A bit of help with a crystal oscillator

[Steve Wilson]

jlarkin@highlandsniptechnology.com wrote:

If a crystal has some sort of static friction effect, namely needs a
mimimum power drive to resonate, how does a crystal oscillator ever
get started? It won't have its 1 mw or whatever required
resonant-frequency drive until it's already oscillating.

I don't understand your premise. If an oscillator won't start, it won't
start.

There are many reasons why an oscillator won't start. Some may start if you
tap the crystal case or touch a probe to the circuit.

Contamination could cause nonlinearities that could hinder startup. There
are many other effects that could hinder oscillation. Some have been
described previously. One that has not been mentioned so far is activity
dips. This is described as follows:

"Activity dips occur where the f vs. T curves of unwanted modes intersect
the f vs. T curve of the wanted mode. Such activity dips are highly
sensitive to drive level and load reactance."

See Vig, April 2012, Page 60

https://www.vcamerica.com/pub/media/documents/vig3.pdf

An activity dip could hinder startup at a particular temperature.

This actually works, at least the single time I tried it:

https://www.dropbox.com/s/xbjmhido66u6slz/XO.JPG?raw=1

I never had the guts to do that in production.

Good idea.

 

[John Larkin]

On Thu, 07 Nov 2019 18:57:46 +0000, Peter <nospam@nospam9876.com>
wrote:

jlarkin@highlandsniptechnology.com wrote

This actually works, at least the single time I tried it:

https://www.dropbox.com/s/xbjmhido66u6slz/XO.JPG?raw=1

I never had the guts to do that in production.

Unbelievable

The power level must be miniscule - just the gate current and what
gets coupled to the gate via parasitic capacitances (Miller effect
etc).

It's fairly common to leave jfet gates open, and let them bias
themselves, like in electret microphones. This one even AGCs itself.
The drain pullup came from the TTL gate input circuit. So the DC
situation isn't totally absurd.

But why it oscillates is still a mystery to me. It did make a nice
clock at the TTL gate output.

--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[John Larkin]

On Thu, 7 Nov 2019 18:36:27 -0000 (UTC), Steve Wilson <no@spam.com>
wrote:

jlarkin@highlandsniptechnology.com wrote:

If a crystal has some sort of static friction effect, namely needs a
mimimum power drive to resonate, how does a crystal oscillator ever
get started? It won't have its 1 mw or whatever required
resonant-frequency drive until it's already oscillating.

I don't understand your premise. If an oscillator won't start, it won't
start.

There are many reasons why an oscillator won't start. Some may start if you
tap the crystal case or touch a probe to the circuit.

Contamination could cause nonlinearities that could hinder startup. There
are many other effects that could hinder oscillation. Some have been
described previously. One that has not been mentioned so far is activity
dips. This is described as follows:

"Activity dips occur where the f vs. T curves of unwanted modes intersect
the f vs. T curve of the wanted mode. Such activity dips are highly
sensitive to drive level and load reactance."

See Vig, April 2012, Page 60

https://www.vcamerica.com/pub/media/documents/vig3.pdf

An activity dip could hinder startup at a particular temperature.

This actually works, at least the single time I tried it:

https://www.dropbox.com/s/xbjmhido66u6slz/XO.JPG?raw=1

I never had the guts to do that in production.

Good idea.

You snipped too much for this conversation to make sense.

--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[Peter]

jlarkin@highlandsniptechnology.com wrote

This actually works, at least the single time I tried it:

https://www.dropbox.com/s/xbjmhido66u6slz/XO.JPG?raw=1

I never had the guts to do that in production.

Unbelievable

The power level must be miniscule - just the gate current and what
gets coupled to the gate via parasitic capacitances (Miller effect
etc).

 

[Steve Wilson]

John Larkin <jlarkin@highland_atwork_technology.com> wrote:

On Thu, 7 Nov 2019 18:36:27 -0000 (UTC), Steve Wilson <no@spam.com
wrote:

jlarkin@highlandsniptechnology.com wrote:

If a crystal has some sort of static friction effect, namely needs a
mimimum power drive to resonate, how does a crystal oscillator ever
get started? It won't have its 1 mw or whatever required
resonant-frequency drive until it's already oscillating.

