A bit of help with a crystal oscillator

[Rick C]

On Friday, November 8, 2019 at 5:21:20 PM UTC-5, Steve Wilson wrote:

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.

A little searching turned up a few.

https://mirror.thelifeofkenneth.com/lib/electronics_archive/Understanding_Crystal_Oscillators.pdf

I found several guides on oscillators that refer to MOSFETs as being suitable "transconductance" devices.

Why would a MOSFET be a problem?

--

Rick C.

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

 

[Rick C]

On Friday, November 8, 2019 at 4:43:42 PM UTC-5, Jeroen Belleman 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.

Jeroen Belleman

+1

 

[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.

Jeroen Belleman

I did a google search. MOSFET Colpitts do exist. For example, see

"Loop Gain of the Common-Drain Colpitts Oscillator"

https://www.degruyter.com/downloadpdf/j/eletel.2010.56.issue-4/v10177-010-
0057-5/v10177-010-0057-5.pdf

However, the problem with a Colpitts is you need two pins. One for the
gate, the other for the source or drain whichever configuration is used. If
you put the capacitors on the chip, you only need one pin for the external
inductor that is also connected to ground. But I have always heard putting
capacitors on an ic are very expensive due to the area required.

An alternative is to use a factory-trimmed RC oscillator if the frequency
requirements are not too strict. Another alternative for precision work is
to mount the oscillator externally and use one pin to drive the ic.

Design is full of tradeoffs.

 

[Rick C]

On Friday, November 8, 2019 at 6:31:35 PM UTC-5, Steve Wilson wrote:

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.

Jeroen Belleman

I did a google search. MOSFET Colpitts do exist. For example, see

"Loop Gain of the Common-Drain Colpitts Oscillator"

https://www.degruyter.com/downloadpdf/j/eletel.2010.56.issue-4/v10177-010-
0057-5/v10177-010-0057-5.pdf

However, the problem with a Colpitts is you need two pins. One for the
gate, the other for the source or drain whichever configuration is used. If
you put the capacitors on the chip, you only need one pin for the external
inductor that is also connected to ground. But I have always heard putting
capacitors on an ic are very expensive due to the area required.

I believe the inductor is not used when the crystal sets the frequency.

An alternative is to use a factory-trimmed RC oscillator if the frequency
requirements are not too strict. Another alternative for precision work is
to mount the oscillator externally and use one pin to drive the ic.

Design is full of tradeoffs.

If an oscillator circuit (not any discussed here) is a bit more optimized for a single digital pin, the input threshold crossing can be detected, the output then can be stimulated with an impulse of opposite polarity (equivalent of AC coupling) and on the opposite phase of the crossing the same thing can happen with the opposite polarity impulse.

Oscillation can be initiated by simply driving the circuit with impulses of approximately correct timing. Rather than this being a stimulus in the AC domain, this is really to get the DC operating point established. If this were a dedicated output a series resistance would be provided for the output internally on the chip.

This is very much analogous to driving a clock pendulum with an impulse at mid swing. The Shortt–Synchronome free pendulum clock did this with a gravity lever that was controlled electrically by the synchronized slave pendulum. Other clocks do this using electromagnets, often controlled by a watch crystal oscillator.

--

Rick C.

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

 

[Steve Wilson]

Steve Wilson <no@spam.com> wrote:

John Larkin <jlarkin@highland_atwork_technology.com> wrote:
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.

Here is a Schmitt XO along with the PLT file. The oscillation is chaotic at
first until you apply enough drive power to get to oscillate through the
crystal.

The drive power may be excessive for most crystals, so you probably would
revert back to a standard inverter. But the sym shows it can be done, it's
just not practical.

