LTSpice transformer mystery

[M. Hamed]

Any idea why the circuit below (also at the posted link) behaves like this?

The load impedance at the secondary is 100K. At resonance I expect this to get reflected to the primary as 1K. Now with 50 Ohms of series resistance, the voltage at the primary should be pretty close to 1 V. However SPICE shows about 700mV. Any idea why this happens?

https://www.dropbox.com/s/t6vt868q3xwl8c7/Transformer%20with%20a%20Cap%20in%20the%20Secondary.asc

Version 4
SHEET 1 880 680
WIRE 96 64 -176 64
WIRE 208 64 176 64
WIRE 384 64 272 64
WIRE 528 64 384 64
WIRE 528 96 528 64
WIRE -176 112 -176 64
WIRE 208 112 208 64
WIRE 272 112 272 64
WIRE 384 128 384 64
FLAG -176 192 0
FLAG 208 192 0
FLAG 272 192 0
FLAG 384 192 0
FLAG 528 176 0
SYMBOL cap 368 128 R0
SYMATTR InstName C
SYMATTR Value 2.5n
SYMBOL ind2 256 96 R0
SYMATTR InstName L1
SYMATTR Value 100n
SYMATTR Type ind
SYMBOL ind2 192 96 R0
SYMATTR InstName L2
SYMATTR Value 1n
SYMATTR Type ind
SYMBOL voltage -176 96 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V1
SYMATTR Value SINE(0 1 10000k)
SYMBOL res 80 80 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName R1
SYMATTR Value 50
SYMBOL res 512 80 R0
SYMATTR InstName R2
SYMATTR Value 100k
TEXT 176 272 Left 2 !K1 L1 L2 1
TEXT 14 292 Left 2 !.tran 0 40u 38u

 

[M. Hamed]

Actually 10 MHz is not the real resonance frequency. When I used the actual resonance frequency, the voltage at the primary drops to 70mV! I was guessing it's the inductance series resistance but I set those to 0 and still get the same result.

 

[Tim Wescott]

On Fri, 09 Jan 2015 22:40:39 -0800, M. Hamed wrote:

Actually 10 MHz is not the real resonance frequency. When I used the
actual resonance frequency, the voltage at the primary drops to 70mV! I
was guessing it's the inductance series resistance but I set those to 0
and still get the same result.

The primary side will "see" an inductance in series with a cap, or an
inductance in series with a parallel combination of cap and inductor.

Google around for the equivalent circuit of a transformer -- a
transformer with both coils grounded looks like a pi arrangement of three
coils, with the coupling coil's value determined by the mutual
inductance. The details escape me, but the equivalent circuit should
make things more clear.

--
www.wescottdesign.com

 

[John Larkin]

On Fri, 9 Jan 2015 22:22:47 -0800 (PST), "M. Hamed"
<mhdpublic@gmail.com> wrote:

Any idea why the circuit below (also at the posted link) behaves like this?

The load impedance at the secondary is 100K. At resonance I expect this to get reflected to the primary as 1K. Now with 50 Ohms of series resistance, the voltage at the primary should be pretty close to 1 V. However SPICE shows about 700mV. Any idea why this happens?

https://www.dropbox.com/s/t6vt868q3xwl8c7/Transformer%20with%20a%20Cap%20in%20the%20Secondary.asc

There are a couple of things going on here:

1. The default Spice time step is wrong, whatever it is. LT Spice goes
for IC analysis speed, and does a bad job on resonant circuits. Force
the time step down, 1 ns or less.

2. This resonant circuit has a high Q, like 15K or so. You'll need to
sim it for a long time until it settles. Combine that with a small
time step, and things get slow.

Better to an an AC analysis.

I'm playing with a coaxial ceramic resonator Colpitts oscillator in LT
Spice, and I am skeptical about the analysis. It does things that
shouldn't be possible.


--

John Larkin Highland Technology, Inc
picosecond timing laser drivers and controllers

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

 

[M. Hamed]

On Saturday, January 10, 2015 at 11:37:51 AM UTC-7, John Larkin wrote:

On Fri, 9 Jan 2015 22:22:47 -0800 (PST), "M. Hamed" wrote:

Any idea why the circuit below (also at the posted link) behaves like this?

The load impedance at the secondary is 100K. At resonance I expect this to get reflected to the primary as 1K. Now with 50 Ohms of series resistance, the voltage at the primary should be pretty close to 1 V. However SPICE shows about 700mV. Any idea why this happens?

https://www.dropbox.com/s/t6vt868q3xwl8c7/Transformer%20with%20a%20Cap%20in%20the%20Secondary.asc

There are a couple of things going on here:

1. The default Spice time step is wrong, whatever it is. LT Spice goes
for IC analysis speed, and does a bad job on resonant circuits. Force
the time step down, 1 ns or less.

