Sensing small inductances

[bitrex]

On 8/28/19 9:47 PM, bitrex wrote:

On 8/28/19 9:29 PM, Steve Wilson wrote:
John Larkin <jlarkin@highland_atwork_technology.com> wrote:

On Wed, 28 Aug 2019 22:35:44 GMT, Steve Wilson <no@spam.com> wrote:
That's his circcuit. Try a Colpitts.
Any passive resonator that oscillates is seeing a negative resistance.
Energy is conserved, and the resonator loss has to be made up for.
Where did you get that? Try measuring the input impedance of a
Colpitts. It
is always positive.

Measure when and how? what the Colpitts oscillator circuit's small
signal input impedance is when not oscillating and into the amplifier
with a test voltage isn't the same as its large-signal input impedance
when it is oscillating, its large-signal input impedance will be a
non-linear function of time

what does it matter measuring various parameters of circuits under
conditions they never experience when they're operating

 

[bitrex]

On 8/28/19 9:14 AM, Steve Wilson wrote:

Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

On 8/27/19 4:07 PM, Steve Wilson wrote:
Where do you measure the negative resistance and how do you control it?

You measure it by watching the follower oscillate with no external
feedback. Put a small pot in series with the base and watch where the
oscillation stops.

Cheers

Phil Hobbs

Thanks. This has been very illuminating. First, I find the term
negative resistance has nothing to do with the classical definition,
where an increasing voltage causes decreasing current. An example is
tunnel diode oscillators.

I'd go so far as to say when a system is oscillating all these
small-signal classical-EM and physics derived terms "input impedance",
"negative resistance" etc. have no meaning at all

 

[piglet]

On 28/08/2019 16:03, bitrex wrote:

Well the issue is the inductances I wanna measure, in the single digital
microhenries, to within a nanohenry, say, have a Q of about 0.3 in the
low MHz.

when you can get them to work as part of a standard oscillator tank that
oscillates at all, an octave and a half below their self-resonant
frequency, the stability is poor. if you have say a 4uH resonating with
a 10n cap to get 5MHz a 1 nH difference in the L is only a few hundred
Hz shift. But the oscillator is drifting around by several kHz over 20
minutes

Huh? My calculator makes 4uH 10nF come to 800kHz? For 5MHz at 4uH it
needs C=250pF

Are these nH changes in your low-Q uH inductor that you want to track
happening fast or slowly? Are you after absolute values or just watching
deltas?

piglet

 

[Jeroen Belleman]

Steve Wilson wrote:

John Larkin <jlarkin@highland_atwork_technology.com> wrote:

On Wed, 28 Aug 2019 22:35:44 GMT, Steve Wilson <no@spam.com> wrote:
That's his circcuit. Try a Colpitts.

Any passive resonator that oscillates is seeing a negative resistance.
Energy is conserved, and the resonator loss has to be made up for.

Where did you get that? Try measuring the input impedance of a Colpitts. It
is always positive. The negative resistance is a complete misnomer. It has
nothing to do with the voltage increasing and the current decreasing. It is
only the resistance needed to stop oscillations.

The Barkhausen criteria states the phase shift has to be zero or multiples of
360 degrees, and the loop gain has to be equal to or greater than 1. No
negative resistance is needed.

The Barkhausen criterion and the negative resistance view are
really just two ways of looking at the problem, completely
equivalent.

You have to get rid of this "voltage increasing and the current
decreasing" idea. Impedance is a complex function of frequency.
If its real part goes negative in some range of frequency, it's
completely valid to talk of negative resistance and it will
happily supply energy to an LC resonator if it happens to be
tuned within that range. This often happens by accident in
emitter followers.

It's really trivial to turn my LTspice emitter follower example
into a CC Colpitts oscillator. Just replace the AC source by a
200nH coil and increase the series resistor to 500 Ohms or so,
to limit the rate of growth of the oscillator amplitude to
something reasonable. Note that the equivalent coil Q is really,
really bad! Set an initial condition ".ic v(in)=1" to get it
going. Do a .tran simulation of the first 50ns or so and you'll
see a healthy exponentially growing sinusoid, in spite of the
poor Q.

Of course, there is nothing in this AC equivalent circuit to limit
the amplitude, so it just keeps growing.

Jeroen Belleman

 

[Steve Wilson]

Jeroen Belleman <jeroen@nospam.please> wrote:

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

On Wed, 28 Aug 2019 22:35:44 GMT, Steve Wilson <no@spam.com> wrote:
That's his circcuit. Try a Colpitts.

Any passive resonator that oscillates is seeing a negative resistance.
Energy is conserved, and the resonator loss has to be made up for.

Where did you get that? Try measuring the input impedance of a
Colpitts. It is always positive. The negative resistance is a complete
misnomer. It has nothing to do with the voltage increasing and the
current decreasing. It is only the resistance needed to stop
oscillations.

The Barkhausen criteria states the phase shift has to be zero or
multiples of 360 degrees, and the loop gain has to be equal to or
greater than 1. No negative resistance is needed.

The Barkhausen criterion and the negative resistance view are
really just two ways of looking at the problem, completely
equivalent.

You have to get rid of this "voltage increasing and the current
decreasing" idea. Impedance is a complex function of frequency.
If its real part goes negative in some range of frequency, it's
completely valid to talk of negative resistance and it will
happily supply energy to an LC resonator if it happens to be
tuned within that range. This often happens by accident in
emitter followers.

It's really trivial to turn my LTspice emitter follower example
into a CC Colpitts oscillator. Just replace the AC source by a
200nH coil and increase the series resistor to 500 Ohms or so,
to limit the rate of growth of the oscillator amplitude to
something reasonable. Note that the equivalent coil Q is really,
really bad! Set an initial condition ".ic v(in)=1" to get it
going. Do a .tran simulation of the first 50ns or so and you'll
see a healthy exponentially growing sinusoid, in spite of the
poor Q.

Of course, there is nothing in this AC equivalent circuit to limit
the amplitude, so it just keeps growing.

