states "In > >>> fact, > >>>> were it not for this slight non-linearity, it would be virtually > >>>> impossible to build a simple lamp-stabilized RC oscillator with good > >>>> envelope stability over a wide frequency range." rather than "In fact, > >>> were > >>>> it not for [amplifier] nonlinearity, it would be impossible to build a > >>>> simple oscillator with good envelope stability." The meaning changes > a > >>>> little bit. > >>>> > >>>> Thanks Bob, > >>>> > >>>> Even looking at Tim's article, they are talking about a low degree of > >>>> distortion with an RC oscillator. I am assuming that the Q of the RC > >>> would > >>>> be quite low with respect to the overtone crystals you speak, and yet > the > >>>> RC oscillator described here has low distortion from the oscillator > >>> stage. > >>> > >>> The objective of an RC lab oscillator design **is** low harmonic > distortion. > >>> They > >>> have awful phase noise. > >>> > >>>> > >>>> Putting a filter in the feedback path with the high Q crystal seems > like > >>>> you would be de-Q-ing the crystal and losing the high Q > characteristics > >>> of > >>>> the crystal. > >>> > >>> The oscillator must be a closed loop to operate. There will **always** > be > >>> things > >>> “in series” with the crystal. > >>> > >>>> Any changes of filter components over time seems like it > >>>> would necessarily add drift to the oscillator. > >>> > >>> Since you **must** tune the oscillator on frequency and you **must** > select > >>> the overtone, you will have caps and inductors in the loop. > >>> > >>>> What do you think? Of > >>>> course I am not saying that you can't put filters in the crystal > circuit > >>>> but rather that is something I would never recommend doing that in a > >>>> precision oscillator design. > >>> > >>> Except you have to do it. Since you have to do it, every example out > there > >>> of a low phase noise oscillator has at least some caps in series with > the > >>> crystal. The vast majority have both coils and caps. > >>> > >>>> > >>>> I realize what the impedance plot looks like of AT-cut and SC-cut > >>> crystals > >>>> but my question was specifically about harmonics. That is the topic > of > >>>> this thread. Are you thinking that crystals are rich in harmonics? > I am > >>>> not really seeing an idea of where you are saying the harmonic > components > >>>> come from in these high precision oscillators in the oscillator > circuit. > >>> > >>> The limiting action in the oscillator device creates harmonics. > >>> > >>>> > >>>> What are the "impedance properties" of the crystal? > >>> > >>> There are literally thousands of papers on this. The simple answer is > that > >>> they have **many** resonant modes. > >>> > >>>> Why use a crystal > >>>> rather than slapping a cap and a coil in there to get your desired > >>>> frequency? > >>> > >>> 1) Because it’s Q is higher > >>> 2) Because it’s more stable > >>> > >>>> > >>>> When you "pick off" the collector current, wouldn't that include the > >>>> amplified base to emitter junction noise inherent in simple transistor > >>>> oscillator circuits? > >>> > >>> Again, it’s a loop. The current goes around in circles. There is no > magic > >>> “clean here” current. If you are looking at an OCXO that doubles the > >>> crystal > >>> before the output is created, it’s a really good bet they pulled the > signal > >>> off the collector of the oscillator. The net result is still a low > phase > >>> noise > >>> oscillator. > >>> > >>>> Would that be the same as the crystal current? > >>> > >>> You can’t have an oscillator with just a crystal. You also need other > >>> “stuff”…. > >>> > >>> Bob > >>> > >>>> > >>>> Thanks. > >>>> > >>>> Jerry > >>>> _________________________________________________ > >>>> time-nuts mailing list -- [email protected] > >>>> To unsubscribe, go to > >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > >>>> and follow the instructions there. > >>> > >>> _________________________________________________ > >>> time-nuts mailing list -- [email protected] > >>> To unsubscribe, go to > >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > >>> and follow the instructions there. > >>> > >> _________________________________________________ > >> time-nuts mailing list -- [email protected] > >> To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > >> and follow the instructions there. > > > > _________________________________________________ > > time-nuts mailing list -- [email protected] > > To unsubscribe, go to > > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
