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 Posted: Oct 09, 2010 - 08:54 PM |
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Joined: Apr 25, 2004
Posts: 3809
Location: Denmark
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Posted: Oct 10, 2010 - 12:24 AM |
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Joined: Nov 22, 2002
Posts: 12045
Location: Tangent, OR, USA
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The outstanding presentation that Mike Englehardt, the author of LTspice, gives on road shows is here:
http://LTspice.linear.com/software/LTsp ... ur2009.zip
Great explanations and shows how easy it is to use.
Jim |
_________________
Jim Wagner
Oregon Research Electronics, Consulting Div.
Tangent, OR, USA
"The only thing standing between us and victory is defeat" P.G.Wodhouse in Wooster & Jeeves series
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Posted: Oct 10, 2010 - 08:22 AM |
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Joined: Nov 29, 2007
Posts: 3219
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Posted: Oct 12, 2010 - 04:24 PM |
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Joined: Apr 25, 2004
Posts: 3809
Location: Denmark
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Thanx for those links 
/Bingo |
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Posted: Oct 28, 2010 - 05:53 PM |
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Joined: Dec 11, 2007
Posts: 6848
Location: Cleveland, OH
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Anyone have time to answer a dumb LT Spice beginner's question?
I've downloaded it to check it out, after having heard and read about it for a long time.
I drew up a simple RC LPF and put a voltage source, AC, 1 V, on the input. R = 151 K, C = 0.15 uF. 1/(2PiRC) gives about 7 Hz Fc by MY calculations.
If I click on the Voltage label the pop up box says:
BW 23.4 Hz, Power BW 10.44 Hz.
When I blow up the image, and put a cursor on it, the 3dB point is 7 Hz, as I expected.
What, then are the BW and Power BW calculations actually calculating? I looked through the LTSpice User's Guide and the LTspiceGettingStartedGuide but must have missed the definitions.
Thanks
JC |
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Posted: Oct 28, 2010 - 07:28 PM |
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Joined: Nov 22, 2002
Posts: 12045
Location: Tangent, OR, USA
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Not sure. There is a good Yahoo users group which I think is referenced in the Help files.
Jim |
_________________
Jim Wagner
Oregon Research Electronics, Consulting Div.
Tangent, OR, USA
"The only thing standing between us and victory is defeat" P.G.Wodhouse in Wooster & Jeeves series
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Posted: Oct 28, 2010 - 08:15 PM |
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Joined: Jan 09, 2007
Posts: 1868
Location: Arlington, Texas, U.S.A.
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DocJC wrote:
Anyone have time to answer a dumb LT Spice beginner's question?
Yes and not dumb 
DocJC wrote:
What, then are the BW and Power BW calculations actually calculating? I looked through the LTSpice User's Guide and the LTspiceGettingStartedGuide but must have missed the definitions.
Help -> Help Topics, Waveform Viewer -> Waveform Arithmetic, "2. Compute the average or RMS of a trace." |
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Posted: Oct 28, 2010 - 08:43 PM |
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Joined: Dec 11, 2007
Posts: 6848
Location: Cleveland, OH
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I appreciate the responses!
I'm not sure the Help->... really answers the question, however. I see that it defines the foundation for user generated expressions to be calculated/plotted, but I do not see that it defines the two terms that LTSpice intrinsically provides: BW and Power BW.
I don't see that either term is defined in the Waveform Arithmetic list.
The BW and Power BW pop up when one is viewing a Bode Plot and click on the label at the top of the plot.
Unfortunately, the Help index doesn't include Bode, Bandwidth, Power Bandwidth, Filter, Spectrum, etc...
JC |
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Posted: Oct 28, 2010 - 09:16 PM |
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Joined: Nov 29, 2007
Posts: 3219
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| I don't get that popup and have never seen it. When clicking on a label in the waveform viewer I get the cursor window - like it is described in the help. |
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Stealing Proteus doesn't make you an engineer.
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Posted: Oct 28, 2010 - 09:27 PM |
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Joined: Aug 04, 2002
Posts: 1704
Location: Brisbane, Australia
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| Post your schematic ( or attach the file ) |
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Posted: Oct 28, 2010 - 10:58 PM |
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Joined: Dec 11, 2007
Posts: 6848
Location: Cleveland, OH
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A screen shot is attached.
AC 1 V source into an RC Low Pass Filter.
R = 151K, C = 0.15uF
I put the white cross hair cursor on the -3dB point and the Freq is 7 Hz, (as I expected).
Left Control Click on the small green text label at the top center of the Bode Plot and the small box to the left of the schematic pops up. That is where it says BW = 55.7 Hz, Power BW = 11.02 Hz.
