L oscillator stability

J

J. Hunter

Guest
Hi !

I'm using the oscillator part of a 74HC4060 in a home made inductance
meter. One cap to ground at both input and output of the CMOS oscillator
gate. The inductance to be measured between the input and output in series
with a reference inductance of 10uH. A microcontroller reads the frequency
from one of the output of the HC4060, calculates the inductance value and
displays it on a LCD screen. It should read between 1uH (129Mhz) to about 1H

It seemed to work well until I found out I has a slight oscillator
instability when measuring a 4700uH coil.

The oscillator runs about 60Khz with that inductance. If I connect the coil,
it can oscillate at say 58Khz and stay at that frequency. If I disconnect
and reconnect the coil, it ma oscillate at a sligthly higher or lower rate
and stay at that new frequency. Weird!

I experimented a bit to correct that problem and found out that connecting a
small network in series with the indcutance to be measured (a 100 ohms in
parrallel with a 1uF, values not critical) stabilizes the reading.
Smaller inductance values are unaffected by this network. Higher inductance
values, I don't know yet...

My questions are:

What could be involved in the instablity?
Is my solution (which is empirical) a good one?
Is there a better solution if any?

I know this is lot of questions.

J. Hunter
 
J. Hunter wrote:

Hi !

I'm using the oscillator part of a 74HC4060 in a home made inductance
meter. One cap to ground at both input and output of the CMOS oscillator
gate. The inductance to be measured between the input and output in series
with a reference inductance of 10uH. A microcontroller reads the frequency
from one of the output of the HC4060, calculates the inductance value and
displays it on a LCD screen. It should read between 1uH (129Mhz) to about 1H

It seemed to work well until I found out I has a slight oscillator
instability when measuring a 4700uH coil.

The oscillator runs about 60Khz with that inductance. If I connect the coil,
it can oscillate at say 58Khz and stay at that frequency. If I disconnect
and reconnect the coil, it ma oscillate at a sligthly higher or lower rate
and stay at that new frequency. Weird!

I experimented a bit to correct that problem and found out that connecting a
small network in series with the indcutance to be measured (a 100 ohms in
parrallel with a 1uF, values not critical) stabilizes the reading.
Smaller inductance values are unaffected by this network. Higher inductance
values, I don't know yet...

My questions are:

What could be involved in the instablity?
Is my solution (which is empirical) a good one?
Is there a better solution if any?

I know this is lot of questions.
Click to expand...
Not really, it is simple.
Th inductance is involving an area. The connecting
wires are part of the enclosed area.

Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net
 
"Rene Tschaggelar" <none@none.net> wrote in message
news:4291e5d6$0$1150$5402220f@news.sunrise.ch...
J. Hunter wrote:

Hi !

I'm using the oscillator part of a 74HC4060 in a home made inductance
meter. One cap to ground at both input and output of the CMOS oscillator
gate. The inductance to be measured between the input and output in
series with a reference inductance of 10uH. A microcontroller reads the
frequency from one of the output of the HC4060, calculates the inductance
value and displays it on a LCD screen. It should read between 1uH
(129Mhz) to about 1H

It seemed to work well until I found out I has a slight oscillator
instability when measuring a 4700uH coil.

The oscillator runs about 60Khz with that inductance. If I connect the
coil, it can oscillate at say 58Khz and stay at that frequency. If I
disconnect and reconnect the coil, it ma oscillate at a sligthly higher
or lower rate and stay at that new frequency. Weird!

I experimented a bit to correct that problem and found out that
connecting a small network in series with the indcutance to be measured
(a 100 ohms in parrallel with a 1uF, values not critical) stabilizes the
reading.
Smaller inductance values are unaffected by this network. Higher
inductance values, I don't know yet...

My questions are:

What could be involved in the instablity?
Is my solution (which is empirical) a good one?
Is there a better solution if any?

I know this is lot of questions.

Not really, it is simple.
Th inductance is involving an area. The connecting
wires are part of the enclosed area.
Click to expand...
I don't quite understand as the setup is the same from one reading to the
next. Even with few uH coils, it is stable. Wiring should be less of a
problem with higher inductance values.
Shouldn't be...?


Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net
Click to expand...
 
On Mon, 23 May 2005 11:28:10 -0400, the renowned "J. Hunter"
<j2hunter@yahoo.com> wrote:

"Rene Tschaggelar" <none@none.net> wrote in message
news:4291e5d6$0$1150$5402220f@news.sunrise.ch...
J. Hunter wrote:

Hi !

I'm using the oscillator part of a 74HC4060 in a home made inductance
meter. One cap to ground at both input and output of the CMOS oscillator
gate. The inductance to be measured between the input and output in
series with a reference inductance of 10uH. A microcontroller reads the
frequency from one of the output of the HC4060, calculates the inductance
value and displays it on a LCD screen. It should read between 1uH
(129Mhz) to about 1H

It seemed to work well until I found out I has a slight oscillator
instability when measuring a 4700uH coil.