2. This resonant circuit has a high Q, like 15K or so. You'll need to
sim it for a long time until it settles. Combine that with a small
time step, and things get slow.

Better to an an AC analysis.

I'm playing with a coaxial ceramic resonator Colpitts oscillator in LT
Spice, and I am skeptical about the analysis. It does things that
shouldn't be possible.

Yes, I think there is some kind of bug in SPICE. As evidenced by the fact that I would get some result, then close the waveform window, and resimulate and I get a different result.

I also think because of the high Q, any minor deviation from the resonance frequency causes extreme attenuation of the output. Problem is that the resonance frequency isn't what would be calculated from L and C. It seems a bit different.

Finally, AC analysis seems to show the expected result, but at a third resonance frequency.

 

Looks like you're talking how much the mutual inductance affects two tuned circuits that are tuned to different frequencies. Or am I wrong ?

If i am right, seems like that might be tough on a simulator.

[now see in the old days we didn'tt use no ocnfouded simulators, we just blew shit up until it worked - then it was "years in development" - a selling point]

 

[John Larkin]

On Sat, 10 Jan 2015 16:47:30 -0800 (PST), "M. Hamed"
<mhdpublic@gmail.com> wrote:

On Saturday, January 10, 2015 at 11:37:51 AM UTC-7, John Larkin wrote:
On Fri, 9 Jan 2015 22:22:47 -0800 (PST), "M. Hamed" wrote:

Any idea why the circuit below (also at the posted link) behaves like this?

The load impedance at the secondary is 100K. At resonance I expect this to get reflected to the primary as 1K. Now with 50 Ohms of series resistance, the voltage at the primary should be pretty close to 1 V. However SPICE shows about 700mV. Any idea why this happens?

https://www.dropbox.com/s/t6vt868q3xwl8c7/Transformer%20with%20a%20Cap%20in%20the%20Secondary.asc

There are a couple of things going on here:

1. The default Spice time step is wrong, whatever it is. LT Spice goes
for IC analysis speed, and does a bad job on resonant circuits. Force
the time step down, 1 ns or less.

2. This resonant circuit has a high Q, like 15K or so. You'll need to
sim it for a long time until it settles. Combine that with a small
time step, and things get slow.

Better to an an AC analysis.

I'm playing with a coaxial ceramic resonator Colpitts oscillator in LT
Spice, and I am skeptical about the analysis. It does things that
shouldn't be possible.



Yes, I think there is some kind of bug in SPICE. As evidenced by the fact that I would get some result, then close the waveform window, and resimulate and I get a different result.

Well, all simulators have bugs.

I also think because of the high Q, any minor deviation from the resonance frequency causes extreme attenuation of the output. Problem is that the resonance frequency isn't what would be calculated from L and C. It seems a bit different.

Again, the transient sim time step will affect the apparent resonant
frequency.

Finally, AC analysis seems to show the expected result, but at a third resonance frequency.

That's probably the right one! Use a lot of frequency steps, over a
narrow sweep range, to nail the frequency. (Or use a calculator.)


--

John Larkin Highland Technology, Inc
picosecond timing laser drivers and controllers

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

 

[John Larkin]

On Sat, 10 Jan 2015 21:42:12 -0800 (PST), jurb6006@gmail.com wrote:

>Looks like you're talking how much the mutual inductance affects two tuned circuits that are tuned to different frequencies. Or am I wrong ?

M=1 here, and there's one cap, so there's only one resonance.


--

John Larkin Highland Technology, Inc
picosecond timing laser drivers and controllers

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

 

[M. Hamed]

On Saturday, January 10, 2015 at 10:42:17 PM UTC-7, jurb...@gmail.com wrote:

Looks like you're talking how much the mutual inductance affects two tuned circuits that are tuned to different frequencies. Or am I wrong ?

If i am right, seems like that might be tough on a simulator.

[now see in the old days we didn'tt use no ocnfouded simulators, we just blew shit up until it worked - then it was "years in development" - a selling point]

I think my main Gripe with the simulator is that results have changed just by closing and reopening the waveform window. On the other hand, I now realize the Q may have been so high that what seemed to be minor deviations from the resonance frequency may have had extreme effects.

 

[M. Hamed]

So I decided to try a similar circuit live. I wound 10 turns of #18 enamel on a ferrite donut, and another 10 turns on top of them. Inductance L came to about 48uH. I chose a 600pF to resonate around 1 MHz. Turn ration here should be 1 since both primary and secondary are 10 turns. I connected 2.2K at the load and 2.2K as the source resistance. I applied about 4V pk-pk and was expecting 2V pk-pk (at resonance since resonance will cancel the winding inductance).

Instead, I got about 1V pk-pk which meant the reflected impedance wasn't the 2.2K I'm expecting (ZL/n^2), it was closer to 1K!