Jeroen Belleman

The exponential increase in amplitude as oscillations start is a normal
function of oscillators. The amplitude increases until the energy delivered
to the tank matches the energy lost in the tank.

If the idea of negative resistance were true, there would be nothing to
limit the amplitude of oscillations, and no way to control it.

Negative resistance means an increase in voltage causes a decrease in
current.

There is no portion of the cycle in a cc Colpitts where this is true. An
increase in voltage causes an increase in current. That is positive
resistance.

The idea of negative resistance and the Barkhausen criteria being
equivalent is false. There is no portion of the oscillator cycle where an
increase in voltage causes a decrease in current.

The oscillation amplitude in a cc Colpitts is controlled by the current
through the emitter follower. This adjusted by changing the emitter
resistor. The amplitude will increase until the energy delivered to the
tank matches the loss in the tank. There is no limiting effect where the
amplitude is clipped by forward biasing the base-collector junction so it
conducts into the VCC supply.

Parasitic oscillations are caused by inductance in the base combined with
stray capacitances to form a Colpitts oscillator.

I am including the readme from my Oscillators.zip file. It shows how to
design the tank, as well as explicit instruction on how to adjust the
amplitude.

----------------------------------------------------------------------

A Brief Survey Of RF Oscillators in LTspice

Steve Wilson

V1.0 Oct 2018

The Colpitts oscillator was invented in 1918 by Edwin Colpitts. It
is one of the most vigorous oscillators known, and often finds its
way where it is not wanted, such as in parasitic oscillations.

People often have difficulty getting their oscillator to run. Here
are a series of working examples that can be used as a template, and
to gain further understanding of how the oscillator works.

These examples have been tested in LTspice IV and XVII.

01.ASC Classic Colpitts
~~~~~~~~~~~~~~~~~~~~~~~
This is the simplest and easiest to get working. Here are the steps:

1. Select the operating frquency, fo. In this example, fo = 5 MHz
and Q = 40. C is the combined value of C1 and C2 in series.

2. Set XL = 50 ohms

3. L = XL / (2 * pi * fo)
= 50 / (2 * pi * 5e6)
= 1.5915 uH

4. ESR = XL / Q
= 50 / 4
= 1.25 Ohms

5. C = 1 / (2 * pi * fo * XC)
= 1 / (2 * pi * 5e6 * 50)
= 6.3661e-10

6. C1/C2 ratio = 1:1

7. C1 = 2 * C
= 2 * 6.3661e-10
= 1.2732 nF

8. Select a transistor with a ft greater than fo

9. Adjust the operating point by changing the emitter resistor, R1

Run the LTspice model.

NOTE: In all these oscillators, you should monitor the signal on the
base to ensure it doesn't approach VCC, and check the base-emitter
voltage at the negative peak to make sure it doesn't exceed the
reverse breakdown voltage of the transistor.

02.ASC Colpitts Q=1
~~~~~~~~~~~~~~~~~~~
Parasitic oscillations are caused by having an inductance in the
base of an emitter follower that combines with the base-emitter
capacitance and capacitance from the emitter to ground. This forms a
a voltage divider made of two capacitors in series across the
inductor, which makes a classic Colpitts.

The inductance is often very lossy, which gives the circuit a low
qualty factor, or Q. This is no problem for a Colpitts. Here is one
running with a Q of 1.

It is probably a good idea to add a resistor to the base in every
circuit that could potentially form a parasitic. The resistor could
range from 5 Ohms to perhaps 50 ohms. You will quickly find the
correct value for a particular transistor and circuit arrangement.

03.ASC 5 MHz Clapp
~~~~~~~~~~~~~~~~~~
One problem with the Colpitts is the difficulty of adjusting the
frequency to a desired value. The Clapp oscillator puts a small
capacitor, C3, in series with the inductor, which gives the
oscillator a limited tuning range.

This changes the design procedure, which follows:

1. select the desired series capacitor, C3. We use 32pf and fo = 5MHz

2. set the desired C3:C ratio. We use 10:1

3. determne the total series capacitance

CT = (32e-12 * 320e-12) / (32e-12 + 320e-12)
= 29.09 pf

4. set the ratio of C1:C2. We set CX to 470pf

CX * CT / (CX - CT)
= 470 * 320 / (470 - 320) = 1002pf

5. find the inductance

XC = 1 / (2 * pi * fo * CT)
= 1 / (2 * pi * 5e6 * 29.09e-12)
= 1,094.19Ohms

L = XC / (2 * pi * fo)
= 1094 / (2 * pi * 5e6)
= 34.82uH

Adjust the emitter resistor, R1, for the desired operating point.

04.ASC JFET 155MHz Clapp Osc
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Some people think JFETs are only good for low fequency audio. Here's
one running quite well at 155 MHz.

05.ASC 10MHz Slow Start Xtal Osc 94 seconds
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
High Q oscillators take a long time to start up and settle. This is
a typical result. It requires 94 seconds to stabilize.

This means it is virtually imposssible to view the actual waveforms
at various points in the oscillator in order to optimize them.

06.ASC 10MHz Fast Start Xtal Osc 0.030 seconds
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Various methods have been proposed to speed up the process. One
method is to perturb the oscillator so it starts oscillating faster.
However, it is difficult to inject enough energy into the tank to
make much difference, and it still takes a long time to settle.

Here is a method I developed a long time ago to greatly speed the
process.

It uses the .IC command to initialize a current into the inductor
that is equal to the peak current after the oscillator has
stabilized.

Note you cannot simply initialize the inductor with a current. This
generates a very large current in the tank and doesn't start the
oscillator properly. This is shown later in 11.ASC.

The solution is to use the equivalent resistance in the crystal. A
voltage is established across the inductor and resistor to set the
desired current. This reduces the startup time from 94 seconds to
0.030 seconds, which is an improvement of a factor of 3,133.

Now you can examine the oscillator waveforms in detail and make
whatever optimizations you desire.

07.ASC 10MHz Low Noise Oscillator
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The speedup even works in complicated feedback circuits that can
take a while to stabilize. Here is a circuit by Bruce Griffith that
stabilizes the operating point to reduce low frequency noise.

08.ASC Pierce crystal oscillator with B-source
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The speedup technique even works for Pierce oscillators. Here is a
circuit with a bit lower Q but still takes 14 seconds to stabilize.

It is impractical to make detailed observations of the waveforms to
see what can be done to optimize the oscillator.

09.ASC Fast Start Pierce crystal oscillator
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The Fast Start method is applied to the Pierce oscillator. The total
elapsed time is 0.010 seconds, an improvement of a factor of 1,400.

10.ASC Pierce oscillator with Current Pulse
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Some have tried to speed up a crystal oscillator by injecting a
pulse of current into the inductor after the analysis has started.

This does not work. I have not been able to produce the desired
current by making any change to the injected pulse.

11.ASC Pierce oscillator with Initial Condition
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Using the .IC command to inject a current into the inductor prior to
the analysis does not work.

In this case, it produces a current of 2e17 Amps in the inductor.

The Fast Start method is the only one that starts the analysis with
the desired current.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Change Log
~~~~~~~~~~