Version 4
SHEET 1 1008 716
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FLAG 400 544 C1C2
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WINDOW 0 32 32 VTop 2
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SYMATTR InstName C1
SYMATTR Value 0.1p
SYMBOL cap 432 560 R0
SYMATTR InstName C2
SYMATTR Value 45p
SYMBOL cap -80 560 R0
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SYMATTR Value 45p
SYMBOL res 608 528 R90
WINDOW 0 0 56 VBottom 2
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SYMATTR Value 10m
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WINDOW 0 0 56 VBottom 2
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SYMATTR InstName R5
SYMATTR Value 25
SYMBOL cap 160 448 R90
WINDOW 0 0 32 VBottom 2
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SYMATTR InstName C5
SYMATTR Value 4p
SYMBOL 74hc14 272 96 R0
SYMATTR InstName U1
TEXT 40 -16 Left 2 !.tran 0 2m 0 1n
TEXT 40 -48 Left 2 ;'Pierce Schmitt Crystal Oscillator
TEXT 544 -16 Left 2 !.options plotwinsize=0
TEXT 752 -16 Left 2 !.ic V(OscIn) = 0
TEXT 544 8 Left 2 !.inc 74hc.lib
TEXT 160 256 Left 2 ;May be overdriving the crystal
RECTANGLE Normal 32 112 32 112 2

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[keith wright]

On Friday, 8 November 2019 14:21:20 UTC-8, Steve Wilson wrote:
....

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

I've seen one device that does use a single pin for a crystal connection in a 433MHz OOK receiver.

https://img.ozdisan.com/ETicaret_Dosya/453934_1928180.pdf

kw

 

On Fri, 08 Nov 2019 10:58:29 -0800, 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.

The question is, does the oscillator start.

A normal oscillator is merely a noise amplifier with a frequency
selective positive feedback. For each time the original white thermal
noise around the feedback loop is amplified, the original white
thermal noise is amplified only in the filter passband increasing the
amplitude.

For each time around the loop, the spectral peak becomes narrower and
narrower, much narrower than the filter Q would suggest, finally
ending up as a single spectral line. The amplitude can't grow
forester, either the amplifier gain is reduced or the signal is
clipped.

While initially starting to amplify the white noise, it is essential
the amplifier is in class A or AB. After a few iterations, when the
amplitude has grown close to saturation, the amplifier can fall into
class C and oscillate happily forever.

If the amplifier is initially deeply in class C, it doesn't amplify
the thermal noise and the oscillator will not start. The Schmitt
trigger makes starting even more unlikely.

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

Many faulty oscillator constructions starts simply by the operating
voltage turn-on transient, putting the amplifier momentarily into
class AB. Such oscillator may start when powered from battery and the
DC voltage is applied by a switch, since the start transient is fast,
but might not start when a mains powered device is plugged into mains,
due to the slow charging of the storage capacitors.

 

[Steve Wilson]

keith wright <keith@kjwdesigns.com> wrote:

On Friday, 8 November 2019 14:21:20 UTC-8, Steve Wilson wrote:
...

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

I've seen one device that does use a single pin for a crystal connection
in a 433MHz OOK receiver.

https://img.ozdisan.com/ETicaret_Dosya/453934_1928180.pdf

kw

Interesting. Thanks.

It says the oscillator is a Colpitts and the series caps are internal. But it
doesn't say of the oscillator is a MOSFET or bipolar.

I would suspect bipolar to maintain process compatibility with the rest of
the circuitry.

 

On Sat, 09 Nov 2019 07:08:52 +0200, upsidedown@downunder.com wrote:

On Fri, 08 Nov 2019 10:58:29 -0800, 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.

The question is, does the oscillator start.

A normal oscillator is merely a noise amplifier with a frequency
selective positive feedback. For each time the original white thermal
noise around the feedback loop is amplified, the original white
thermal noise is amplified only in the filter passband increasing the
amplitude.

A Schmitt RC oscillator doesn't work that way. It doesn't need noise
to start.

For each time around the loop, the spectral peak becomes narrower and
narrower, much narrower than the filter Q would suggest, finally
ending up as a single spectral line. The amplitude can't grow
forester, either the amplifier gain is reduced or the signal is
clipped.

While initially starting to amplify the white noise, it is essential
the amplifier is in class A or AB. After a few iterations, when the
amplitude has grown close to saturation, the amplifier can fall into
class C and oscillate happily forever.

If the amplifier is initially deeply in class C, it doesn't amplify
the thermal noise and the oscillator will not start. The Schmitt
trigger makes starting even more unlikely.

Funny, they always work for me.

