Are there SC-crystals out there in the wild that are not Overtone?
Reference on high quality crystal oscillators:
https://www.bartelsos.de/dk7jb.php/colpitts-oszillator-rohde
On Fri, Feb 28, 2020 at 11:35 AM Attila Kinali attila@kinali.ch wrote:
On Thu, 27 Feb 2020 08:45:16 -0800
"Richard (Rick) Karlquist" richard@karlquist.com wrote:
OTOH, you could build a simple Colpitts
oscillator and see where it oscillates.
That's what they did back in the dark
ages.
Any time nut should be up for that.
Yes, but how many of us can build a time-nuts quality oscillator?
I'm still lacking that paper/book that teaches me how to build
a high stability oscillator.
I have a couple of 5MHz 3rd OT SC cut crystals in HC-37 case sitting
in a box, waiting to be used as some oscillator, I just lack the knowledge
to make good use of them.
Attila Kinali
--
In science if you know what you are doing you should not be doing it.
In engineering if you do not know what you are doing you should not be doing it.
-- Richard W. Hamming, The Art of Doing Science and Engineering
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On 2/28/20 2:34 AM, Attila Kinali wrote:
On Thu, 27 Feb 2020 08:45:16 -0800
"Richard (Rick) Karlquist" richard@karlquist.com wrote:
OTOH, you could build a simple Colpitts
oscillator and see where it oscillates.
That's what they did back in the dark
ages.
Any time nut should be up for that.
Yes, but how many of us can build a time-nuts quality oscillator?
I'm still lacking that paper/book that teaches me how to build
a high stability oscillator.
I think to a large extent that is more art than science. High
performance electronics (low noise, high stability, mass production, you
name it) is always a combination of tradeoffs of non-ideal behavior,
much of which is not necessarily modelable in a systems sense. So the
trades get made by "gut feel" developed from experience.
Driving along that path a bit further, the really fundamental
improvements come when someone figures out how to get better performance
without needing art and skill. Movable type brought the written word to
everyone. Offset printing brought high quality image reproduction to the
masses. Silicon lithography brought computation to all of us.
I have a couple of 5MHz 3rd OT SC cut crystals in HC-37 case sitting
in a box, waiting to be used as some oscillator, I just lack the knowledge
to make good use of them.
Experiment. Build a fet or MMIC oscillator and see how it works.
Attila Kinali
Hi
On Feb 28, 2020, at 9:33 AM, jimlux jimlux@earthlink.net wrote:
On 2/28/20 2:34 AM, Attila Kinali wrote:
On Thu, 27 Feb 2020 08:45:16 -0800
"Richard (Rick) Karlquist" richard@karlquist.com wrote:
OTOH, you could build a simple Colpitts
oscillator and see where it oscillates.
That's what they did back in the dark
ages.
Any time nut should be up for that.
Yes, but how many of us can build a time-nuts quality oscillator?
I'm still lacking that paper/book that teaches me how to build
a high stability oscillator.
I think to a large extent that is more art than science. High performance electronics (low noise, high stability, mass production, you name it) is always a combination of tradeoffs of non-ideal behavior, much of which is not necessarily modelable in a systems sense. So the trades get made by "gut feel" developed from experience.
There are papers that dive into some of the tradeoffs. Things like
phase noise get a lot of attention. However, every attempt that I’m
aware of to write an “oscillator cookbook” has either become very
narrow ( = how to use this chip in a clock oscillator) or has hit a wall
before becoming useful.
The normal way high precision oscillators are “learned" is to either
learn it from the guy on the next bench (who already knows) or to
do a whole lot of experimentation. Even there, the process is pretty
narrowly focused.
Bob
Driving along that path a bit further, the really fundamental improvements come when someone figures out how to get better performance without needing art and skill. Movable type brought the written word to everyone. Offset printing brought high quality image reproduction to the masses. Silicon lithography brought computation to all of us.
I have a couple of 5MHz 3rd OT SC cut crystals in HC-37 case sitting
in a box, waiting to be used as some oscillator, I just lack the knowledge
to make good use of them.
Experiment. Build a fet or MMIC oscillator and see how it works.