I don't understand your premise. If an oscillator won't start, it won't
start.

There are many reasons why an oscillator won't start. Some may start if
you tap the crystal case or touch a probe to the circuit.

Contamination could cause nonlinearities that could hinder startup.
There are many other effects that could hinder oscillation. Some have
been described previously. One that has not been mentioned so far is
activity dips. This is described as follows:

"Activity dips occur where the f vs. T curves of unwanted modes
intersect the f vs. T curve of the wanted mode. Such activity dips are
highly sensitive to drive level and load reactance."

See Vig, April 2012, Page 60

https://www.vcamerica.com/pub/media/documents/vig3.pdf

An activity dip could hinder startup at a particular temperature.

This actually works, at least the single time I tried it:

https://www.dropbox.com/s/xbjmhido66u6slz/XO.JPG?raw=1

I never had the guts to do that in production.

Good idea.

You snipped too much for this conversation to make sense.

That's a complaint you don't hear often.

 

[Jan Panteltje]

The easy to use internal oscillator also works good enough for me so
far. It may not work so well in a high speed digital circuit, but that's
someone else's problem, not mine. LOL.

Thank you, 73,

If you look at the issue from a 'math' POV:
For an UART that samples at half the bitrate for 1 bit the error
can be plus and minus 1/2 bit, so +-50 %.
Normally you sent like 1 start bit 8, data bits, and 1 stop bit.
That makes 10 bits and the error accumulates (sample point shift) so
then is +- 5%.
If you assume a similar error for the other side you need +- 2.5 %.

Check it with your chips's internal oscillator spec over the temperature
range you will be using.
Or just send some character in a loop and measure the length - and change of
of length of a frame while using the heat gun or fridge on the chip ;-).
All that just in case you design for space or something ..

 

[Jeroen Belleman]

jlarkin@highlandsniptechnology.com wrote:

On Thu, 7 Nov 2019 10:52:44 -0000 (UTC), Steve Wilson <no@spam.com
wrote:

jlarkin@highlandsniptechnology.com wrote:

Other failure modes are shown in VIG, April 2012, Page 7-8,
https://www.vcamerica.com/pub/media/documents/vig3.pdf
Note, if the drive level is too low, the crystal may not start. This is a
case where the oscillator can meet the Barkhausen Critera, and the
oscillator won't start. Other failure modes are shown above.
I think it's a matter of gain, not amplitude. A crystal is linear.
Contamination can cause nonlinearities

See Page 3-32, Vig, April 2012

Contamination control
contamination can adversely affect
- nonlinearities and resistance anomalies (high starting
resistance)

For more info on contamination, see Wikipedia:

https://en.wikipedia.org/wiki/Crystal_oscillator#Production

For low drive level problems, the cure is to increase the drive level.

The problem is too high a drive level may cause mode skipping, spurious
frequencies, or even crystal fracture.

The cure is to operate within the manufacturer's drive level
specifications. However, it may be difficult to measure if it is too low or
beyond the current probe frequency range.

The cure is to model the oscillator in LTspice. See

08.ASC, Pierce crystal oscillator with B-source

https://drive.google.com/open?id=1ZsbpkV0aaKS5LURIb1dfu_ndshsSaYtf

Modeling allows changing any of the circuit parameters very easily. You can
even model spurious resonances by adding one or more crystals in parallel
with the main one.

For spurious frequencies, see Vig, April 2012, Pages 58, 59 and 67.


If a crystal has some sort of static friction effect, namely needs a
mimimum power drive to resonate, how does a crystal oscillator ever
get started? It won't have its 1 mw or whatever required
resonant-frequency drive until it's already oscillating.


This actually works, at least the single time I tried it:

https://www.dropbox.com/s/xbjmhido66u6slz/XO.JPG?raw=1

I never had the guts to do that in production.