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

Many faulty oscillator constructions starts simply by the operating
voltage turn-on transient, putting the amplifier momentarily into
class AB. Such oscillator may start when powered from battery and the
DC voltage is applied by a switch, since the start transient is fast,
but might not start when a mains powered device is plugged into mains,
due to the slow charging of the storage capacitors.

Again, a Schmitt RC oscillator always starts. It must oscillate
because neither state is stable, and drives hard towards the opposite
state. Linear analysis makes no sense.

http://electronics-course.com/schmitt-trigger-oscillator



--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[Jan Panteltje]

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%.

It depends.
There are good and bad implementations of that.
You can write one in software that measures the almost exact bit time, and then uses that
to set either a software delay or programs the baudrate downcounters,
that also works with non standard baudrates,


I was thinking[tm] OP is using 4800 Bd,
that is very low these days, only context I use 4800 Bd is with GPS modules.
Those have a crystal and the baudrate is very precise,
Never had any problem with that with PICs running from internal oscillator,
even outside in the cold, my drone's auto-pilot does just that
http://panteltje.com/panteltje/quadcopter/index.html

My GPS clock / radiation logger does that:
http://panteltje.com/panteltje/pic/gm_pic2/

Also have a Raspberry Pi 2 with /dev/ttyAMA0 getting time and position from a GPS module at 4800 Bd,
but the raspi clock is crystal based.

Most serial projects here run on 19200 or 115200 Bd.
To make it easier for myself most send a 'hello' at power up so you can see if your baudrate matches or not.

 

On Fri, 08 Nov 2019 22:05:17 -0800, jlarkin@highlandsniptechnology.com
wrote:

On Sat, 09 Nov 2019 07:08:52 +0200, upsidedown@downunder.com wrote:

On Fri, 08 Nov 2019 10:58:29 -0800, 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.

The question is, does the oscillator start.

A normal oscillator is merely a noise amplifier with a frequency
selective positive feedback. For each time the original white thermal
noise around the feedback loop is amplified, the original white
thermal noise is amplified only in the filter passband increasing the
amplitude.

A Schmitt RC oscillator doesn't work that way. It doesn't need noise
to start.

The simplest form of a relaxation RC oscillator is implemented with a
neon lamp, which has a higher turn-on voltage and a lower turn-off
voltage, so there is hysteresis.

It is hard to imagine a crystal oscillators as an relaxation
oscillator.

 

[Peter]

Steve Wilson <no@spam.com> wrote

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.

Also Class A might be a problem if you are building a CPU which has
very low power modes. You can't really throw away say 1mA.

 

[Peter]

That's a really good explanation, I think.

upsidedown@downunder.com wrote

A normal oscillator is merely a noise amplifier with a frequency
selective positive feedback. For each time the original white thermal
noise around the feedback loop is amplified, the original white
thermal noise is amplified only in the filter passband increasing the
amplitude.

For each time around the loop, the spectral peak becomes narrower and
narrower, much narrower than the filter Q would suggest, finally
ending up as a single spectral line. The amplitude can't grow
forester, either the amplifier gain is reduced or the signal is
clipped.

While initially starting to amplify the white noise, it is essential
the amplifier is in class A or AB. After a few iterations, when the
amplitude has grown close to saturation, the amplifier can fall into
class C and oscillate happily forever.

If the amplifier is initially deeply in class C, it doesn't amplify
the thermal noise and the oscillator will not start. The Schmitt
trigger makes starting even more unlikely.

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

Many faulty oscillator constructions starts simply by the operating
voltage turn-on transient, putting the amplifier momentarily into
class AB. Such oscillator may start when powered from battery and the
DC voltage is applied by a switch, since the start transient is fast,
but might not start when a mains powered device is plugged into mains,
due to the slow charging of the storage capacitors.

 

On Sat, 09 Nov 2019 07:08:52 +0200, upsidedown@downunder.com wrote:

On Fri, 08 Nov 2019 10:58:29 -0800, 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.

The question is, does the oscillator start.

A normal oscillator is merely a noise amplifier with a frequency
selective positive feedback. For each time the original white thermal
noise around the feedback loop is amplified, the original white
thermal noise is amplified only in the filter passband increasing the
amplitude.