The oscillator runs about 60Khz with that inductance. If I connect the
coil, it can oscillate at say 58Khz and stay at that frequency. If I
disconnect and reconnect the coil, it ma oscillate at a sligthly higher
or lower rate and stay at that new frequency. Weird!

I experimented a bit to correct that problem and found out that
connecting a small network in series with the indcutance to be measured
(a 100 ohms in parrallel with a 1uF, values not critical) stabilizes the
reading.
Smaller inductance values are unaffected by this network. Higher
inductance values, I don't know yet...

My questions are:

What could be involved in the instablity?
Is my solution (which is empirical) a good one?
Is there a better solution if any?

I know this is lot of questions.

Not really, it is simple.
Th inductance is involving an area. The connecting
wires are part of the enclosed area.


I don't quite understand as the setup is the same from one reading to the
next. Even with few uH coils, it is stable. Wiring should be less of a
problem with higher inductance values.
Shouldn't be...?
Click to expand...
Yes. Have you looked at the waveform with an oscilloscope? Maybe the
distributed capacitance of that relatively high value is causing some
problems-- you might see high-frequency hash during parts of the
oscillator wavform, for example, so you could have an oscillator on
top of an oscillator.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
 
Hi !

I'm using the oscillator part of a 74HC4060 in a home made inductance
meter. One cap to ground at both input and output of the CMOS
oscillator
gate. The inductance to be measured between the input and output in
series with a reference inductance of 10uH. A microcontroller reads the
frequency from one of the output of the HC4060, calculates the
inductance
value and displays it on a LCD screen. It should read between 1uH
(129Mhz) to about 1H

It seemed to work well until I found out I has a slight oscillator
instability when measuring a 4700uH coil.

The oscillator runs about 60Khz with that inductance. If I connect the
coil, it can oscillate at say 58Khz and stay at that frequency. If I
disconnect and reconnect the coil, it ma oscillate at a sligthly higher
or lower rate and stay at that new frequency. Weird!

I experimented a bit to correct that problem and found out that
connecting a small network in series with the indcutance to be measured
(a 100 ohms in parrallel with a 1uF, values not critical) stabilizes
the
reading.
Smaller inductance values are unaffected by this network. Higher
inductance values, I don't know yet...

My questions are:

What could be involved in the instablity?
Is my solution (which is empirical) a good one?
Is there a better solution if any?

I know this is lot of questions.

Not really, it is simple.
Th inductance is involving an area. The connecting
wires are part of the enclosed area.


I don't quite understand as the setup is the same from one reading to the
next. Even with few uH coils, it is stable. Wiring should be less of a
problem with higher inductance values.
Shouldn't be...?

Yes. Have you looked at the waveform with an oscilloscope? Maybe the
distributed capacitance of that relatively high value is causing some
problems-- you might see high-frequency hash during parts of the
oscillator wavform, for example, so you could have an oscillator on
top of an oscillator.
Click to expand...
That's a good point. I'm gonna check this very carefully.
If this is the case, maybe the small RC I'm adding in series lowers the
oscillator gain.

J. Hunter

Best regards,
Spehro Pefhany
Click to expand...
 
On Mon, 23 May 2005 16:16:31 +0200, Rene Tschaggelar wrote:

J. Hunter wrote:

Hi !

I'm using the oscillator part of a 74HC4060 in a home made inductance
meter. One cap to ground at both input and output of the CMOS oscillator
gate. The inductance to be measured between the input and output in series
with a reference inductance of 10uH. A microcontroller reads the frequency
from one of the output of the HC4060, calculates the inductance value and
displays it on a LCD screen. It should read between 1uH (129Mhz) to about 1H

It seemed to work well until I found out I has a slight oscillator
instability when measuring a 4700uH coil.

The oscillator runs about 60Khz with that inductance. If I connect the coil,
it can oscillate at say 58Khz and stay at that frequency. If I disconnect
and reconnect the coil, it ma oscillate at a sligthly higher or lower rate
and stay at that new frequency. Weird!

I experimented a bit to correct that problem and found out that connecting a
small network in series with the indcutance to be measured (a 100 ohms in
parrallel with a 1uF, values not critical) stabilizes the reading.
Smaller inductance values are unaffected by this network. Higher inductance
values, I don't know yet...

My questions are:

What could be involved in the instablity?
Is my solution (which is empirical) a good one?
Is there a better solution if any?

I know this is lot of questions.

Not really, it is simple.
Th inductance is involving an area. The connecting
wires are part of the enclosed area.
Click to expand...
Good Answer. I wanted to say "Parasitics," and I wanted to suggest hanging
a couple of feet (1/2-1 meter) of wire off one or the other end of the DUT
and wave your hands around, to see if what you actually have is a theremin.
:)

Good Luck!
Rich
 
On Mon, 23 May 2005 09:29:50 -0400, J. Hunter wrote:

Hi !