Don't you dare! ;-)

Jeroen Belleman

 

[Steve Wilson]

Jeroen Belleman <jeroen@nospam.please> wrote:

jlarkin@highlandsniptechnology.com wrote:

This actually works, at least the single time I tried it:

https://www.dropbox.com/s/xbjmhido66u6slz/XO.JPG?raw=1

I never had the guts to do that in production.

Don't you dare! ;-)

Jeroen Belleman

Actually, with a bit of care, this could be made into a nice 155 MHz Clapp
oscillator. See 04.ASC JFET 155MHz Clapp Osc in Oscillators.zip

https://drive.google.com/open?id=1ZsbpkV0aaKS5LURIb1dfu_ndshsSaYtf

 

On Fri, 8 Nov 2019 12:00:23 -0000 (UTC), Steve Wilson <no@spam.com>
wrote:

Jeroen Belleman <jeroen@nospam.please> wrote:

jlarkin@highlandsniptechnology.com wrote:

This actually works, at least the single time I tried it:

https://www.dropbox.com/s/xbjmhido66u6slz/XO.JPG?raw=1

I never had the guts to do that in production.

Don't you dare! ;-)

Jeroen Belleman

Actually, with a bit of care, this could be made into a nice 155 MHz Clapp
oscillator. See 04.ASC JFET 155MHz Clapp Osc in Oscillators.zip

https://drive.google.com/open?id=1ZsbpkV0aaKS5LURIb1dfu_ndshsSaYtf

I thought it would be cool to make an XO from a schmitt trigger
inverter, a crystal to ground at the input, and a feedback resistor.
This must oscillate, and always does, just nowhere close to the
expected frequency. The crystal is just an expensive capacitor.



--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[Rick C]

On Friday, November 8, 2019 at 10:22:07 AM UTC-5, jla...@highlandsniptechnology.com wrote:

On Fri, 8 Nov 2019 12:00:23 -0000 (UTC), Steve Wilson <no@spam.com
wrote:

Jeroen Belleman <jeroen@nospam.please> wrote:

jlarkin@highlandsniptechnology.com wrote:

This actually works, at least the single time I tried it:

https://www.dropbox.com/s/xbjmhido66u6slz/XO.JPG?raw=1

I never had the guts to do that in production.

Don't you dare! ;-)

Jeroen Belleman

Actually, with a bit of care, this could be made into a nice 155 MHz Clapp
oscillator. See 04.ASC JFET 155MHz Clapp Osc in Oscillators.zip

https://drive.google.com/open?id=1ZsbpkV0aaKS5LURIb1dfu_ndshsSaYtf

I thought it would be cool to make an XO from a schmitt trigger
inverter, a crystal to ground at the input, and a feedback resistor.
This must oscillate, and always does, just nowhere close to the
expected frequency. The crystal is just an expensive capacitor.

Why "nowhere close to the expected frequency"? This is in essence a digital oscillator and has been made to work. Heck, it's been done through software connecting an input port to an output port... on the same pin.

--

Rick C.

- Get 1,000 miles of free Supercharging
- Tesla referral code - https://ts.la/richard11209

 

[Steve Wilson]

jlarkin@highlandsniptechnology.com wrote:

On Fri, 8 Nov 2019 12:00:23 -0000 (UTC), Steve Wilson <no@spam.com
wrote:

Jeroen Belleman <jeroen@nospam.please> wrote:

jlarkin@highlandsniptechnology.com wrote:

This actually works, at least the single time I tried it:

https://www.dropbox.com/s/xbjmhido66u6slz/XO.JPG?raw=1

I never had the guts to do that in production.

Don't you dare! ;-)

Jeroen Belleman

Actually, with a bit of care, this could be made into a nice 155 MHz
Clapp oscillator. See 04.ASC JFET 155MHz Clapp Osc in Oscillators.zip

https://drive.google.com/open?id=1ZsbpkV0aaKS5LURIb1dfu_ndshsSaYtf

I thought it would be cool to make an XO from a schmitt trigger
inverter, a crystal to ground at the input, and a feedback resistor.
This must oscillate, and always does, just nowhere close to the
expected frequency. The crystal is just an expensive capacitor.

You are doing it wrong. You have a RC oscillator. You need to get 180
degree phase reversal for positive feedback. A pi network will do this.
Just add the series load capacitors and stick the crystal between them.