V1.0 Initial Release Oct 2018

 

On Thu, 29 Aug 2019 09:47:27 +0200, Jeroen Belleman
<jeroen@nospam.please> wrote:

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

On Wed, 28 Aug 2019 22:35:44 GMT, Steve Wilson <no@spam.com> wrote:
That's his circcuit. Try a Colpitts.

Any passive resonator that oscillates is seeing a negative resistance.
Energy is conserved, and the resonator loss has to be made up for.

Where did you get that? Try measuring the input impedance of a Colpitts. It
is always positive. The negative resistance is a complete misnomer. It has
nothing to do with the voltage increasing and the current decreasing. It is
only the resistance needed to stop oscillations.

The Barkhausen criteria states the phase shift has to be zero or multiples of
360 degrees, and the loop gain has to be equal to or greater than 1. No
negative resistance is needed.


The Barkhausen criterion and the negative resistance view are
really just two ways of looking at the problem, completely
equivalent.

You have to get rid of this "voltage increasing and the current
decreasing" idea. Impedance is a complex function of frequency.
If its real part goes negative in some range of frequency, it's
completely valid to talk of negative resistance and it will
happily supply energy to an LC resonator if it happens to be
tuned within that range. This often happens by accident in
emitter followers.

It's really trivial to turn my LTspice emitter follower example
into a CC Colpitts oscillator. Just replace the AC source by a
200nH coil and increase the series resistor to 500 Ohms or so,
to limit the rate of growth of the oscillator amplitude to
something reasonable. Note that the equivalent coil Q is really,
really bad! Set an initial condition ".ic v(in)=1" to get it
going. Do a .tran simulation of the first 50ns or so and you'll
see a healthy exponentially growing sinusoid, in spite of the
poor Q.

Of course, there is nothing in this AC equivalent circuit to limit
the amplitude, so it just keeps growing.

Jeroen Belleman

It's a terminology thing. The mechanization of the Barkhausen
criteria, namely provide a feedback in phase with the resonance and a
multiple of 360 degrees, with power gain avove 1, is in fact negative
resistance.

The oscillation criterion that net loop gain be exactly 1 is met by
something going nonlinear before it goes nuclear.

 

[Jeroen Belleman]

Steve Wilson wrote:

Jeroen Belleman <jeroen@nospam.please> wrote:

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

On Wed, 28 Aug 2019 22:35:44 GMT, Steve Wilson <no@spam.com> wrote:
That's his circcuit. Try a Colpitts.

Any passive resonator that oscillates is seeing a negative resistance.
Energy is conserved, and the resonator loss has to be made up for.

Where did you get that? Try measuring the input impedance of a
Colpitts. It is always positive. The negative resistance is a complete
misnomer. It has nothing to do with the voltage increasing and the
current decreasing. It is only the resistance needed to stop
oscillations.

The Barkhausen criteria states the phase shift has to be zero or
multiples of 360 degrees, and the loop gain has to be equal to or
greater than 1. No negative resistance is needed.

The Barkhausen criterion and the negative resistance view are
really just two ways of looking at the problem, completely
equivalent.

You have to get rid of this "voltage increasing and the current
decreasing" idea. Impedance is a complex function of frequency.
If its real part goes negative in some range of frequency, it's
completely valid to talk of negative resistance and it will
happily supply energy to an LC resonator if it happens to be
tuned within that range. This often happens by accident in
emitter followers.

It's really trivial to turn my LTspice emitter follower example
into a CC Colpitts oscillator. Just replace the AC source by a
200nH coil and increase the series resistor to 500 Ohms or so,
to limit the rate of growth of the oscillator amplitude to
something reasonable. Note that the equivalent coil Q is really,
really bad! Set an initial condition ".ic v(in)=1" to get it
going. Do a .tran simulation of the first 50ns or so and you'll
see a healthy exponentially growing sinusoid, in spite of the
poor Q.

Of course, there is nothing in this AC equivalent circuit to limit
the amplitude, so it just keeps growing.

Jeroen Belleman

The exponential increase in amplitude as oscillations start is a normal
function of oscillators. The amplitude increases until the energy delivered
to the tank matches the energy lost in the tank.

If the idea of negative resistance were true, there would be nothing to
limit the amplitude of oscillations, and no way to control it.

Isn't that what I wrote, just a few lines higher?

Negative resistance means an increase in voltage causes a decrease in
current.

There is no portion of the cycle in a cc Colpitts where this is true. An
increase in voltage causes an increase in current. That is positive
resistance.

The idea of negative resistance and the Barkhausen criteria being
equivalent is false. There is no portion of the oscillator cycle where an
increase in voltage causes a decrease in current.

I give up. This is a waste of time.

