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Re: Determine phase and gain margin in filter/amp

monettsys
 

--- In LTspice@..., <frank_wiedmann@...> wrote:

If you want to measure phase margin or gain margin with a .measure command, you can take a look at Measure_Margins.asc from . I recommend that you download the archive LoopGain_Probe.zip so that you have all the required files.

Best regards,

Frank
Thanks, Frank,

The Tian method uses equations that are far too long and error-prone. I would never be able to find a typo. Also, it requires an additional subcircuit that has to be included in each circuit, plus changes to the Control Panel defaults. These all take time to incorporate and to troubleshoot when something goes wrong.

I much prefer the method shown by Kris Lokere in "LTspice IV: Stability of Op Amp Circuits". It is much simpler.



I will translate your .measure statements to Lokere's demo and see if it gives the same results.

Thanks,

Mike
Re: Singleton amplifier loop gain simulation(Stability margins)

 

ok.everything solved.thank you all for your help.
Re: XVAR specification

 




---In ltspice@..., <rsrakeshbharath@...> wrote:

... I try to use the variable resistor(XVAR) from LTSPICE library.

What kind of resistor?
You used a varistor, is that you intended?
Look for the example
Schematic
in our group library.

hws
Re: How to create a spice model using only the datasheet

 

thx a lot
Re: Determine phase and gain margin in filter/amp

 

---In LTspice@..., <405a82e5@...> wrote:

Rick, How did you manage to get three digits of resolution? I tried using the cursor but could not locate 0dB or measure the phase angle that accurately. I assume you used the .measure command - could you post it for those of us who are interested?
If you want to measure phase margin or gain margin with a .measure command, you can take a look at Measure_Margins.asc from??. I recommend that you download the archive LoopGain_Probe.zip so that you have all the required files.

Best regards,

Frank
XVAR specification

 

Dear all,


I am designing a circuit in LTSPICE. I need a variable resistor to use it in my circuit. I try to use the variable resistor(XVAR) from LTSPICE library. After designing the circuit after using this variable resistor (XVAR) from LTspice library; I am unable to run the simulation. I am always getting this error-"Unknown sub-circuit called in xu8 n003 n008 50k". My variable resistance value is 50Kohm. Can anyone help me to solve this issue and let me know the following:

1.) What component has to be used for simulating variable resistor in the LTspice library?
2.) How to set the value of variable resistance for XVAR component?

Thanks & Regards,
M.Rakesh Sharma
Re: FFT Resolution

 

ronw6wo?wrote:

To gain some FFT experience? I checked? the bandwidth of individual harmonics of a 14MHz square wave? They show as having a bandwidth of 1.5 MHz which is ridiculous I have tried many simulation settings but they all produce the same results?? What Pulse and tran settings should I be using ?

I don't use FFTs regularly, but some things you should be doing include:

Disable waveform compression (.option plotwinsize=0).

The simulation must run for an exact integer multiple of cycles, and turn off Windowing in the FFT. ?Either of those can make the components wider.

More data points at finer time increments is usually better. ?Specify a Maximum Timestep in the .TRAN command.

Many circuits start with a burp, which you might not always notice. ?Remember, a transient simulation begins with the variable sources turned off, and then suddenly they are turned on. ?That can cause a bias shift if signals are AC-coupled anywhere ... which translates into a widening of the FFT components. ?To avoid that when it happens, don't start saving data until after the bias has settled or the transient has died out ("Time to Start Saving Data" in the .TRAN command). ?Remember to adjust the stop time accordingly so you still have an integral number of cycles saved.

Also, don't forget, you can't make perfect "square" waves. ?The Pulse waveform rise and fall times should not be zero.

Andy
Re: FFT Resolution

 

Hi
If you upload the your schematics, some will give you a comment, I think.
?
Shiggy
?


On Tue, Dec 10, 2013 at 1:56 PM, <ronw6wo@...> wrote:
?

To gain some FFT experience? I checked? the bandwidth of individual harmonics of a 14MHz square wave? They show as having a bandwidth of 1.5 MHz which is ridiculous I have tried many simulation settings but they all produce the same results?? What Pulse and tran settings should I be using ?
FFT Resolution

 

To gain some FFT experience? I checked? the bandwidth of individual harmonics of a 14MHz square wave? They show as having a bandwidth of 1.5 MHz which is ridiculous I have tried many simulation settings but they all produce the same results?? What Pulse and tran settings should I be using ?
Re: How to create a spice model using only the datasheet

 

Hello,

The diode model is on Vishay's web page.

.MODEL ETX06 D (IS=1.8807P N=2.5 BV=680 IBV=100U RS=34.2934M TT=26.4698N
+ CJO=175.851P VJ=700M M=459.591M EG=2.5 XTI=2 RL=25.4602MEG))
There is no model as usual for the capacitor.
You could model it with its capacitance + some nano henry inductance (Lser 20nH?).

Best regards,
Helmut
Re: Determine phase and gain margin in filter/amp

 

Mike,

I enlarged the plot at the point where the gain crossed 0dB and used a cursor.

Rick
Re: How to create a spice model using only the datasheet

John Woodgate
 

In message <l84qps+nq6dbm@...>, dated Mon, 9 Dec 2013, aymen.dardouri1990@... writes:


i hope that s.b does have the models, even if i think its a bit hard.

in fact i only need 2 models:

-diode: VISHAY VS-8ETX06FPPBF

-capacitor: VISHAY MKP1840422104M
They may not be in the archives, but models of diodes and capacitors can be made from data sheets plus a little more information that Vishay may well supply if you ask. I can't help with the diode but there are people here on the list who can.