See file 08.ASC, "Pierce crystal oscillator with B-source", in
Oscillators.zip, at

https://drive.google.com/open?id=1ZsbpkV0aaKS5LURIb1dfu_ndshsSaYtf

It is designed for 1/(2 * pi * sqrt(10e-3 * 0.1e-12)) = 5,032,921 Hz, not
counting the series load caps and crystal capacitance. You can find the
calculations in Vig.

It runs at 5 MHz according to LTspice.

 

[John Larkin]

On Fri, 8 Nov 2019 16:14:26 -0000 (UTC), Steve Wilson <no@spam.com>
wrote:

jlarkin@highlandsniptechnology.com wrote:

On Fri, 8 Nov 2019 12:00:23 -0000 (UTC), Steve Wilson <no@spam.com
wrote:

Jeroen Belleman <jeroen@nospam.please> wrote:

jlarkin@highlandsniptechnology.com wrote:

This actually works, at least the single time I tried it:

https://www.dropbox.com/s/xbjmhido66u6slz/XO.JPG?raw=1

I never had the guts to do that in production.

Don't you dare! ;-)

Jeroen Belleman

Actually, with a bit of care, this could be made into a nice 155 MHz
Clapp oscillator. See 04.ASC JFET 155MHz Clapp Osc in Oscillators.zip

https://drive.google.com/open?id=1ZsbpkV0aaKS5LURIb1dfu_ndshsSaYtf

I thought it would be cool to make an XO from a schmitt trigger
inverter, a crystal to ground at the input, and a feedback resistor.
This must oscillate, and always does, just nowhere close to the
expected frequency. The crystal is just an expensive capacitor.

You are doing it wrong.

I'm not doing it. It's just something I tried once.

What's wrong with trying things?

--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[Steve Wilson]

John Larkin <jlarkin@highland_atwork_technology.com> wrote:

On Fri, 8 Nov 2019 16:14:26 -0000 (UTC), Steve Wilson <no@spam.com
wrote:

jlarkin@highlandsniptechnology.com wrote:

On Fri, 8 Nov 2019 12:00:23 -0000 (UTC), Steve Wilson <no@spam.com
wrote:

Jeroen Belleman <jeroen@nospam.please> wrote:

jlarkin@highlandsniptechnology.com wrote:

This actually works, at least the single time I tried it:

https://www.dropbox.com/s/xbjmhido66u6slz/XO.JPG?raw=1

I never had the guts to do that in production.

Don't you dare! ;-)

Jeroen Belleman

Actually, with a bit of care, this could be made into a nice 155 MHz
Clapp oscillator. See 04.ASC JFET 155MHz Clapp Osc in Oscillators.zip

https://drive.google.com/open?id=1ZsbpkV0aaKS5LURIb1dfu_ndshsSaYtf

I thought it would be cool to make an XO from a schmitt trigger
inverter, a crystal to ground at the input, and a feedback resistor.
This must oscillate, and always does, just nowhere close to the
expected frequency. The crystal is just an expensive capacitor.

You are doing it wrong.

I'm not doing it. It's just something I tried once.

What's wrong with trying things?

I was referring to your schmitt inverter. If you knew anything about
Barkhausen Criteria, you wouldn't even waste your time and come to the
wrong conclusion. I fixed it for you.

I thought it would be cool to make an XO from a schmitt trigger
inverter, a crystal to ground at the input, and a feedback resistor.
This must oscillate, and always does, just nowhere close to the
expected frequency. The crystal is just an expensive capacitor.

 

[Peter]

Jan Panteltje <pNaOnStPeAlMtje@yahoo.com> wrote

If you look at the issue from a 'math' POV:
For an UART that samples at half the bitrate for 1 bit the error
can be plus and minus 1/2 bit, so +-50 %.
Normally you sent like 1 start bit 8, data bits, and 1 stop bit.
That makes 10 bits and the error accumulates (sample point shift) so
then is +- 5%.
If you assume a similar error for the other side you need +- 2.5 %.