Attila Kinali
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Am 28.02.20 um 11:34 schrieb Attila Kinali:
Yes, but how many of us can build a time-nuts quality oscillator?
I'm still lacking that paper/book that teaches me how to build
a high stability oscillator.
Especially if you see THIS in a tutorial and 5 cm below the OOOH
so precise phase noise simulation results of this circuit:
----- < short.png > -----
That builds confidence.
But Bernd was very precise with his prediction where the fundamental
of the SC crystal was to be expected. The sweep took abt. 25 minutes @
1Hz RBW.
The step on the left side is where the next measurement happens.
The crystal laid openly on the end of the port cable on the table at
room temperature.
--- < fundamental.png > ---
I have a couple of 5MHz 3rd OT SC cut crystals in HC-37 case sitting
in a box, waiting to be used as some oscillator, I just lack the knowledge
to make good use of them.
regards, Gerhard
Many crystals possess spurious modes not terribly far from the desired Hi-Q
mode.
Since the spurious mode(s) are lower Q, oscillation on one of these can
build up
faster than oscillation in the desired mode, driving the sustaining
amplifier into
compression before the desired oscillating mode really gets going. This
will leave
only the fastest-growing mode as the winner. This is not speculation- I've
seen
it happen.
My point is that just building an oscillator with an unknown crystal has no
assurance
of running where you really want it to, thus leading you astray.
Discovering all these
modes is a big part of the benefit of studying the crystal with a VNA or
similar instrument
before building anything. Forewarned is forearmed- you then have a better
chance of
building an oscillator that does what you want it to do.
Dana
On Thu, Feb 27, 2020 at 10:46 AM Richard (Rick) Karlquist <
richard@karlquist.com> wrote:
OTOH, you could build a simple Colpitts
oscillator and see where it oscillates.
That's what they did back in the dark
ages.
Any time nut should be up for that.
Rick N6RK
On 2/27/2020 5:35 AM, Bob kb8tq wrote:
Hi
Ok, so just to run the math:
5 MHz / 2.9 = 1.724 MHz
If the Q at the fundamental is 500K (a wild guess) then 1.724 MHz /
500,000 = 3.4 Hz
In a world where a synthesized sweeper might be stepping in 10Hz
steps, that’s an
easy one to miss.
Bob
On Feb 26, 2020, at 11:40 PM, Bernd Neubig BNeubig@t-online.de wrote:
Hi Gerhard,
I am rather sure that it is a 5 MHz 3rd overtone crystal.
the resistance should be in the 80 to 110 Ohm range and Q about 1.5
million. You can see the resonance without ringing in a span of 100 Hz or
smaller with a sweep time of 10 sec minimum.
See attached the response of a 5 MHz SC3 crystal in HC-40/U package.
Indeed the 5.45 MHz is the B-mode which has a temperature coefficient
of -30 ppm/K
Because the crystal blank has a plano-convex shape. The overtones are
quite far away from 3 times or 5 times the fundamental mode. 3rd overtone
is about (rough guess) 2.9 time of fundamental mode.
To find them you must really carefully sweep around a few 10 to 100 kHz
span with slow sweep time a narrow bandwidth
Regards
Bernd
DK1AG
-----Ursprüngliche Nachricht-----
Von: time-nuts [mailto:time-nuts-bounces@lists.febo.com] Im Auftrag
von Gerhard Hoffmann via time-nuts
Gesendet: Mittwoch, 26. Februar 2020 01:42
To get a first impression, I soldered the crystal to an SMA plug and
put it on an
R&S ZVB-8 network analyzer and measured S11. I could see the 5 MHz
resonance
as a 15 dB dip. There was also a resonance at 5.45 and a smaller one
another 90 KHz
higher. the +10% suggest that it is an SC cut.
But I could not see anything at 1 or 1.6666 MHz, so it should be a
fundamental crystal?
Is that common?
I made most measurements at room temperature. I can turn the hot air
solder
station down to 91°C which is not far away from the crystal's 87.7°C
inflection point, and I could see some variation on the 5.45 MHz
resonance vs. temp.
