[SI-LIST] Re: S-parameter passivity... Interpreting the results.
- From: "Hill, John" <John.Hill@xxxxxxxxxx>
- To: <ray.anderson@xxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Fri, 21 Jul 2006 11:28:30 -0400
Ray,
We are mixing the context of some of the e-mails. There are two threads
that are being mixed together here. I am to blame for this, but there is
a reason behind the madness. First was the idea of increasing the Q of a
crystal with negative resistance. The second is the doubling of the Q in
a balanced configuration of two Colpitts oscillators.=20
A Colpitts oscillator works with negative resistance at the base of the
transistor. This can be measured as a S11 that is positive. This
provides for a loop gain that is positive even with the loss incurred in
the resonator. In an oscillator higher Q is good as long as you are not
concerned for the ring up time. In the balanced oscillator design, the
losses of the single resonator are shared between the two oscillators.
Hence the Q of the resonator as seen by each of the oscillators is
doubled. Each oscillator sees half of the ESR.=20
In the e-mail from Xilei he has not indicated that he is making an
oscillator. Hence, a high Q may or may not be what he wants or needs.
The point was not to be a slave to Q. It can and should be controlled.
We should not just allow it to happen.
As to your point one; yes, at resonates in the Colpitts oscillator my
expectation is that the resistance of the bias network will be in
parallel with the much lower ESR of the resonator.=20
As to your point two; yes, my claim is that the degradation of Q due to
the bias network is not major. The circuit fulfills the requirements of
the application very well. Fussing over the Q of the circuit is a great
academic subject, but has little value. After all the bias network is
required to make the circuit work.=20
The point to remember is that the ring up time of the oscillator is
important in this application. As the Q increases, the ring up time will
increase and that is not always good. It needs to be controlled and this
is done by the bias network. I have not calculated the reduction in Q
because it is not important to the application. The ring up time is more
important and it can be measured easily.
As to your point three; no, the claim is that having two oscillators 180
degrees out of phase provides for a sharing of the ESR of the resonator
and effectively doubles the Q as seen by each oscillator. In an
oscillator the loop gain will be placed into compression by clipping
until it is at unity at the frequency of oscillation. But this does
bring up the idea that negative resistance can affect the Q of a
network. This is the madness I referred to earlier in this e-mail. To me
this is an interesting thought that may lead Xilei to a novel solution
to his problem.
But remember the gain is changing as the oscillator is ringing up and is
mostly compressed when it is running. This is why analyzing oscillators
with SPICE is not as good as using a frequency domain simulator that can
simulate the non-linear variations in gain and junction capacitance as
the oscillator rings up. If you think about it, the Q is changing as the
oscillator is ringing up.
As to your point four; not every circuit is improved by a higher Q. The
point is to control the Q. The Balance Oscillator has a higher effective
Q than a single Colpitts and hence has a higher yield and the
manufacturing guys like that. However, it will also have the tendency to
have a longer ring up time and that can be a problem if you allow the Q
to get too high.=20
I hope this clears thing up a bit. The idea is to control the Q and not
to be controlled by it. Don't leave it to chance and consider that the
right Q is what our circuits need, not high Q or low Q for that matter.=20
Best regards,
John
=20
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-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Ray Anderson
Sent: Thursday, July 20, 2006 7:01 PM
To: si-list@xxxxxxxxxxxxx
Cc: Ray Anderson
Subject: [SI-LIST] Re: S-parameter passivity... Interpreting the
results.
This has been an interesting thread to follow.
John, let me know if I have these items correct:
1) Bias resistors (R6+R7) and (R5+R8) are in parallel and =3D3D ~ 20k =
ohms
2) You maintain that the 20K is in parallel with several ohms of ESR
in=3D20
the SAW resonator and will only degrade the resonator Q by a =
very=3D20
small amount.
3) You claim that a negative resistance reflected from each transistors
base
effectively increases the Q of the resonator. So you have some
increase
in Q by means of your 'negative resistance' argument which is only
diminished slightly by the parallel bias resistor issue.
4) You mention in one of your e-mails that "the higher the Q, the harder
it=3D20
will be to control the circuit in production.", but in another
message
you say: "The manufacturing guys liked the circuit because it had
=3D20
a higher yield" (referring to increased resonator Q). Can you
comment?
I read through your patent and didn't notice the Q enhancement as one of
your claims. Did I miss it or was it not claimed due to prior art or
other considerations?
Regards,
-Ray Anderson
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Mike Monett
Sent: Thursday, July 20, 2006 3:24 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: S-parameter passivity... Interpreting the
results.
>Mike,
> Keyless Entry systems are very popular today and most of them have
> a Colpitts Oscillator in them. You probably have a Colpitts
> Oscillator with the bias network you have issues with in your
> pocket right now.
> These Colpitts Oscillators work from -40 degrees C to +85. The
> automotive industry makes a few million of these transmitters a
> year and the warranty return rates are very low.
> At RF frequencies the bias resistors you are identifying are in
> parallel with the resonator not series and yes they do limit the
> Q. To be specific R6 in series with R7 along with R5 in series
> with R8 are in parallel with the resonator. They are there to bias
> the transistor into the active region. The circuit ends up with
> 20,000 ohms or more in parallel with the ohms of resistance of the
> resonator at resonance. It is not a big deal.
> If you would like to address your issues with Colpitts Oscillators
> with the current designers, I can forward an e-mail to them out
> side of this list. However, please be specific. Your statements
> will not hold much weight with people that have spent years
> working full time with the Colpitts oscillator unless you are
> specific.
> I am currently doing a vision system and am back to being a
> Digital Designer except for the EMC and SI work. So if you don't
> mind, I would like to get back to Signal Integrity issues.
>Best regards,
>John
John,
Earlier, you claimed your patent 5,486,793 doubles the Q of the
crystal. I asked for clarification, but your answer failed to
support your claim.
As I stated before, the Thevenin equivalent of both bias networks
are in series with the crystal. This will significantly reduce the
Q. You acknowledge this, but you now claim it is not a big deal.
Thanks for your new information.
Regards,
Mike Monett
Antiviral, Antibacterial Silver Solution:
http://silversol.freewebpage.org/index.htm
SPICE Analysis of Crystal Oscillators:
http://silversol.freewebpage.org/spice/xtal/clapp.htm
Noise-Rejecting Wideband Sampler:
http://www3.sympatico.ca/add.automation/sampler/intro.htm
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