>"Steve Wilson" wrote in message
>news:616d5312-f036-416f-bb2b-6b134ea47129@googlegroups.com...
>>On Saturday, October 13, 2018 at 3:35:12 AM UTC-4, Kevin Aylward wrote:
> >>"Steve Wilson" wrote in message
> >>news:e37a690b-e9b3-44c0-a990-805d688aaa26@googlegroups.com...
>
> >>On Saturday, October 13, 2018 at 12:08:58 AM UTC-4, Steve Wilson wrote:
>
> >> De-Qing doesn't work. As you reduce the Q, the amplitude after settling
> >> also decreases. For example, with CS = 10p, the current through Rs is
> >> 17.52 mA p-p instead of >the required 20 mA p-p, and the settling time
> >> is
> >> still too long, at 55.079 seconds. Here is the LTspice file. Please let
> >> me
> > >know if you find any errors.
>
> >>The frequency is wrong also. It should be 10MHz. It's around 11.5MHz.
> >>Something is drastically wrong with this method.
>
>> Sure, the frequency is moved by (1 + Cs/2(CL+Cp)), where CL is the xtal
>> load (e.g. in a Colpitts the series value of the caps either side of the
>> xtal), but this is a known correction, and not really a significant
>> factor
> >in the design phase. It can be automatically included in the xtal
> >subcircuit
> >if desired. I wouldn't recommend using a de Q cap of 10p for a 10 fF cs
> >though! Typically I would say 1pF is the max one would want to use for a
>> xtal oscillator.
> >As I noted, DeQing is the standard way, and it works extremely well, and
>> rationally, it is the only practical way to design high Q oscillators. It
>> keeps the same basic loop gain and phase for starters.
>?? You have a warped idea of working extremely well.
Oh dear.... I guess you must have missed posts that explain who I am. I have
cut down on my posts though.
... Now for the trumpet.... I am Principle Analog IC Designer at
www.rakon.com
There aint a lot of world leading precision, low noise, xtal oscillator
vendors. Probably countable on one hand. Its a very niche market. Pretty
much no non specialist attempts such oscillators. They are actually somewhat
complicated, considering the mere name "Oscillator".
Ovens...linearisers...chebychev compansation functions...DC-DC voltage
doubles...LDOs... power compensation... all in a 7 x 5 package...:-)
>DeQing introduces serious errors into the analysis.
No it doesn't. One just needs to understand how DeQing effects the results.
I already explained some of this.
>There is another way to design high Q oscillators. Use the Fast Start
>method.
Just not viable as an only tool in commercial oscillator design. It might
have a few uses though.
For example, every time a bias changing component changes, you would need to
rerun at high Q to get the exact data to initialise for the fast start.
>> For example, when one actually gets to use the expensive, $100k per seat
>> per
>> year Cadence Periodic Steady State and Phase Noise analysis, for 10
>> years,
>> one gains a bit of knowledge how oscillators work. :-)
>You don't need a $100k per sea and 10 years.
You do need to pay the money if you want to do accurate phase noise and have
it work as designed on first pass silicon. There are no cheapo LTSpices for
PSS and Phase Noise analysis.
Today, there is no realistic hope of being competitive in high performance
oscillator design without expensive design tools.
>> Calculating phase noise is very complicated. Noise at every point of the
>> cycle needs to be determined, so the currents and voltages on DeQed
>> circuits
>> need to be, essentially, the same as the full Q circuit as noise depends
>> on
>> instantaneous currents. Phase noise simulations consistently shows that
>> the
>> low frequency (e.g. 1/f) slope phase noise moves reliably with the DeQed
>> factor. For example if the full Q PN at 10Hz is -100dBc, it is -80dBc
>> with a
>> DeQ factor of 10. HF Flat band noise stays the same whatever the DeQed
>> ratio
>> is.
>> I have some somewhat original tutorial papers on Oscillator Phase Noise
>> here:
> >http://www.kevinaylward.co.uk/ee/phasenoise/phasenoise.html
>> If I get the time, I will try and address your specific points of your
>> other
>> posts.
>Don't bother. We already know all we need. Here's the comparison:
Oh.....
>De-Qing
>1. Gives the wrong frequency
>2. Gives the wrong amplitude
>3. Has a long startup
>4. Cannot be used in AC analysis
I guess you haven't bothered to actually run a comparisons of say, the AC
gain/phase through an oscillator Pierce pi network at various Qs. Its
actually quite interesting. It just don't matter whether deQ is, the peak
gain remains the same.
So, yes, it works wonderfully for AC. Full Q AC makes it really difficult to
actually plot and read the data. Way, way to spikey.
I already explained the offset in frequency isn't a problem in a real
design. Its a known quantity. Same goes for accurate phase noise, which is
what matters. Variation in amplitude is not usually significant. I haven't
spent much time investigating why you are getting major amplitude variations
in LTSpice. When a 5fF cap is DeQed to 50ff, 500fF, amplitude variation is
not normally an issue.
I have ran your circuit in SuperSpice at CS=0.1p with L=2.5m and CS=1p with
L=0.25m with cp=1p and both amplitudes are around 4.8V, on a 2ms run
However, in LTSpice I get around 2.5V and 4.8V with those same C/L values,
so it looks like something is up with LTSpice.
I do note that your example has a pretty big cp at 5pf. Typically is would
be 1pF. Even a big 10MHz 3rd overtone is pushing it to be 3pF.
>Fast Start
>1. Gives the exact frequency
>2. Gives the exact amplitude
It doesn't. As I explained. It only gives exact data, if it is measures
accurately first.
For example, in commercial ASIC designs, ones must do process, temperature
and supply corners. For example, see
http://www.anasoft.co.uk/images/worst_case.png
Typically, one does 10 thousands of runs for AC and TRAN. The operating
conditions are different each time, meaning you don't know how to initialise
the fast start. How it actually starts up is also of major significant.
Commercial designs involve a lot more work then running a few sims in
LTSpice.
>3. Starts instantly
>4. Has no effect in AC analysis
Other issues are, for example, in 3rd Overtone designs. Typically, xtals
have an unwanted B mode about 10% higher in frequency than the wanted C
mode. This fast start technique, essentially, doesn't allow that part of the
B Mode suppression to be evaluated. For vast chunks of a design, the start
up behaviour actually needs to be evaluated. This means DeQing for the most
part.
I would agree that for setting up spice demos fast starting with this method
has some merits.
-- Kevin Aylward
http://www.anasoft.co.uk - SuperSpice
http://www.kevinaylward.co.uk/ee/index.html