Jeroen Belleman

 

[Steve Wilson]

jlarkin@highlandsniptechnology.com wrote:

It's a terminology thing. The mechanization of the Barkhausen
criteria, namely provide a feedback in phase with the resonance and a
multiple of 360 degrees, with power gain avove 1, is in fact negative
resistance.

Barkhausen describes the conditions necessary for oscillations to start.

There is nothing in Barkhausen that shows negative resistance in the
oscillator.

Power gain is supplied by the active circuit - the emitter follower in a cc
Dolpitts, and amplifier in a Pierce.

The oscillation criterion that net loop gain be exactly 1 is met by
something going nonlinear before it goes nuclear.

Oscillations increase until the energy delivered to the tank matches the
energy lost in the tank. This is controlled in a cc Colpitts by adjusting
the emitter resistor, which changes the emitter current, which changes the
energy delivered to the tank.

If the idea of negative resistance were true, there would be no way to
limit the amplitude of oscillations, and no way to control it.

Please see the readme in my Oscillators.zip

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

I posted it earlier to Jeroen.

 

[Steve Wilson]

Jeroen Belleman <jeroen@nospam.please> wrote:

Steve Wilson wrote:
Jeroen Belleman <jeroen@nospam.please> wrote:

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

On Wed, 28 Aug 2019 22:35:44 GMT, Steve Wilson <no@spam.com> wrote:
That's his circcuit. Try a Colpitts.

Any passive resonator that oscillates is seeing a negative
resistance. Energy is conserved, and the resonator loss has to be
made up for.

Where did you get that? Try measuring the input impedance of a
Colpitts. It is always positive. The negative resistance is a
complete misnomer. It has nothing to do with the voltage increasing
and the current decreasing. It is only the resistance needed to stop
oscillations.

The Barkhausen criteria states the phase shift has to be zero or
multiples of 360 degrees, and the loop gain has to be equal to or
greater than 1. No negative resistance is needed.

The Barkhausen criterion and the negative resistance view are
really just two ways of looking at the problem, completely equivalent.

You have to get rid of this "voltage increasing and the current
decreasing" idea. Impedance is a complex function of frequency.
If its real part goes negative in some range of frequency, it's
completely valid to talk of negative resistance and it will
happily supply energy to an LC resonator if it happens to be
tuned within that range. This often happens by accident in emitter
followers.

It's really trivial to turn my LTspice emitter follower example
into a CC Colpitts oscillator. Just replace the AC source by a
200nH coil and increase the series resistor to 500 Ohms or so, to
limit the rate of growth of the oscillator amplitude to something
reasonable. Note that the equivalent coil Q is really, really bad! Set
an initial condition ".ic v(in)=1" to get it going. Do a .tran
simulation of the first 50ns or so and you'll see a healthy
exponentially growing sinusoid, in spite of the poor Q.

Of course, there is nothing in this AC equivalent circuit to limit the
amplitude, so it just keeps growing.

Jeroen Belleman

The exponential increase in amplitude as oscillations start is a normal
function of oscillators. The amplitude increases until the energy
delivered to the tank matches the energy lost in the tank.

If the idea of negative resistance were true, there would be nothing to
limit the amplitude of oscillations, and no way to control it.

Isn't that what I wrote, just a few lines higher?

That was in your simulation. In a real oscillator, the amplitude of
oscillations increases until the energy delivered to the tank matches the
energy lost.

In a cc Colpitts, the energy delivered to the tank is controlled by the
emitter current, which is adjusted by changing the emitter resistor.

This proves the idea of negative resistance is false.

Negative resistance means an increase in voltage causes a decrease in
current.

There is no portion of the cycle in a cc Colpitts where this is true.
An increase in voltage causes an increase in current. That is positive
resistance.

The idea of negative resistance and the Barkhausen criteria being
equivalent is false. There is no portion of the oscillator cycle where
an increase in voltage causes a decrease in current.

I give up. This is a waste of time.

You cannot provide an example of negative resistance in a real oscillator,
or how to control the amplitude of oscillation. Until yo do, your theories
don't match reality.

> Jeroen Belleman

 

[George Herold]

On Thursday, August 29, 2019 at 10:44:45 AM UTC-4, Steve Wilson wrote:

Jeroen Belleman <jeroen@nospam.please> wrote:

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

On Wed, 28 Aug 2019 22:35:44 GMT, Steve Wilson <no@spam.com> wrote:
That's his circcuit. Try a Colpitts.

Any passive resonator that oscillates is seeing a negative resistance.
Energy is conserved, and the resonator loss has to be made up for.

Where did you get that? Try measuring the input impedance of a
Colpitts. It is always positive. The negative resistance is a complete
misnomer. It has nothing to do with the voltage increasing and the
current decreasing. It is only the resistance needed to stop
oscillations.

The Barkhausen criteria states the phase shift has to be zero or
multiples of 360 degrees, and the loop gain has to be equal to or
greater than 1. No negative resistance is needed.

The Barkhausen criterion and the negative resistance view are
really just two ways of looking at the problem, completely
equivalent.

You have to get rid of this "voltage increasing and the current
decreasing" idea. Impedance is a complex function of frequency.
If its real part goes negative in some range of frequency, it's
completely valid to talk of negative resistance and it will
happily supply energy to an LC resonator if it happens to be
tuned within that range. This often happens by accident in
emitter followers.

It's really trivial to turn my LTspice emitter follower example
into a CC Colpitts oscillator. Just replace the AC source by a
200nH coil and increase the series resistor to 500 Ohms or so,
to limit the rate of growth of the oscillator amplitude to
something reasonable. Note that the equivalent coil Q is really,
really bad! Set an initial condition ".ic v(in)=1" to get it
going. Do a .tran simulation of the first 50ns or so and you'll
see a healthy exponentially growing sinusoid, in spite of the
poor Q.

Of course, there is nothing in this AC equivalent circuit to limit
the amplitude, so it just keeps growing.

Jeroen Belleman

The exponential increase in amplitude as oscillations start is a normal
function of oscillators. The amplitude increases until the energy delivered
to the tank matches the energy lost in the tank.