For each time around the loop, the spectral peak becomes narrower and
narrower, much narrower than the filter Q would suggest, finally
ending up as a single spectral line. The amplitude can't grow
forester, either the amplifier gain is reduced or the signal is
clipped.

While initially starting to amplify the white noise, it is essential
the amplifier is in class A or AB. After a few iterations, when the
amplitude has grown close to saturation, the amplifier can fall into
class C and oscillate happily forever.

If the amplifier is initially deeply in class C, it doesn't amplify
the thermal noise and the oscillator will not start. The Schmitt
trigger makes starting even more unlikely.

For those who like to play with LTSpice, one could unroll the feedback
loop by creating a few dozen modules containing an amplifier stage, an
attenuator (to simulate feedback factor and filter losses) followed by
a bandpass filter. Connect these modules in series.

At the input of this long chain, connect a resistor to ground,creating
a thermal white noise density of -174 dBm/Hz. Observe the amplitude
and spectrum after each stage.

If in a stage the attenuation is grater than amplifier gain, no output
is produced from the chain. If the amplifiers are class C no output is
produced from the chain.

 

On Fri, 08 Nov 2019 22:05:17 -0800, jlarkin@highlandsniptechnology.com
wrote:

Again, a Schmitt RC oscillator always starts. It must oscillate
because neither state is stable, and drives hard towards the opposite
state. Linear analysis makes no sense.

http://electronics-course.com/schmitt-trigger-oscillator

This relaxation oscillator assumes that the capacitance to ground is
sufficiently large and that the DC voltage to the inverter is applied
fast enough so that the voltage across C1 is less than Vil and hence
the output is "high" starting to charge the capacitor through R1.

The equivalent circuit of a crystal is a series resonance circuit
consisting of Cs and L at Fs with a very small parallel capacitance
Cp forming the parallel resonance Fp with the inductance L.

The question is, does the parallel capacitance Cp with some internal
feedback resistor R1 parallel create sufficiently fast relaxation
oscillations to excite the crystal parallel resonance Fp to produce a
voltage swing larger than the hysteresis and hence maintain
oscillation.

 

[Steve Wilson]

Peter <nospam@nospam9876.com> wrote:

Steve Wilson <no@spam.com> wrote

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.

Also Class A might be a problem if you are building a CPU which has
very low power modes. You can't really throw away say 1mA.

Watch crystals use an inverter running off a tiny battery. They last for
years.

 

[Steve Wilson]

Peter <nospam@nospam9876.com> wrote:

That's a really good explanation, I think.

upsidedown@downunder.com wrote

A normal oscillator is merely a noise amplifier with a frequency
selective positive feedback. For each time the original white thermal
noise around the feedback loop is amplified, the original white
thermal noise is amplified only in the filter passband increasing the
amplitude.

For each time around the loop, the spectral peak becomes narrower and
narrower, much narrower than the filter Q would suggest, finally
ending up as a single spectral line. The amplitude can't grow
forester, either the amplifier gain is reduced or the signal is
clipped.

While initially starting to amplify the white noise, it is essential
the amplifier is in class A or AB. After a few iterations, when the
amplitude has grown close to saturation, the amplifier can fall into
class C and oscillate happily forever.

If the amplifier is initially deeply in class C, it doesn't amplify
the thermal noise and the oscillator will not start. The Schmitt
trigger makes starting even more unlikely.

The Schmitt has no stable state so it has no choice but to oscillate.

Getting it to oscillate at the crystal frequency may take a lot of drive
power. This may exceed the crystal's capability. Here's an example. You
need Helmut's 74hc.lib to run it.