I must build a fixture for the hot air because the sweep time at 1 Hz
bandwidth
is close to eternal.
Is the un-harmonicity (???) between fundamental and overtones stronger
with SC-cuts
than normal AT? I also could not see anything at 15 MHz. Next I'll make
a board
for the PI fixture as described by Bernd Neubig in his crystal cookbook.
BTW I could see some more dips with >= 10 Hz resolution. I hope that
does not mean
and follow the instructions there.
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Basically you are right. One of the problem is, that SC-cut crystals possess
a strong "B-mode" resonance just 9% above the desired stable "C-mode". The
B-mode has a similar or even a bit lower resistance than the C-mode, and has
a temperature coefficient of -30 ppm/K. It is a temperature sensing mode,
and sometimes used for that purpose.
The existence of two close modes with similar resistance makes SC-cut
oscillator design a bit of an art, but not magic.
Bernd
-----Ursprüngliche Nachricht-----
Von: time-nuts [mailto:time-nuts-bounces@lists.febo.com] Im Auftrag von
Bruce Griffiths
Gesendet: Freitag, 28. Februar 2020 12:08
The idea is to see what frequency the crystal oscillates at not to build a
high stability oscillator.
If can be made to oscillate at a frequency somewhere around 5MHz/3 then its
likely a third overtone crystal.
I am glad you could confirm my rough estimate.
The critical point when measuring SC-cut crystals at room temperature is, that their f(T) response is rather steep at room temperature. There even minor temperature variations during the sweep (or between two sweeps) will cause a shift of the impedance response.
This is the reason for the small edge at the left side of Gerhard's picture.
Best regards
Bernd
-----Ursprüngliche Nachricht-----
Von: time-nuts [mailto:time-nuts-bounces@lists.febo.com] Im Auftrag von Gerhard Hoffmann
Gesendet: Freitag, 28. Februar 2020 16:44
An: time-nuts@lists.febo.com
Betreff: Re: [time-nuts] Are there SC-crystals out there in the wild that are not Overtone?
...
But Bernd was very precise with his prediction where the fundamental
of the SC crystal was to be expected. The sweep took abt. 25 minutes @ 1Hz RBW.
The step on the left side is where the next measurement happens.
The crystal laid openly on the end of the port cable on the table at
room temperature.
--- < fundamental.png > ---
You are right. As I am usually stating in my crystal seminars: "You are ordering a crystal with one particular frequency, but the manufacturer supplies with the crystals free of charge a bunch of additional frequencies, which are not mentioned on the marking."
There is a multitude of spurious in real crystals:
- So-called an-harmonic spurious resonances, which are all above the desired frequency. For plano-convex crystals, as the usual high-precision overtone crystals at 5 MHz or 10 MHz, there are two or three (at least) very strong spuriii about 100 kHz ~ 200 kHz above. In poorly designed crystals these modes could be as strong or even stronger than the main mode.
- The other overtones including the fundamental mode with their an-harmonics are also always present. Higher overtones usually have higher resistance than the main mode, but the fundamental mode of a 3rd overtone could have a lower resistance than the desired 3rd OT. The B-Mode is a temperature sensor mode with -30 ppm/K f(T) slope
- SC-cut crystals have a strong "B-mode only 9% above the main mode, which has comparable or even lower resistance than the desired "C-mode".
- And finally there are the higher overtones of the low frequency vibration modes such as face-shear mode etc. Those can interfere with the main mode within a small temperature interval and will cause frequency dips and activity dips ("band breaks")
If you try to build an oscillator with overtone crystals you must always include a kind of trap or other selective circuits to allow only the desired overtone to work. For an oscillator using a SC-cut crystal you need to add additional selectivity to avoid operation at (or jumping to) the B-mode. This could be very tricky.
Best regards
Bernd
-----Ursprüngliche Nachricht-----
Von: time-nuts [mailto:time-nuts-bounces@lists.febo.com] Im Auftrag von Dana Whitlow
Gesendet: Samstag, 29. Februar 2020 00:47
Many crystals possess spurious modes not terribly far from the desired Hi-Q mode.