If the idea of negative resistance were true, there would be nothing to
limit the amplitude of oscillations, and no way to control it.

AS JL said, I think you're just having a terminology disagreement.
All the oscillators I've built grow until the amplitude hits the power
rails.. or is limited by some other non-linear element that is in the
loop/ circuit. (Famously a light bulb in the HP200)

George h.

Negative resistance means an increase in voltage causes a decrease in
current.

There is no portion of the cycle in a cc Colpitts where this is true. An
increase in voltage causes an increase in current. That is positive
resistance.

The idea of negative resistance and the Barkhausen criteria being
equivalent is false. There is no portion of the oscillator cycle where an
increase in voltage causes a decrease in current.

The oscillation amplitude in a cc Colpitts is controlled by the current
through the emitter follower. This adjusted by changing the emitter
resistor. The amplitude will increase until the energy delivered to the
tank matches the loss in the tank. There is no limiting effect where the
amplitude is clipped by forward biasing the base-collector junction so it
conducts into the VCC supply.

Parasitic oscillations are caused by inductance in the base combined with
stray capacitances to form a Colpitts oscillator.

I am including the readme from my Oscillators.zip file. It shows how to
design the tank, as well as explicit instruction on how to adjust the
amplitude.

----------------------------------------------------------------------

A Brief Survey Of RF Oscillators in LTspice

Steve Wilson

V1.0 Oct 2018

The Colpitts oscillator was invented in 1918 by Edwin Colpitts. It
is one of the most vigorous oscillators known, and often finds its
way where it is not wanted, such as in parasitic oscillations.

People often have difficulty getting their oscillator to run. Here
are a series of working examples that can be used as a template, and
to gain further understanding of how the oscillator works.

These examples have been tested in LTspice IV and XVII.

01.ASC Classic Colpitts
~~~~~~~~~~~~~~~~~~~~~~~
This is the simplest and easiest to get working. Here are the steps:

1. Select the operating frquency, fo. In this example, fo = 5 MHz
and Q = 40. C is the combined value of C1 and C2 in series.

2. Set XL = 50 ohms

3. L = XL / (2 * pi * fo)
= 50 / (2 * pi * 5e6)
= 1.5915 uH

4. ESR = XL / Q
= 50 / 4
= 1.25 Ohms

5. C = 1 / (2 * pi * fo * XC)
= 1 / (2 * pi * 5e6 * 50)
= 6.3661e-10

6. C1/C2 ratio = 1:1

7. C1 = 2 * C
= 2 * 6.3661e-10
= 1.2732 nF

8. Select a transistor with a ft greater than fo

9. Adjust the operating point by changing the emitter resistor, R1

Run the LTspice model.

NOTE: In all these oscillators, you should monitor the signal on the
base to ensure it doesn't approach VCC, and check the base-emitter
voltage at the negative peak to make sure it doesn't exceed the
reverse breakdown voltage of the transistor.

02.ASC Colpitts Q=1
~~~~~~~~~~~~~~~~~~~
Parasitic oscillations are caused by having an inductance in the
base of an emitter follower that combines with the base-emitter
capacitance and capacitance from the emitter to ground. This forms a
a voltage divider made of two capacitors in series across the
inductor, which makes a classic Colpitts.

The inductance is often very lossy, which gives the circuit a low
qualty factor, or Q. This is no problem for a Colpitts. Here is one
running with a Q of 1.

It is probably a good idea to add a resistor to the base in every
circuit that could potentially form a parasitic. The resistor could
range from 5 Ohms to perhaps 50 ohms. You will quickly find the
correct value for a particular transistor and circuit arrangement.

03.ASC 5 MHz Clapp
~~~~~~~~~~~~~~~~~~
One problem with the Colpitts is the difficulty of adjusting the
frequency to a desired value. The Clapp oscillator puts a small
capacitor, C3, in series with the inductor, which gives the
oscillator a limited tuning range.

This changes the design procedure, which follows:

1. select the desired series capacitor, C3. We use 32pf and fo = 5MHz

2. set the desired C3:C ratio. We use 10:1

3. determne the total series capacitance

CT = (32e-12 * 320e-12) / (32e-12 + 320e-12)
= 29.09 pf

4. set the ratio of C1:C2. We set CX to 470pf

CX * CT / (CX - CT)
= 470 * 320 / (470 - 320) = 1002pf

5. find the inductance

XC = 1 / (2 * pi * fo * CT)
= 1 / (2 * pi * 5e6 * 29.09e-12)
= 1,094.19Ohms

L = XC / (2 * pi * fo)
= 1094 / (2 * pi * 5e6)
= 34.82uH

Adjust the emitter resistor, R1, for the desired operating point.

04.ASC JFET 155MHz Clapp Osc
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Some people think JFETs are only good for low fequency audio. Here's
one running quite well at 155 MHz.

05.ASC 10MHz Slow Start Xtal Osc 94 seconds
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
High Q oscillators take a long time to start up and settle. This is
a typical result. It requires 94 seconds to stabilize.

This means it is virtually imposssible to view the actual waveforms
at various points in the oscillator in order to optimize them.

06.ASC 10MHz Fast Start Xtal Osc 0.030 seconds
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Various methods have been proposed to speed up the process. One
method is to perturb the oscillator so it starts oscillating faster.
However, it is difficult to inject enough energy into the tank to
make much difference, and it still takes a long time to settle.

Here is a method I developed a long time ago to greatly speed the
process.

It uses the .IC command to initialize a current into the inductor
that is equal to the peak current after the oscillator has
stabilized.

Note you cannot simply initialize the inductor with a current. This
generates a very large current in the tank and doesn't start the
oscillator properly. This is shown later in 11.ASC.

The solution is to use the equivalent resistance in the crystal. A
voltage is established across the inductor and resistor to set the
desired current. This reduces the startup time from 94 seconds to
0.030 seconds, which is an improvement of a factor of 3,133.

Now you can examine the oscillator waveforms in detail and make
whatever optimizations you desire.

07.ASC 10MHz Low Noise Oscillator
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The speedup even works in complicated feedback circuits that can
take a while to stabilize. Here is a circuit by Bruce Griffith that
stabilizes the operating point to reduce low frequency noise.

08.ASC Pierce crystal oscillator with B-source
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The speedup technique even works for Pierce oscillators. Here is a
circuit with a bit lower Q but still takes 14 seconds to stabilize.

It is impractical to make detailed observations of the waveforms to
see what can be done to optimize the oscillator.

09.ASC Fast Start Pierce crystal oscillator
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The Fast Start method is applied to the Pierce oscillator. The total
elapsed time is 0.010 seconds, an improvement of a factor of 1,400.

10.ASC Pierce oscillator with Current Pulse
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Some have tried to speed up a crystal oscillator by injecting a
pulse of current into the inductor after the analysis has started.

This does not work. I have not been able to produce the desired
current by making any change to the injected pulse.

11.ASC Pierce oscillator with Initial Condition
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Using the .IC command to inject a current into the inductor prior to
the analysis does not work.

In this case, it produces a current of 2e17 Amps in the inductor.

The Fast Start method is the only one that starts the analysis with
the desired current.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Change Log
~~~~~~~~~~

