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

Bob Camp kb8tq at n1k.org
Tue Jul 28 07:05:33 EDT 2015


Hi

Yup, the second cap goes to ground not in parallel with the first cap.


> On Jul 27, 2015, at 10:42 PM, jerry shirᴀr <radio.n9xr at gmail.com> wrote:
> 
> Thanks Bob,
> 
> I had no intention of being offensive.  It is just my manner.  Forgive me.

I get a bit feisty from time to time - sorry about that.

> 
> First of all, I think you had intended to say "100pF from emitter to
> ground" rather than "100pF from emitter to base."  I think that's a typo.
> 
> Next, how much crystal drive level is there?  I guess it depends on the
> crystal resistance, but I think there will be a lot of crystal current.

Which is not unintentional ….that circuit will have pretty good phase noise for a
simple to wire circuit. 

> 
> Then I don't know why you would purposely want to clip the signal of the
> oscillator, but it is the designer's prerogative.

Ok, how would you propose to build an oscillator that does not have limiting,
since 99.999% of all precision (and otherwise) crystal oscillators have that feature?

Without a limiter in the feedback loop, there is no way to meet Barkenhausen’s criteria 
on a fast enough basis.

>  For low noise
> applications, I have always used an AGC so the oscillator will have a lot
> of drive at startup and then backs off greatly for steady state operation.

Which speeds up startup.

> You can also more easily control the drive level to a small enough amount
> so that the drive won't mess up the frequency accuracy.  

Except all the AGC does is to eliminate the massive extra gain at startup. It’s
far to slow to take care of the loop it’s self.

> Also the buffer
> from the oscillator stage buffers the actual crystal current so the base to
> emitter noise does not get amplified to the collector and propagated down
> the line.  I could beat a -145dBc/Hz spec at 100Hz and -168dBc/Hz at
> 10kHz.

Both of which are pretty noisy compared to the stuff I do these days. A lot depends on 
the frequency ...

> 
> I don't have anything here to breadboard these things up.  I don't
> understand why when there is already a clean crystal current in the
> circuit,

Possibly for one of three reasons:

1) They don’t want to degrade the Q (or more correctly phase slope) of the crystal by putting the resistance of the buffer in series with it.
2) They want better isolation in a single oscillator stage than they get with a series arrangement.
3) They want to double (or triple) the output frequency 

*and* do any / all of these things without degrading (and actually in case 1 improving) the phase noise. 

> someone wouldn't want to take advantage of that signal and use
> that rather than add filters and poor biasing to the mix to claim that
> something can be done when we are talking about ideal situations where the
> lowest phase noise is desired.

Your claim is that harmonics in the oscillator will keep you from having low phase noise. That simply is not correct. 

> 
> Someday you might end up needing to design a low noise oscillator and you
> will need to consider these design obstacles.

Well oddly enough, I have designed a few (thousand) low noise oscillators over the
last 4 or 5 decades. I’ve also torn down more oscillators than I can count and looked
at how they are done. The “no limiting” crystal oscillator has never popped up in any 
of that. It’s also never shown up in any of the papers, going back into the 1920’s. 

Put simply:

You (even with an AGC) will have a few db of excess gain. That gain causes part of the 
loop current to be converted to harmonic energy. That energy does not circulate with the
same efficiency as the main mode. (crystals have “harmonics”, or more commonly acoustic
overtones). To say that there is one part of the oscillator that is the crystal
and another part that is the sustaining stage is fine. To say that the oscillator is only one
or the other half of that complete circuit is not correct. 

> 
> Take care, Bob.

Have fun

Bob

> 
> Jerry
> 
> On Mon, Jul 27, 2015 at 5:39 PM, Bob Camp <kb8tq at n1k.org> 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
>>>>> _______________________________________________
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