After a few agonizing hours, a lot of head scratching, and numerous LTSpice simulation + Scilab calculations, I finally figured it out...

 

[Maynard A. Philbrook Jr.]

In article <be80d9d8-e07f-44da-89a8-26eed583102d@googlegroups.com>,
jurb6006@gmail.com says...

Looks like you're talking how much the mutual inductance affects two tuned circuits that are tuned to different frequencies. Or am I wrong ?

If i am right, seems like that might be tough on a simulator.

[now see in the old days we didn'tt use no ocnfouded simulators, we just blew shit up until it worked - then it was "years in development" - a selling point]

Yes, they call that doing real bench work, using real equipment and
maybe a cak-culator or some sort, abacus, fingers, slide rule, what
ever..

Jamie

 

[Maynard A. Philbrook Jr.]

In article <e6360124-27ad-4ef8-afe5-c2f15b9b68d7@googlegroups.com>,
mhdpublic@gmail.com says...

On Saturday, January 10, 2015 at 10:42:17 PM UTC-7, jurb...@gmail.com wrote:
Looks like you're talking how much the mutual inductance affects two tuned circuits that are tuned to different frequencies. Or am I wrong ?

If i am right, seems like that might be tough on a simulator.

[now see in the old days we didn'tt use no ocnfouded simulators, we just blew shit up until it worked - then it was "years in development" - a selling point]

I think my main Gripe with the simulator is that results have changed just by closing and reopening the waveform window. On the other hand, I now realize the Q may have been so high that what seemed to be minor deviations from the resonance frequency may have had extreme effects.

I didn't look at your sim however, the default model assumes the ratio
of L to be the ratio of turns.. You may want to fiddle with that one.

There is another model you can use that allows you to specify closer
to your real world circuit.

Open the Help and look at the "L" and you'll see the other model.

Jamie

 

[John Larkin]

On Mon, 12 Jan 2015 12:54:54 -0800 (PST), "M. Hamed"
<mhdpublic@gmail.com> wrote:

On Saturday, January 10, 2015 at 10:42:17 PM UTC-7, jurb...@gmail.com wrote:
Looks like you're talking how much the mutual inductance affects two tuned circuits that are tuned to different frequencies. Or am I wrong ?

If i am right, seems like that might be tough on a simulator.

[now see in the old days we didn'tt use no ocnfouded simulators, we just blew shit up until it worked - then it was "years in development" - a selling point]

I think my main Gripe with the simulator is that results have changed just by closing and reopening the waveform window. On the other hand, I now realize the Q may have been so high that what seemed to be minor deviations from the resonance frequency may have had extreme effects.

Try this:

https://dl.dropboxusercontent.com/u/53724080/Circuits/Filters/LC1.asc

Run it, probe the LC, and zoom the pulse tops. There is AM modulation,
entirely a Spice artifact.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

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

 

[M. Hamed]

On Monday, January 12, 2015 at 3:40:54 PM UTC-7, John Larkin wrote:

Try this:

https://dl.dropboxusercontent.com/u/53724080/Circuits/Filters/LC1.asc

Run it, probe the LC, and zoom the pulse tops. There is AM modulation,
entirely a Spice artifact.

I see about 4 mV variance.

 

[John Larkin]

On Tue, 13 Jan 2015 08:32:24 -0800 (PST), "M. Hamed"
<mhdpublic@gmail.com> wrote:

On Monday, January 12, 2015 at 3:40:54 PM UTC-7, John Larkin wrote:

Try this:

https://dl.dropboxusercontent.com/u/53724080/Circuits/Filters/LC1.asc

Run it, probe the LC, and zoom the pulse tops. There is AM modulation,
entirely a Spice artifact.


I see about 4 mV variance.

Try the Gear algorithm!

The frequency is off by roughly 1%, with any of the integration
methods, using the default (automatic) time step. Speed kills!


--

John Larkin Highland Technology, Inc
picosecond timing laser drivers and controllers

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

 

[John Larkin]

On Mon, 12 Jan 2015 17:14:40 -0500, "Maynard A. Philbrook Jr."
<jamie_ka1lpa@charter.net> wrote:

In article <be80d9d8-e07f-44da-89a8-26eed583102d@googlegroups.com>,
jurb6006@gmail.com says...

Looks like you're talking how much the mutual inductance affects two tuned circuits that are tuned to different frequencies. Or am I wrong ?

If i am right, seems like that might be tough on a simulator.

[now see in the old days we didn'tt use no ocnfouded simulators, we just blew shit up until it worked - then it was "years in development" - a selling point]

Yes, they call that doing real bench work, using real equipment and
maybe a cak-culator or some sort, abacus, fingers, slide rule, what
ever..

Jamie

One of my guys, a lot younger than me, still uses a slide rule now and
then.

I sure don't miss slide rules!


--

John Larkin Highland Technology, Inc
picosecond timing laser drivers and controllers

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