I'm using the oscillator part of a 74HC4060 in a home made inductance
meter. One cap to ground at both input and output of the CMOS oscillator
gate. The inductance to be measured between the input and output in series
with a reference inductance of 10uH. A microcontroller reads the frequency
from one of the output of the HC4060, calculates the inductance value and
displays it on a LCD screen. It should read between 1uH (129Mhz) to about 1H

It seemed to work well until I found out I has a slight oscillator
instability when measuring a 4700uH coil.

The oscillator runs about 60Khz with that inductance. If I connect the coil,
it can oscillate at say 58Khz and stay at that frequency. If I disconnect
and reconnect the coil, it ma oscillate at a sligthly higher or lower rate
and stay at that new frequency. Weird!

I experimented a bit to correct that problem and found out that connecting a
small network in series with the indcutance to be measured (a 100 ohms in
parrallel with a 1uF, values not critical) stabilizes the reading.
Smaller inductance values are unaffected by this network. Higher inductance
values, I don't know yet...

My questions are:

What could be involved in the instablity?
Is my solution (which is empirical) a good one?
Is there a better solution if any?
Click to expand...
If it works, leave it. There's nothing wrong with empirical design. The
PHDs want to wrap higher-order equations around everything, but if it
works, it works. :)

Especially if it's repatable, and especially especially if you're getting
the "right" reading while poking around the circuit with your fingers. :)

Cheers!
Rich
 
"J. Hunter" <j2hunter@yahoo.com> wrote in message
news:cEmke.24531$3R6.1089303@weber.videotron.net...

[clip for brevity]

The way I read it, you have the inductor(s) wired in the same position as
the oscillator crystal would normally sit.
I was initially going to reply that it could not possibly oscillate at low
frequencies due to loss of phase shift in the fixed caps. Then realised I
was assuming a cap in // with the inductor. Then realised there wasn't one.
Then realised series LC resonance would be needed anyway . Then realised
stray C around the L's would give a number of additional oscillation modes.
Then figured the inverter output impedance and input limiting diodes will
also give trouble.
Then gave up and simmed it :).
Surprisingly it can work all the way upto those 1H inductors but you now
have an oscillator that readily insists on oscillating but for all manner
of awkward reasons. AC voltage levels vary all over the place as the
frequency varies and as the other guys mention, stray capacitance can have a
sizeable frequency effect even with the big inductors (the circuit will
"squegg").
The circuit functions but not really well enough as a measuring meter.
It's far better to resonate the Ls against a known, fixed, decent sized, C
value. Use the arrangement in the original AADE meter and it's clones
(LM317?) and -then- feed into the counter chip for subsequent counting. Even
then, there will be problems trying to get 'low Q' coils to resonate.
A final solution needs more complex oscillator electronics, resulting in an
even bigger headache than wading through those resonant frequency sums
inside the micro :)
regards
john
 
In message <pVkke.42815$zo5.968920@wagner.videotron.net>, J. Hunter
<j2hunter@yahoo.com> writes
Hi !

I'm using the oscillator part of a 74HC4060 in a home made inductance
meter. One cap to ground at both input and output of the CMOS oscillator
gate. The inductance to be measured between the input and output in series
with a reference inductance of 10uH. A microcontroller reads the frequency
from one of the output of the HC4060, calculates the inductance value and
displays it on a LCD screen. It should read between 1uH (129Mhz) to about 1H

It seemed to work well until I found out I has a slight oscillator
instability when measuring a 4700uH coil.

The oscillator runs about 60Khz with that inductance. If I connect the coil,
it can oscillate at say 58Khz and stay at that frequency. If I disconnect
and reconnect the coil, it ma oscillate at a sligthly higher or lower rate
and stay at that new frequency. Weird!

I experimented a bit to correct that problem and found out that connecting a
small network in series with the indcutance to be measured (a 100 ohms in
parrallel with a 1uF, values not critical) stabilizes the reading.
Smaller inductance values are unaffected by this network. Higher inductance
values, I don't know yet...

My questions are:

What could be involved in the instablity?
Is my solution (which is empirical) a good one?
Is there a better solution if any?

I know this is lot of questions.

J. Hunter


Your system has more than one frequency which meets the zero phase+ loop
Click to expand...
gain ^1 criteria.
The oscillator has a varying gain with amplitude as it goes into
limiting so once its going it will lock also oscillation builds per
cycle so that the higher frequency if the gains the same will build
first.
All this means reduce capacitive coupling due to the self capacity on
the inductance best done by reducing the termination resistance both
sides on the inductance, this probably means a better oscillator.
Finally You want most or nearly all the oscillation current through the
inductance so it could usefully be placed in one arm of an AC bridge
with a compensating capacitor and resistor in the other arm and set this
into the oscillator circuit.



--
dd
 
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