[time-nuts] Square to sine wave symmetrical conversion (part 2)

Tue Jul 28 11:35:56 UTC 2015

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 <time at radio.sent.com> 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 <radio.n9xr at gmail.com>
> 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" <kb8tq at n1k.org> 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 <radio.n9xr at gmail.com>
> 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 -- time-nuts at febo.com
> >>>> To unsubscribe, go to
> >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> >>>> and follow the instructions there.
> >>>
> >>> _________________________________________________
> >>> time-nuts mailing list -- time-nuts at febo.com
> >>> To unsubscribe, go to
> >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> >>> and follow the instructions there.
> >>>
> >> _________________________________________________
> >> time-nuts mailing list -- time-nuts at febo.com
> >> To unsubscribe, go to
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> >> and follow the instructions there.
> >
> > _________________________________________________
> > time-nuts mailing list -- time-nuts at febo.com
> > To unsubscribe, go to
> > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> > and follow the instructions there.
>
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
>