Sorry, I left the the Left Control out of the click instructions above.
JC |
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Posted: Oct 28, 2010 - 11:48 PM |
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Joined: Dec 11, 2007
Posts: 6848
Location: Cleveland, OH
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If you expand the image within your web browser it is readable...
I eMailed Linear Tech, also, and received a very prompt reply:
Quote:
Power BW should be self explanitory. BW is the
information bandwidth.
3dB points are not related to integration techniques
so there is no reason for the results to be the same.
Use 3dB points if that is what you are comfortable
with and don't use the integration techniques.
Regards,
I guess if it was "self explanitory" I would not have asked the question...
And I asked for a reference within their manual or literature, or the explicit equation(s) used to generate the provided BW & Power BW, neither of which were provided.
Clearly someone things these specific numbers are useful, or they would not be included.
I'm left not quite sure what to think...
JC |
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Posted: Oct 29, 2010 - 12:25 AM |
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Joined: Dec 11, 2007
Posts: 6848
Location: Cleveland, OH
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| OK, /Rant, time to move on... |
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Posted: Oct 29, 2010 - 01:11 AM |
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Joined: Nov 29, 2007
Posts: 3219
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The answer is typical for Linear (may I guess, M.E.?) "Don't use it, if you don't get it" ...
The value is indeed the same one as LTSpice gives you with a .MEAS integral calculation over the visible (selected) waveform frequency range
Code:
.MEAS AC integ INTEG V(n002)
But the resulting value should have the Unit V * Hz = V / s (integral of a voltage function along a frequency). However, LTSpice give the unit as Hz.
I have never seen anything with the unit V / s called an (information) bandwith. Stuff like slew rate has V / s.
And I don't remember having ever heard about that value or its significance in judging or describing a filter or a quadrupole in general. |
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Stealing Proteus doesn't make you an engineer.
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Posted: Oct 29, 2010 - 02:19 AM |
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Joined: Mar 24, 2006
Posts: 191
Location: Canada
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I imagine the bandwidth shown in the integration result is the equivalent bandwidth if the filter were a rectangular filter. This is useful for noise measurements
You have to simulate the AC response far beyond the cutoff frequency to get the full integration curve then when you <CTRL> R_CLICK the trace name you get a figure PI/2 time bigger than the 3dB bandwidth (for a first order filter).
I didn't think information bandwidth could be specified without knowing the noise level. Maybe that's what the reference level is.
Klave |
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Posted: Oct 29, 2010 - 02:30 AM |
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Joined: Dec 11, 2007
Posts: 6848
Location: Cleveland, OH
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Hi Arnold,
Thank you for the feedback!
Yes, the Tech Support responder was M.E. Looks like the quality of his responses has left a lasting impression on others besides myself, and left him with a lasting reputation!
I know that I am a "little rusty" ( ) when it comes to LaPlace Transforms and Bode Plots, etc., but I always thought the BW for a LPF was pretty well defined.
When one is using a program for the first time, and it gives "odd" results, you have to wonder if the well known program that many people are using is "wrong", or if you have failed to understand what it is doing, or both.
Both, I think, applies in this case.
That said, the reason I actually downloaded and used it was a current project has a bunch of filter stages, and LTSpice did truely make the frequency response analysis MUCH easier, once I learned to ignore it's BW report!
JC
Edit: Hi Klave, Thanks for the input. I will have to spend a little more time with the program to better understand it's capabilities and methodologies. |
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Posted: Oct 29, 2010 - 04:34 AM |
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Joined: Jan 23, 2010
Posts: 840
Location: Edmonton, Alberta
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V(s) = 1/(1+sRC)
I(s) = V * sC/(1+sRC)
P(s) = V/Z = sC/(1+sRC)^2
MATLAB magic:
Code:
EDU>> R=151e3; C=0.1e-6;
EDU>> V=tf(1,[R*C 1])
Transfer function:
1
------------
0.0151 s + 1
EDU>> P=tf([C 0],[R^2*C^2 2*R*C 1])
Transfer function:
1e-07 s
---------------------------
0.000228 s^2 + 0.0302 s + 1
EDU>> bode(V,'r',P,'b'); legend('V(s)','P(s)'); grid
Power bandwidth contains some high % of the power (I've heard 90% and 99%) while gain bandwidth (voltage) is what we're used to - BW between V/1.41 points. |
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Posted: Oct 29, 2010 - 06:45 PM |
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Joined: Dec 11, 2007
Posts: 6848
Location: Cleveland, OH
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TLucas,
Thanks for the above. Power Bandwidth wasn't a term I was familar with, and Wiki's definition has to do with audio amplifier performance...
Live and learn.
JC |
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