Since the spurious mode(s) are lower Q, oscillation on one of these can build up faster than oscillation in the desired mode, driving the sustaining amplifier into compression before the desired oscillating mode really gets going. This will leave only the fastest-growing mode as the winner. This is not speculation- I've seen it happen.
My point is that just building an oscillator with an unknown crystal has no assurance of running where you really want it to, thus leading you astray.
Discovering all these
modes is a big part of the benefit of studying the crystal with a VNA or similar instrument before building anything. Forewarned is forearmed- you then have a better chance of building an oscillator that does what you want it to do.
Dana
Hi
As long as we are beating up on the poor old SC for mode issues:
We have an C mode ( = the one that normally gets used in a precision oscillator)
We have a B mode ( = the “thermometer” mode that gets used in some MCXO’s)
Hmmm …. why did they start with B … hmmm ….
Well yes indeed there is an A mode. For a given overtone it’s above the B mode.
Exactly why they are lettered high to low … at least they are in order.
Page 3-15 in:
Quartz Resonator & Oscillator Tutorial - Time and Frequency ...tf.nist.gov › sim › 2010_Seminar › vig3 https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=2ahUKEwiYxa7Z-vbnAhWJlnIEHaTSCAkQFjABegQIARAB&url=https%3A%2F%2Ftf.nist.gov%2Fsim%2F2010_Seminar%2Fvig3.ppt&usg=AOvVaw0LXXz8jKe0sNHtk3Cj18ok
Gives a nice plot of how it fits in. Why mention it? Well it’s the answer to the next up
question “why is the oscillator passband so tight?” ( = you have to kill a very close mode just above
the 3rd OT C mode and a closer than you would think mode below it).
Bob
On Feb 29, 2020, at 6:30 AM, Bernd Neubig BNeubig@t-online.de wrote:
You are right. As I am usually stating in my crystal seminars: "You are ordering a crystal with one particular frequency, but the manufacturer supplies with the crystals free of charge a bunch of additional frequencies, which are not mentioned on the marking."
There is a multitude of spurious in real crystals:
- So-called an-harmonic spurious resonances, which are all above the desired frequency. For plano-convex crystals, as the usual high-precision overtone crystals at 5 MHz or 10 MHz, there are two or three (at least) very strong spuriii about 100 kHz ~ 200 kHz above. In poorly designed crystals these modes could be as strong or even stronger than the main mode.
- The other overtones including the fundamental mode with their an-harmonics are also always present. Higher overtones usually have higher resistance than the main mode, but the fundamental mode of a 3rd overtone could have a lower resistance than the desired 3rd OT. The B-Mode is a temperature sensor mode with -30 ppm/K f(T) slope
- SC-cut crystals have a strong "B-mode only 9% above the main mode, which has comparable or even lower resistance than the desired "C-mode".
- And finally there are the higher overtones of the low frequency vibration modes such as face-shear mode etc. Those can interfere with the main mode within a small temperature interval and will cause frequency dips and activity dips ("band breaks")
If you try to build an oscillator with overtone crystals you must always include a kind of trap or other selective circuits to allow only the desired overtone to work. For an oscillator using a SC-cut crystal you need to add additional selectivity to avoid operation at (or jumping to) the B-mode. This could be very tricky.
Best regards
Bernd
-----Ursprüngliche Nachricht-----
Von: time-nuts [mailto:time-nuts-bounces@lists.febo.com] Im Auftrag von Dana Whitlow
Gesendet: Samstag, 29. Februar 2020 00:47
Many crystals possess spurious modes not terribly far from the desired Hi-Q mode.
Since the spurious mode(s) are lower Q, oscillation on one of these can build up faster than oscillation in the desired mode, driving the sustaining amplifier into compression before the desired oscillating mode really gets going. This will leave only the fastest-growing mode as the winner. This is not speculation- I've seen it happen.
My point is that just building an oscillator with an unknown crystal has no assurance of running where you really want it to, thus leading you astray.
Discovering all these
modes is a big part of the benefit of studying the crystal with a VNA or similar instrument before building anything. Forewarned is forearmed- you then have a better chance of building an oscillator that does what you want it to do.
Dana
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