Check it with your chips's internal oscillator spec over the temperature
range you will be using.
Or just send some character in a loop and measure the length - and change of
of length of a frame while using the heat gun or fridge on the chip ;-).
All that just in case you design for space or something ..

Yes; UARTs don't need an accurate clock.

Well, until the other end is doing auto baud detection Modems used
to autobaud on the "AT" characters and with many of them you need a
clock accurate to a fraction of 1%.

 

[John Larkin]

On Fri, 8 Nov 2019 18:32:49 -0000 (UTC), Steve Wilson <no@spam.com>
wrote:

John Larkin <jlarkin@highland_atwork_technology.com> wrote:

On Fri, 8 Nov 2019 16:14:26 -0000 (UTC), Steve Wilson <no@spam.com
wrote:

jlarkin@highlandsniptechnology.com wrote:

On Fri, 8 Nov 2019 12:00:23 -0000 (UTC), Steve Wilson <no@spam.com
wrote:

Jeroen Belleman <jeroen@nospam.please> wrote:

jlarkin@highlandsniptechnology.com wrote:

This actually works, at least the single time I tried it:

https://www.dropbox.com/s/xbjmhido66u6slz/XO.JPG?raw=1

I never had the guts to do that in production.

Don't you dare! ;-)

Jeroen Belleman

Actually, with a bit of care, this could be made into a nice 155 MHz
Clapp oscillator. See 04.ASC JFET 155MHz Clapp Osc in Oscillators.zip

https://drive.google.com/open?id=1ZsbpkV0aaKS5LURIb1dfu_ndshsSaYtf

I thought it would be cool to make an XO from a schmitt trigger
inverter, a crystal to ground at the input, and a feedback resistor.
This must oscillate, and always does, just nowhere close to the
expected frequency. The crystal is just an expensive capacitor.

You are doing it wrong.

I'm not doing it. It's just something I tried once.

What's wrong with trying things?

I was referring to your schmitt inverter. If you knew anything about
Barkhausen Criteria, you wouldn't even waste your time and come to the
wrong conclusion. I fixed it for you.

Classical concepts like Barkhausen are useless in strongly nonlinear
circuits, like Schmitt oscillators.

I tried the Schmitt XO before Spice was available. It might actually
work, with a fast Schmitt and the right crystal.

--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[Jeroen Belleman]

On 2019-11-08 16:21, jlarkin@highlandsniptechnology.com wrote:

On Fri, 8 Nov 2019 12:00:23 -0000 (UTC), Steve Wilson <no@spam.com
wrote:

Jeroen Belleman <jeroen@nospam.please> wrote:

jlarkin@highlandsniptechnology.com wrote:

This actually works, at least the single time I tried it:

https://www.dropbox.com/s/xbjmhido66u6slz/XO.JPG?raw=1

I never had the guts to do that in production.

Don't you dare! ;-)

Jeroen Belleman

Actually, with a bit of care, this could be made into a nice 155 MHz Clapp
oscillator. See 04.ASC JFET 155MHz Clapp Osc in Oscillators.zip

https://drive.google.com/open?id=1ZsbpkV0aaKS5LURIb1dfu_ndshsSaYtf

I thought it would be cool to make an XO from a schmitt trigger
inverter, a crystal to ground at the input, and a feedback resistor.
This must oscillate, and always does, just nowhere close to the
expected frequency. The crystal is just an expensive capacitor.

I suppose you've got a bit of an impedance mismatch there.
The crystal can resonate all it wants, but the Schmitt trigger
remains impervious to that.

I've always wondered why ICs invariably use the Pierce.
Wouldn't they rather use a topology that has one end of
the crystal at GND and use the pin thus freed for something
interesting? OK, you'd need a pair of caps inside the package,
but is that a real problem?