V1.0 Initial Release Oct 2018

 

[Steve Wilson]

George Herold <gherold@teachspin.com> wrote:

On Thursday, August 29, 2019 at 10:44:45 AM UTC-4, Steve Wilson wrote:
If the idea of negative resistance were true, there would be nothing to
limit the amplitude of oscillations, and no way to control it.

AS JL said, I think you're just having a terminology disagreement.
All the oscillators I've built grow until the amplitude hits the power
rails.. or is limited by some other non-linear element that is in the
loop/ circuit. (Famously a light bulb in the HP200)

Terminology disagreement - nonsense. There is no such thing as negative
resistance in an oscillator. The amplitude would increase without limit
until the oscillator railed. This would increase the noise, which is
critical in precision oscillators.

Crystal oscillators requires a carefully controlled amplitude. If it is too
small, the oscillator may not start. Too high and the crystal may fracture.

You cannot control the amplitude if the oscillator runs on negative
resistance.

If your oscillators hit the power rails, you could also be exceeding the
reverse breakdown voltage in the b-e junction. This is not good.

Hitting the rails is simply sloppy design when the amplitude is so easy to
control.

Try some of the oscillators in my article at

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

> George h.

 

[Phil Hobbs]

On 8/29/19 10:41 AM, jlarkin@highlandsniptechnology.com wrote:

On Thu, 29 Aug 2019 09:47:27 +0200, Jeroen Belleman
jeroen@nospam.please> wrote:

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

On Wed, 28 Aug 2019 22:35:44 GMT, Steve Wilson <no@spam.com> wrote:
That's his circcuit. Try a Colpitts.

Any passive resonator that oscillates is seeing a negative resistance.
Energy is conserved, and the resonator loss has to be made up for.

Where did you get that? Try measuring the input impedance of a Colpitts. It
is always positive. The negative resistance is a complete misnomer. It has
nothing to do with the voltage increasing and the current decreasing. It is
only the resistance needed to stop oscillations.

The Barkhausen criteria states the phase shift has to be zero or multiples of
360 degrees, and the loop gain has to be equal to or greater than 1. No
negative resistance is needed.


The Barkhausen criterion and the negative resistance view are
really just two ways of looking at the problem, completely
equivalent.

You have to get rid of this "voltage increasing and the current
decreasing" idea. Impedance is a complex function of frequency.
If its real part goes negative in some range of frequency, it's
completely valid to talk of negative resistance and it will
happily supply energy to an LC resonator if it happens to be
tuned within that range. This often happens by accident in
emitter followers.

It's really trivial to turn my LTspice emitter follower example
into a CC Colpitts oscillator. Just replace the AC source by a
200nH coil and increase the series resistor to 500 Ohms or so,
to limit the rate of growth of the oscillator amplitude to
something reasonable. Note that the equivalent coil Q is really,
really bad! Set an initial condition ".ic v(in)=1" to get it
going. Do a .tran simulation of the first 50ns or so and you'll
see a healthy exponentially growing sinusoid, in spite of the
poor Q.

Of course, there is nothing in this AC equivalent circuit to limit
the amplitude, so it just keeps growing.

Jeroen Belleman

It's a terminology thing. The mechanization of the Barkhausen
criteria, namely provide a feedback in phase with the resonance and a
multiple of 360 degrees, with power gain avove 1, is in fact negative
resistance.

The oscillation criterion that net loop gain be exactly 1 is met by
something going nonlinear before it goes nuclear.

I think Steve is stuck thinking of negative resistance in the time
domain, i.e.

dI_1/dV_1 = -R.

In the frequency domain, an input impedance like that gives an S11
greater than unity at all frequencies.

But in a real oscillator, S11 only has to have a negative real part near
the frequency of the resonator--since only that frequency band is
relevant, it's at least as good as having a negative input resistance at
all frequencies. In fact it's generally better, since the (slow) bias
network doesn't have to cope with the negative resistance at all.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Joseph Gwinn]

On Aug 29, 2019, jlarkin@highlandsniptechnology.com wrote
(in article<hqofmet497f174cr4i1ddbrr64ipf7pao5@4ax.com&gt:

On Thu, 29 Aug 2019 09:47:27 +0200, Jeroen Belleman
jeroen@nospam.please> wrote:

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

On Wed, 28 Aug 2019 22:35:44 GMT, Steve Wilson <no@spam.com> wrote:
That's his circcuit. Try a Colpitts.

Any passive resonator that oscillates is seeing a negative resistance.
Energy is conserved, and the resonator loss has to be made up for.