Version 4
SHEET 1 1008 716
WIRE -32 144 -64 144
WIRE 224 144 -32 144
WIRE 640 144 336 144
WIRE 704 144 640 144
WIRE 752 144 704 144
WIRE -64 368 -64 144
WIRE 224 368 -64 368
WIRE 640 368 640 144
WIRE 640 368 304 368
WIRE -64 464 -64 368
WIRE 96 464 -64 464
WIRE 352 464 160 464
WIRE -64 544 -64 464
WIRE -32 544 -64 544
WIRE 80 544 48 544
WIRE 112 544 80 544
WIRE 240 544 192 544
WIRE 256 544 240 544
WIRE 352 544 352 464
WIRE 352 544 320 544
WIRE 400 544 352 544
WIRE 448 544 400 544
WIRE 512 544 448 544
WIRE 640 544 640 368
WIRE 640 544 592 544
WIRE -64 560 -64 544
WIRE 448 560 448 544
WIRE -64 656 -64 624
WIRE 448 656 448 624
FLAG -64 656 0
FLAG 448 656 0
FLAG 80 544 R5L1
FLAG 240 544 L1C1
FLAG -32 144 OscIn
FLAG 704 144 Out
FLAG 400 544 C1C2
SYMBOL cap 256 560 R270
WINDOW 0 32 32 VTop 2
WINDOW 3 0 32 VBottom 2
WINDOW 39 -28 32 VBottom 2
SYMATTR InstName C1
SYMATTR Value 0.1p
SYMBOL cap 432 560 R0
SYMATTR InstName C2
SYMATTR Value 45p
SYMBOL cap -80 560 R0
SYMATTR InstName C3
SYMATTR Value 45p
SYMBOL res 608 528 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R2
SYMATTR Value 750
SYMBOL res 208 384 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName R3
SYMATTR Value 100k
SYMBOL ind 96 560 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 5 56 VBottom 2
SYMATTR InstName L1
SYMATTR Value 10m
SYMBOL res 64 528 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R5
SYMATTR Value 25
SYMBOL cap 160 448 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C5
SYMATTR Value 4p
SYMBOL 74hc14 272 96 R0
SYMATTR InstName U1
TEXT 40 -16 Left 2 !.tran 0 2m 0 1n
TEXT 40 -48 Left 2 ;'Pierce Schmitt Crystal Oscillator
TEXT 544 -16 Left 2 !.options plotwinsize=0
TEXT 752 -16 Left 2 !.ic V(OscIn) = 0
TEXT 544 8 Left 2 !.inc 74hc.lib
TEXT 160 256 Left 2 ;May be overdriving the crystal
RECTANGLE Normal 32 112 32 112 2

[Transient Analysis]
{
Npanes: 4
Active Pane: 2
{
traces: 1 {524290,0,"V(oscin)"}
X: ('m',1,0,0.0001,0.001)
Y[0]: (' ',1,0.9,0.3,4.2)
Y[1]: ('m',1,1e+308,0.0002,-1e+308)
Volts: (' ',0,0,1,0.9,0.3,4.2)
Log: 0 0 0
GridStyle: 1
},
{
traces: 1 {34603012,0,"I(R5)"}
X: ('m',1,0,0.0001,0.001)
Y[0]: ('m',1,-0.003,0.0005,0.003)
Y[1]: ('ľ',0,1e+308,0.0001,-1e+308)
Amps: ('m',0,0,1,-0.003,0.0005,0.003)
Log: 0 0 0
GridStyle: 1
},
{
traces: 1 {524293,0,"V(c1c2)"}
X: ('m',1,0,0.0001,0.001)
Y[0]: (' ',1,0.6,0.3,4.2)
Y[1]: ('ľ',0,1e+308,0.0001,-1e+308)
Volts: (' ',0,0,1,0.6,0.3,4.2)
Log: 0 0 0
GridStyle: 1
},
{
traces: 1 {524291,0,"V(out)"}
X: ('m',1,0,0.0001,0.001)
Y[0]: (' ',1,-0.5,0.5,5.5)
Y[1]: ('ľ',0,1e+308,0.0001,-1e+308)
Volts: (' ',0,0,1,-0.5,0.5,5.5)
Log: 0 0 0
GridStyle: 1
}
}

 

[Tauno Voipio]

On 9.11.19 01:10, Rick C wrote:

On Friday, November 8, 2019 at 5:21:20 PM UTC-5, Steve Wilson wrote:
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.

A little searching turned up a few.

https://mirror.thelifeofkenneth.com/lib/electronics_archive/Understanding_Crystal_Oscillators.pdf

I found several guides on oscillators that refer to MOSFETs as being suitable "transconductance" devices.

Why would a MOSFET be a problem?

The traditional crystal oscillators use the grid-to-cathode diode
of a tube or gate-to-source diode of a JFET as a means of generating
an amplitude-dependent bias on the control electrode. A MOSFET needs
an external diode to make it work.