Jeroen Belleman

 

[Steve Wilson]

John Larkin <jlarkin@highland_atwork_technology.com> wrote:

On Fri, 8 Nov 2019 18:32:49 -0000 (UTC), Steve Wilson <no@spam.com
wrote:

John Larkin <jlarkin@highland_atwork_technology.com> wrote:

On Fri, 8 Nov 2019 16:14:26 -0000 (UTC), Steve Wilson <no@spam.com
wrote:

jlarkin@highlandsniptechnology.com wrote:

On Fri, 8 Nov 2019 12:00:23 -0000 (UTC), Steve Wilson <no@spam.com
wrote:

Jeroen Belleman <jeroen@nospam.please> wrote:

jlarkin@highlandsniptechnology.com wrote:

This actually works, at least the single time I tried it:

https://www.dropbox.com/s/xbjmhido66u6slz/XO.JPG?raw=1

I never had the guts to do that in production.

Don't you dare! ;-)

Jeroen Belleman

Actually, with a bit of care, this could be made into a nice 155 MHz
Clapp oscillator. See 04.ASC JFET 155MHz Clapp Osc in
Oscillators.zip

https://drive.google.com/open?id=1ZsbpkV0aaKS5LURIb1dfu_ndshsSaYtf

I thought it would be cool to make an XO from a schmitt trigger
inverter, a crystal to ground at the input, and a feedback resistor.
This must oscillate, and always does, just nowhere close to the
expected frequency. The crystal is just an expensive capacitor.

You are doing it wrong.

I'm not doing it. It's just something I tried once.

What's wrong with trying things?

I was referring to your schmitt inverter. If you knew anything about
Barkhausen Criteria, you wouldn't even waste your time and come to the
wrong conclusion. I fixed it for you.


Classical concepts like Barkhausen are useless in strongly nonlinear
circuits, like Schmitt oscillators.

False. You determine the power input to the crystal through the drive
resistor, The input is a square wave, the signal at the crystal is a sine
wave. The other end of the crystal is also a sine wave of approximately the
same amplitude but opposite phase. The difference in amplitude is due to
the input capacitance of the inverter.

If the voltage swing to the input of the inverter is sufficient to drive
the output, you meet Barkhausen and the circuit will oscillate.

I tried the Schmitt XO before Spice was available. It might actually
work, with a fast Schmitt and the right crystal.

Without phase inversion through the pi network, it cannot work.

All you will get is a RC oscillator.

 

[Jeroen Belleman]

On 2019-11-08 22:16, Steve Wilson wrote:

Jeroen Belleman <jeroen@nospam.please> wrote:

On 2019-11-08 16:21, jlarkin@highlandsniptechnology.com wrote:
On Fri, 8 Nov 2019 12:00:23 -0000 (UTC), Steve Wilson <no@spam.com
wrote:

Jeroen Belleman <jeroen@nospam.please> wrote:

jlarkin@highlandsniptechnology.com wrote:

This actually works, at least the single time I tried it:

https://www.dropbox.com/s/xbjmhido66u6slz/XO.JPG?raw=1

I never had the guts to do that in production.

Don't you dare! ;-)

Jeroen Belleman

Actually, with a bit of care, this could be made into a nice 155 MHz
Clapp oscillator. See 04.ASC JFET 155MHz Clapp Osc in Oscillators.zip

https://drive.google.com/open?id=1ZsbpkV0aaKS5LURIb1dfu_ndshsSaYtf

I thought it would be cool to make an XO from a schmitt trigger
inverter, a crystal to ground at the input, and a feedback resistor.
This must oscillate, and always does, just nowhere close to the
expected frequency. The crystal is just an expensive capacitor.




I suppose you've got a bit of an impedance mismatch there.
The crystal can resonate all it wants, but the Schmitt trigger
remains impervious to that.

I've always wondered why ICs invariably use the Pierce.
Wouldn't they rather use a topology that has one end of
the crystal at GND and use the pin thus freed for something
interesting? OK, you'd need a pair of caps inside the package,
but is that a real problem?

Cost. An inverter in a digital chip is essentially free. A Colpitts or
Clapp would require a bipolar or JFET, which may be incompatible with the
process.

An extra pin may be a problem on a small ic. Just go to the next larger
size.

There's no reason why you can't make a Colpitts with MOSFETs. There
are plenty of MOSFETs in an IC. Package pins are a precious limited
resource.