For the capacitor, you can model it as C L R in series. You get the R value from the tan(delta) data. If it varies a lot with frequency, try changing to a parallel resistor (across the C-L combination). If that R vale doesn't vary a lot with frequency, you probably don't need a model with both series and parallel resistors.

For the L value, try to get an impedance/frequency curve from Vishay. The resonance frequency (minimum impedance) gives you the L value. If you can't get that information, use 1 nH per millimetre of distance between the wires. Or perhaps you can measure the resonance frequency of a sample capacitor.

--
OOO - Own Opinions Only. With best wishes. See www.jmwa.demon.co.uk
Nondum ex silvis sumus
John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
Re: How to create a spice model using only the datasheet

 

i hope that s.b does have the models, even if i think its a bit hard.

in fact i only need 2 models:

-diode: VISHAY VS-8ETX06FPPBF

-capacitor: VISHAY MKP1840422104M
Re: Specifying multiple grounds in LTspice

 

I use a self-made symbol, a small square with a zero-length pin in the middle, which simply netlists as a resistor of value {1/Gmin} to ground.
...Jim Thompson

Web Site: <>
LDO LTC1844

[email protected]
 

Hi, I'm trying to do some simulations with the LTC1844 and was surprised it's not included in the Linear devices included in my LTSpice. I did a search in "all_files.htm" that I down loaded and I can't find any mention of it there. If anyone can give me a suggestion on where to get a model I can use in LTSpice, I'd appreciate it. Regards, Rob
Re: How to create a spice model using only the datasheet

 

aymen.dardouri1990?wrote:

i've been told the is a possibility to creat new spice models using only the datasheets of the components

For the very simplest components (individual transistors, diodes), I think it is possible. ?I don't have the skill to do that. ?I think there are software programs that can help with that.

For anything more complicated than a transistor, such as an IC, it takes much more than a datasheet. ?You need to know what's inside the IC, what the basic circuit is and does. ?Once you have a circuit for it, you then might use the datasheet to refine and tweak the electrical characteristics.

Either way, it is much better to start with a real model, than to make one from a datasheet.

Andy
Re: How to create a spice model using only the datasheet

John Woodgate
 

In message <l84jdn+uq5msk@...>, dated Mon, 9 Dec 2013, aymen.dardouri1990@... writes:

I'm trying to simulate a new project using very specific components, but i had a huge problem in finding the right spice models,
i've loooked everywhere but in vain, i've been told the is a possibility to creat new spice models using only the datasheets of the components,
can someone please help me with that? i'll be gratful
It can be done in some cases, but it's not easy in most cases and near impossible in others. You would do better to ask here about models for your devices. There are many models in various files and folders in the group archives.
--
OOO - Own Opinions Only. With best wishes. See www.jmwa.demon.co.uk
Nondum ex silvis sumus
John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
How to create a spice model using only the datasheet

 

Hallo,


I'm trying to simulate a new project using very specific components, but i had a huge problem in finding the right spice models,
i've loooked everywhere but in vain, i've been told the is a possibility to creat new spice models using only the datasheets of the components,
can someone please help me with that? i'll be gratful.

thx
Re: Specifiying multiple grounds in LTspice

 

¿ªÔÆÌåÓý


Le 09/12/2013 00:12, Richard Sawrey a ¨¦crit?:
?
Jerry,

This is not a SPICE or LTspice limitation.?
I understand that.
Yet it's a limitation.

If you had to solve a circuit with a bunch of nodes isolated from ground and wanted to solve the voltages at all the nodes, you couldn't do it, because the?voltages at the floating nodes are undefined.? LTspice can't do it either.
I'm not questioning the validity of the explanation. I was just attempting to explain why why some users don't like that.
If I'm not mistaken, some other sim packages use "meters" that one can connect to a floating branch. I guess the software knows then that it has to put a new reference, distinct from the ground, just like one would do when faced with non-galvanically-connected circuits and applying Ohm/Kirchoff/Thevenin separately.

Rick
?
On Sunday, December 8, 2013 10:12 AM, Jerry Lee Marcel wrote:
?

Le 08/12/2013 14:07, Andy a ¨¦crit?:
?


The only LTspice requirement, is that every circuit node must have a DC path to GND ...

Some people don't seem to like that, if they have a circuit with an isolation transformer that keeps a part of their circuit actually isolated from the GND that appears elsewhere in their circuit.?
I believe the reason is when a transformer is properly modelled with its stray capacitances, the results will be different in function of the way the floating part is made non-floating.
Particularly there will be a big difference with one leg of the output directly connected to ground compared to having both legs connected via 1Meg resistors.

This limitation of LTspice is not a problem for me, since it is part of my routine to evaluate transformers with all sorts of connections, unbalaned in particular.
I must admit this limitation may be a hindrance for users who are not comfortable with transformers intricacies.



Re: Determine phase and gain margin in filter/amp

monettsys
 

--- In LTspice@..., <sawreyrw@...> wrote:

Maarten,


You set up the circuit correctly, but did you determine the phase margin by plotting V(fb)/V(inm)? Doing this I see a phase margin of 72.8 degrees at 9.16 MHz. In think the circuit is ok, but you should be aware that the output impedance of U2 has a significant effect. You can see that V(ref) increases significantly as the frequency goes up.


Rick
Rick, How did you manage to get three digits of resolution? I tried using the cursor but could not locate 0dB or measure the phase angle that accurately. I assume you used the .measure command - could you post it for those of us who are interested?

Thanks,

Mike
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