jens
16b6516d55
git-svn-id: http://moon:8086/svn/projects/ltspice@388 fda53097-d464-4ada-af97-ba876c37ca34
68 lines
2.9 KiB
Plaintext
68 lines
2.9 KiB
Plaintext
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Frequency Response Analysis
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These examples show to how to extract the small-signal, AC open-loop
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gain from time-domain, closed-loop simulations. The technique is a
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subset of the method shown in ../LoopGain.asc and ../LoopGain2.asc.
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The method assumes that the input impedance of the error amplifier is
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infinite so that the loop gain can be computed solely by the voltage
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loop gain.
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The open-loop gain is computed from the closed-loop system by inserting
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a perturbing voltage source in the loop and measuring how well the
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perturbation is servoed out of the the loop via the feedback. The
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open-loop gain is given by the ratio of complex voltages at either side
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of the perturbing voltage source.
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But since the SMPS macromodels are implemented as time-domain models
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that include detailed switching information but don't include
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continuous-time(average) equivalents, the complex voltage is determined
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from Fourier analysis of a time-domain sine wave perturbation.
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A script of .measure statements is used to perform the Fourier analysis
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and computer the open-loop response. To use this script into your own
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SMPS design, follow these steps:
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1. Place these .measure statements on your circuit as a SPICE
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directive:
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.measure Aavg avg V(a)
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.measure Bavg avg V(b)
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.measure Are avg (V(a)-Aavg)*cos(360*time*Freq)
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.measure Aim avg -(V(a)-Aavg)*sin(360*time*Freq)
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.measure Bre avg (V(b)-Bavg)*cos(360*time*Freq)
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.measure Bim avg -(V(b)-Bavg)*sin(360*time*Freq)
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.measure GainMag param 20*log10(hypot(Are,Aim) / hypot(Bre,Bim))
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.measure GainPhi param mod(atan2(Aim,Are)-atan2(Bim,Bre)+180,360)-180
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2. Insert a voltage source in the feedback loop under analysis. Give
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this voltage sourse the value SINE(0 5m {Freq})
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3. Place a SPICE directive on the schmatic that defines Freq:
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.param Freq=10K
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4. Run a .tran command to see how long it takes your circuit to come to
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steady state and then edit the .tran command so that data isn't saved
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until this time.
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5. Rerun the .tran command to do the analysis at the frequency defined
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with the .param statement of step 3.
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6. Execute menu command View=>SPICE Error Log to see the results of this
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analysis. The open-loop response magnitude is given by GainMag[dB]
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and the phase is given by GainPhi[°].
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You can iterate the param Freq to zero dB. That frequency is the loop
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crossover frequency and phase margin will be reported as GainPhi in the
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error log.
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You can set up a .step statement to sweep the parameter Freq. Then you
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can plot the open-loop response be executing menu command
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View=>SPICE Error Log, and then right mouse clicking and executing menu
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command "Plot .step'ed .meas data" Answer yes to the dialog that asks
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if LTspice should combine the real .meas data to complex data. Then you
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can plot the quanity gain to get a Bode plot the open-loop response of
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the system.
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--Mike |