Where did you get that? Try measuring the input impedance of a Colpitts. I

is always positive. The negative resistance is a complete misnomer. It has
nothing to do with the voltage increasing and the current decreasing. It i

only the resistance needed to stop oscillations.

The Barkhausen criteria states the phase shift has to be zero or multiples
of
360 degrees, and the loop gain has to be equal to or greater than 1. No
negative resistance is needed.


The Barkhausen criterion and the negative resistance view are
really just two ways of looking at the problem, completely
equivalent.

You have to get rid of this "voltage increasing and the current
decreasing" idea. Impedance is a complex function of frequency.
If its real part goes negative in some range of frequency, it's
completely valid to talk of negative resistance and it will
happily supply energy to an LC resonator if it happens to be
tuned within that range. This often happens by accident in
emitter followers.

It's really trivial to turn my LTspice emitter follower example
into a CC Colpitts oscillator. Just replace the AC source by a
200nH coil and increase the series resistor to 500 Ohms or so,
to limit the rate of growth of the oscillator amplitude to
something reasonable. Note that the equivalent coil Q is really,
really bad! Set an initial condition ".ic v(in)=1" to get it
going. Do a .tran simulation of the first 50ns or so and you'll
see a healthy exponentially growing sinusoid, in spite of the
poor Q.

Of course, there is nothing in this AC equivalent circuit to limit
the amplitude, so it just keeps growing.

Jeroen Belleman

It's a terminology thing. The mechanization of the Barkhausen
criteria, namely provide a feedback in phase with the resonance and a
multiple of 360 degrees, with power gain avove 1, is in fact negative
resistance.

The oscillation criterion that net loop gain be exactly 1 is met by
something going nonlinear before it goes nuclear.

I do have a question. One can build an oscillator using an EFDA amplifier and
a length of optical fiber in a ring connecting input to output. Where is the
negative resistance here?

This doesn´t use fiber, but the principle is the
same:<https://www.findlight.net/blog/2017/07/18/optical-parametric-
oscillators/>.

Joe Gwinn

 

On Thu, 29 Aug 2019 21:45:29 -0400, Joseph Gwinn
<joegwinn@comcast.net> wrote:

On Aug 29, 2019, jlarkin@highlandsniptechnology.com wrote
(in article<hqofmet497f174cr4i1ddbrr64ipf7pao5@4ax.com&gt:

On Thu, 29 Aug 2019 09:47:27 +0200, Jeroen Belleman
jeroen@nospam.please> wrote:

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

On Wed, 28 Aug 2019 22:35:44 GMT, Steve Wilson <no@spam.com> wrote:
That's his circcuit. Try a Colpitts.

Any passive resonator that oscillates is seeing a negative resistance.
Energy is conserved, and the resonator loss has to be made up for.

Where did you get that? Try measuring the input impedance of a Colpitts. I

is always positive. The negative resistance is a complete misnomer. It has
nothing to do with the voltage increasing and the current decreasing. It i

only the resistance needed to stop oscillations.

The Barkhausen criteria states the phase shift has to be zero or multiples
of
360 degrees, and the loop gain has to be equal to or greater than 1. No
negative resistance is needed.


The Barkhausen criterion and the negative resistance view are
really just two ways of looking at the problem, completely
equivalent.

You have to get rid of this "voltage increasing and the current
decreasing" idea. Impedance is a complex function of frequency.
If its real part goes negative in some range of frequency, it's
completely valid to talk of negative resistance and it will
happily supply energy to an LC resonator if it happens to be
tuned within that range. This often happens by accident in
emitter followers.

It's really trivial to turn my LTspice emitter follower example
into a CC Colpitts oscillator. Just replace the AC source by a
200nH coil and increase the series resistor to 500 Ohms or so,
to limit the rate of growth of the oscillator amplitude to
something reasonable. Note that the equivalent coil Q is really,
really bad! Set an initial condition ".ic v(in)=1" to get it
going. Do a .tran simulation of the first 50ns or so and you'll
see a healthy exponentially growing sinusoid, in spite of the
poor Q.

Of course, there is nothing in this AC equivalent circuit to limit
the amplitude, so it just keeps growing.

Jeroen Belleman

It's a terminology thing. The mechanization of the Barkhausen
criteria, namely provide a feedback in phase with the resonance and a
multiple of 360 degrees, with power gain avove 1, is in fact negative
resistance.

The oscillation criterion that net loop gain be exactly 1 is met by
something going nonlinear before it goes nuclear.

I do have a question. One can build an oscillator using an EFDA amplifier and
a length of optical fiber in a ring connecting input to output. Where is the
negative resistance here?

It's not a 1-port electrical resonator, so I don't know.

 

[bitrex]

On 8/29/19 1:41 AM, piglet wrote:

On 28/08/2019 16:03, bitrex wrote:
Well the issue is the inductances I wanna measure, in the single
digital microhenries, to within a nanohenry, say, have a Q of about
0.3 in the low MHz.

when you can get them to work as part of a standard oscillator tank
that oscillates at all, an octave and a half below their self-resonant
frequency, the stability is poor. if you have say a 4uH resonating
with a 10n cap to get 5MHz a 1 nH difference in the L is only a few
hundred Hz shift. But the oscillator is drifting around by several kHz
over 20 minutes

Huh? My calculator makes 4uH 10nF come to 800kHz? For 5MHz at 4uH it
needs C=250pF

Right, SOMEBODY forgot the two pi while writing that

Are these nH changes in your low-Q uH inductor that you want to track
happening fast or slowly? Are you after absolute values or just watching
deltas?

piglet

Not much faster than 10s of Hz. And absolute values in this case but
it's an inductive transducer would like to know how much it is strained
compared to its relaxed state, so absolute accuracy with respect to some
standard is less important than precision/repeatability.

 

[Steve Wilson]

Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

I think Steve is stuck thinking of negative resistance in the time
domain, i.e.

dI_1/dV_1 = -R.

In the frequency domain, an input impedance like that gives an S11
greater than unity at all frequencies.