--

-TV

 

[Steve Wilson]

jlarkin@highlandsniptechnology.com wrote:

On Sat, 09 Nov 2019 13:45:24 +0200, upsidedown@downunder.com wrote:

On Sat, 09 Nov 2019 07:08:52 +0200, upsidedown@downunder.com wrote:

On Fri, 08 Nov 2019 10:58:29 -0800, 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.

The question is, does the oscillator start.

A normal oscillator is merely a noise amplifier with a frequency
selective positive feedback. For each time the original white thermal
noise around the feedback loop is amplified, the original white
thermal noise is amplified only in the filter passband increasing the
amplitude.

For each time around the loop, the spectral peak becomes narrower and
narrower, much narrower than the filter Q would suggest, finally
ending up as a single spectral line. The amplitude can't grow
forester, either the amplifier gain is reduced or the signal is
clipped.

While initially starting to amplify the white noise, it is essential
the amplifier is in class A or AB. After a few iterations, when the
amplitude has grown close to saturation, the amplifier can fall into
class C and oscillate happily forever.

If the amplifier is initially deeply in class C, it doesn't amplify
the thermal noise and the oscillator will not start. The Schmitt
trigger makes starting even more unlikely.

As I have shown in other posts, a Schmitt has no stable state and must
oscillate. The trick is to channel the energy through the crystal instead
of around it, as in a plain RC oscillator.

For those who like to play with LTSpice, one could unroll the feedback
loop by creating a few dozen modules containing an amplifier stage, an
attenuator (to simulate feedback factor and filter losses) followed by
a bandpass filter. Connect these modules in series.

At the input of this long chain, connect a resistor to ground,creating
a thermal white noise density of -174 dBm/Hz. Observe the amplitude
and spectrum after each stage.

If in a stage the attenuation is grater than amplifier gain, no output
is produced from the chain. If the amplifiers are class C no output is
produced from the chain.

LT Spice doesn't handle nonlinearity in a frequency sim, and parts are
noiseless in a time domain sim. Time-domain oscillators sometimes need
a kick of some sort.

Time domain oscillators start on numerical instability. There are numerous
examples in Oscillators.zip where the oscillation starts at zero amplitude
and builds up to strength.

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

It is extremely difficult to couple a pertubation into the oscillator tank
to kick start an oscillator. The tank simply ignores the energy in an
external pulse. You have to somehow inject the pulse in series with the
tank but I have not found a practical way to do that.

A simpler method may be to simply increase the loop gain by increasing the
drive level.

 

On Sat, 09 Nov 2019 14:07:12 +0200, upsidedown@downunder.com wrote:

On Fri, 08 Nov 2019 22:05:17 -0800, jlarkin@highlandsniptechnology.com
wrote:


Again, a Schmitt RC oscillator always starts. It must oscillate
because neither state is stable, and drives hard towards the opposite
state. Linear analysis makes no sense.

http://electronics-course.com/schmitt-trigger-oscillator

This relaxation oscillator assumes that the capacitance to ground is
sufficiently large and that the DC voltage to the inverter is applied
fast enough so that the voltage across C1 is less than Vil and hence
the output is "high" starting to charge the capacitor through R1.

It will oscillate with any value of capacitance or any value of
resistance (as long as gate DC current doesn't make too much offset in
the resistor) and will oscillate no matter how long it takes for the
supply to come up. It has no stable state.

I've done CMOS schmitt oscillators with no explicit capacitor.

Schmitt inverters also oscillate with no R or C: just connect input to
output. Add a long trace, a PCB delay line, to better define the
frequency.

The equivalent circuit of a crystal is a series resonance circuit
consisting of Cs and L at Fs with a very small parallel capacitance
Cp forming the parallel resonance Fp with the inductance L.

The question is, does the parallel capacitance Cp with some internal
feedback resistor R1 parallel create sufficiently fast relaxation
oscillations to excite the crystal parallel resonance Fp to produce a
voltage swing larger than the hysteresis and hence maintain
oscillation.

Right. That's the issue.



--

John Larkin Highland Technology, Inc

lunatic fringe electronics