Jeroen Belleman

 

[Steve Wilson]

Jeroen Belleman <jeroen@nospam.please> wrote:

On 2019-11-08 16:21, jlarkin@highlandsniptechnology.com wrote:
On Fri, 8 Nov 2019 12:00:23 -0000 (UTC), Steve Wilson <no@spam.com
wrote:

Jeroen Belleman <jeroen@nospam.please> wrote:

jlarkin@highlandsniptechnology.com wrote:

This actually works, at least the single time I tried it:

https://www.dropbox.com/s/xbjmhido66u6slz/XO.JPG?raw=1

I never had the guts to do that in production.

Don't you dare! ;-)

Jeroen Belleman

Actually, with a bit of care, this could be made into a nice 155 MHz
Clapp oscillator. See 04.ASC JFET 155MHz Clapp Osc in Oscillators.zip

https://drive.google.com/open?id=1ZsbpkV0aaKS5LURIb1dfu_ndshsSaYtf

I thought it would be cool to make an XO from a schmitt trigger
inverter, a crystal to ground at the input, and a feedback resistor.
This must oscillate, and always does, just nowhere close to the
expected frequency. The crystal is just an expensive capacitor.




I suppose you've got a bit of an impedance mismatch there.
The crystal can resonate all it wants, but the Schmitt trigger
remains impervious to that.

I've always wondered why ICs invariably use the Pierce.
Wouldn't they rather use a topology that has one end of
the crystal at GND and use the pin thus freed for something
interesting? OK, you'd need a pair of caps inside the package,
but is that a real problem?

Cost. An inverter in a digital chip is essentially free. A Colpitts or
Clapp would require a bipolar or JFET, which may be incompatible with the
process.

An extra pin may be a problem on a small ic. Just go to the next larger
size.

Jeroen Belleman

 

[Steve Wilson]

Jeroen Belleman <jeroen@nospam.please> wrote:

On 2019-11-08 22:16, Steve Wilson wrote:
Jeroen Belleman <jeroen@nospam.please> wrote:

On 2019-11-08 16:21, jlarkin@highlandsniptechnology.com wrote:
On Fri, 8 Nov 2019 12:00:23 -0000 (UTC), Steve Wilson <no@spam.com
wrote:

Jeroen Belleman <jeroen@nospam.please> wrote:

jlarkin@highlandsniptechnology.com wrote:

This actually works, at least the single time I tried it:

https://www.dropbox.com/s/xbjmhido66u6slz/XO.JPG?raw=1

I never had the guts to do that in production.

Don't you dare! ;-)

Jeroen Belleman

Actually, with a bit of care, this could be made into a nice 155 MHz
Clapp oscillator. See 04.ASC JFET 155MHz Clapp Osc in
Oscillators.zip

https://drive.google.com/open?id=1ZsbpkV0aaKS5LURIb1dfu_ndshsSaYtf

I thought it would be cool to make an XO from a schmitt trigger
inverter, a crystal to ground at the input, and a feedback resistor.
This must oscillate, and always does, just nowhere close to the
expected frequency. The crystal is just an expensive capacitor.




I suppose you've got a bit of an impedance mismatch there.
The crystal can resonate all it wants, but the Schmitt trigger
remains impervious to that.

I've always wondered why ICs invariably use the Pierce.
Wouldn't they rather use a topology that has one end of
the crystal at GND and use the pin thus freed for something
interesting? OK, you'd need a pair of caps inside the package, but is
that a real problem?

Cost. An inverter in a digital chip is essentially free. A Colpitts or
Clapp would require a bipolar or JFET, which may be incompatible with
the process.

An extra pin may be a problem on a small ic. Just go to the next larger
size.

There's no reason why you can't make a Colpitts with MOSFETs. There
are plenty of MOSFETs in an IC. Package pins are a precious limited
resource.

Do you know of any examples of Mosfet Colpitts? The only MOSFET
oscillators I know are plain inverters or balanced LC push-pull pairs that
use on-chip inductors.

I have never seen a single-ended MOSFET crystal oscillator.

Additionally, the voltage swing at the output may be too low to drive
digital logic.

> Jeroen Belleman