But in a real oscillator, S11 only has to have a negative real part near
the frequency of the resonator--since only that frequency band is
relevant, it's at least as good as having a negative input resistance at
all frequencies. In fact it's generally better, since the (slow) bias
network doesn't have to cope with the negative resistance at all.

The oscillator responds on a cycle-by-cycle basis to the input signal. See "A
General Theory of Phase Noise in Electrical Oscillators", Ali Hajimiri and
Thomas H. Lee:

https://authors.library.caltech.edu/4917/1/HAJieeejssc98.pdf

You are thinking of a 1-port device reflecting energy back to the source.

A real oscillator has two ports. For example, in a cc Colpitts, you can say
the base is the input and the emitter is the output.

The negative resistance is measured by inserting a resistor at the base. It
is the value needed to shut down oscillations.

However, if the oscillator ran on negative resistance, it would be impossible
to control the amplitude of oscillations. They would increase until the
oscillator railed.

Railing the oscillator increases the noise, which is bad in precision
oscillators.

Crystal oscillators must have careful control of the amplitude. If it is too
low, the crystal may not start. If it is too high, the crystal may fracture.

It is easy to control the amplitude of oscillation in a cc Colpitts. The
emitter resistor controls the emitter current and thus the energy delivered
to the tank. You set the current by changing the value of the emitter
resistor so that the energy delivered to the tank equals the energy lost in
the tank. This also works the same way with crystal oscillators.

This satisfies the Barkhausen criteria by setting the loop gain to unity.

Again, you cannot do this if the oscillator ran on negative resistance. It
has no concept of amplitude.

So the concepts of negative resistance and Barkhausen criteria are not
equivalent, and are not two ways of saying the same thing.

Negative resistance in oscillators does not exist.

Cheers

Phil Hobbs

 

[Jeroen Belleman]

Phil Hobbs wrote:

On 8/29/19 10:41 AM, jlarkin@highlandsniptechnology.com wrote:
On Thu, 29 Aug 2019 09:47:27 +0200, Jeroen Belleman
jeroen@nospam.please> wrote:

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

On Wed, 28 Aug 2019 22:35:44 GMT, Steve Wilson <no@spam.com> wrote:
That's his circcuit. Try a Colpitts.

Any passive resonator that oscillates is seeing a negative resistance.
Energy is conserved, and the resonator loss has to be made up for.
Where did you get that? Try measuring the input impedance of a
Colpitts. It
is always positive. The negative resistance is a complete misnomer.
It has
nothing to do with the voltage increasing and the current
decreasing. It is
only the resistance needed to stop oscillations.

The Barkhausen criteria states the phase shift has to be zero or
multiples of
360 degrees, and the loop gain has to be equal to or greater than 1. No
negative resistance is needed.


The Barkhausen criterion and the negative resistance view are
really just two ways of looking at the problem, completely
equivalent.

You have to get rid of this "voltage increasing and the current
decreasing" idea. Impedance is a complex function of frequency.
If its real part goes negative in some range of frequency, it's
completely valid to talk of negative resistance and it will
happily supply energy to an LC resonator if it happens to be
tuned within that range. This often happens by accident in
emitter followers.

It's really trivial to turn my LTspice emitter follower example
into a CC Colpitts oscillator. Just replace the AC source by a
200nH coil and increase the series resistor to 500 Ohms or so,
to limit the rate of growth of the oscillator amplitude to
something reasonable. Note that the equivalent coil Q is really,
really bad! Set an initial condition ".ic v(in)=1" to get it
going. Do a .tran simulation of the first 50ns or so and you'll
see a healthy exponentially growing sinusoid, in spite of the
poor Q.

Of course, there is nothing in this AC equivalent circuit to limit
the amplitude, so it just keeps growing.

Jeroen Belleman

It's a terminology thing. The mechanization of the Barkhausen
criteria, namely provide a feedback in phase with the resonance and a
multiple of 360 degrees, with power gain avove 1, is in fact negative
resistance.

The oscillation criterion that net loop gain be exactly 1 is met by
something going nonlinear before it goes nuclear.

I think Steve is stuck thinking of negative resistance in the time
domain, i.e.

dI_1/dV_1 = -R.

In the frequency domain, an input impedance like that gives an S11
greater than unity at all frequencies.

But in a real oscillator, S11 only has to have a negative real part near
the frequency of the resonator--since only that frequency band is
relevant, it's at least as good as having a negative input resistance at
all frequencies. In fact it's generally better, since the (slow) bias
network doesn't have to cope with the negative resistance at all.

Cheers

Phil Hobbs

Indeed. He also seems to have trouble distinguishing between
linearized AC-equivalent models and actual circuits. I suppose
it will sink in, eventually.

Jeroen Belleman

 

[Steve Wilson]

Jeroen Belleman <jeroen@nospam.please> wrote:

Indeed. He also seems to have trouble distinguishing between
linearized AC-equivalent models and actual circuits. I suppose
it will sink in, eventually.

Jeroen Belleman

Piping in to serve insults is very juvenile. Fortunately, there is a
solution. Since your theories and comments serve no useful purpose -

Plonk

 

[Steve Wilson]

Jeroen Belleman <jeroen@nospam.please> wrote:

Indeed. He also seems to have trouble distinguishing between
linearized AC-equivalent models and actual circuits. I suppose
it will sink in, eventually.

Jeroen Belleman

I thought you were out.

I have been running SPICE since the DOS days in the 1980's. It's a pretty
good tool if you know its limits and stay within them.

Your models do not reflect reality. They are useless as an analysis tool.
They give no valid predictions of performance. I won't waste my time on them
anymore.

 

[Steve Wilson]

Steve Wilson <no@spam.com> wrote:

George Herold <gherold@teachspin.com> wrote:

AS JL said, I think you're just having a terminology disagreement.
All the oscillators I've built grow until the amplitude hits the power
rails.. or is limited by some other non-linear element that is in the
loop/ circuit.

There is no excuse for railing an oscillator.

Post one of your designs and I'll fix it for you.