how to use LTSpice values in Matlab

26 Dec 2021
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Updated 26 Dec 2021
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hi there,
i need to export the transfer function of a circuit from LTSpice to Matlab as a .txt file, and then i need to calculate some parameters using that transfer function.For example, assuming that H is my transfer function, i need to calculate G = 100*H/(1-H) using Matlab. How can i do it? I'm attaching the .txt file here.

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 Accepted Answer

Star Strider
Star Strider on 26 Dec 2021
Ran in:

1 vote

Ran in:
Try this —
filename = 'https://www.mathworks.com/matlabcentral/answers/uploaded_files/844665/Draft3.txt';
T1 = readtable(filename, 'VariableNamingRule','preserve')
T1 = 401×2 table
Freq. V(n002) ______ ______________________________________________________ 10 {'(-6.02060034191108e+000dB,-1.79999994078238e-002°)'} 10.233 {'(-6.02060036211185e+000dB,-1.84192732265253e-002°)'} 10.471 {'(-6.02060038326466e+000dB,-1.88483131850069e-002°)'} 10.715 {'(-6.02060040541437e+000dB,-1.92873467657410e-002°)'} 10.965 {'(-6.02060042860797e+000dB,-1.97366067499370e-002°)'} 11.22 {'(-6.02060045289464e+000dB,-2.01963313409642e-002°)'} 11.482 {'(-6.02060047832591e+000dB,-2.06667642906489e-002°)'} 11.749 {'(-6.02060050495572e+000dB,-2.11481550285142e-002°)'} 12.023 {'(-6.02060053284056e+000dB,-2.16407587940291e-002°)'} 12.303 {'(-6.02060056203956e+000dB,-2.21448367719392e-002°)'} 12.589 {'(-6.02060059261468e+000dB,-2.26606562307481e-002°)'} 12.882 {'(-6.02060062463075e+000dB,-2.31884906644261e-002°)'} 13.183 {'(-6.02060065815569e+000dB,-2.37286199374180e-002°)'} 13.49 {'(-6.02060069326061e+000dB,-2.42813304330296e-002°)'} 13.804 {'(-6.02060073001998e+000dB,-2.48469152052698e-002°)'} 14.125 {'(-6.02060076851176e+000dB,-2.54256741342303e-002°)'}
V2c = cellfun(@(x)sscanf(x, '(%fdB,%f°'), T1.('V(n002)'), 'Unif',0);
V2m = cell2mat(V2c')';
T2 = table('Size',[size(T1.('Freq.'),1) 3],'VariableTypes',{'double','double','double'}, 'VariableNames',{'FreqHz','MagndB','PhasDg'});
T2.FreqHz = T1.('Freq.');
T2.MagndB = V2m(:,1);
T2.PhasDg = V2m(:,2)
T2 = 401×3 table
FreqHz MagndB PhasDg ______ _______ _________ 10 -6.0206 -0.018 10.233 -6.0206 -0.018419 10.471 -6.0206 -0.018848 10.715 -6.0206 -0.019287 10.965 -6.0206 -0.019737 11.22 -6.0206 -0.020196 11.482 -6.0206 -0.020667 11.749 -6.0206 -0.021148 12.023 -6.0206 -0.021641 12.303 -6.0206 -0.022145 12.589 -6.0206 -0.022661 12.882 -6.0206 -0.023188 13.183 -6.0206 -0.023729 13.49 -6.0206 -0.024281 13.804 -6.0206 -0.024847 14.125 -6.0206 -0.025426
Mag = db2mag(T2.MagndB);
Phs = deg2rad(T2.PhasDg);
Rsp = Mag.*exp(1j*Phs)
Rsp =
0.5000 - 0.0002i 0.5000 - 0.0002i 0.5000 - 0.0002i 0.5000 - 0.0002i 0.5000 - 0.0002i 0.5000 - 0.0002i 0.5000 - 0.0002i 0.5000 - 0.0002i 0.5000 - 0.0002i 0.5000 - 0.0002i 0.5000 - 0.0002i 0.5000 - 0.0002i 0.5000 - 0.0002i 0.5000 - 0.0002i 0.5000 - 0.0002i 0.5000 - 0.0002i 0.5000 - 0.0002i 0.5000 - 0.0002i 0.5000 - 0.0002i 0.5000 - 0.0002i 0.5000 - 0.0002i 0.5000 - 0.0003i 0.5000 - 0.0003i 0.5000 - 0.0003i 0.5000 - 0.0003i 0.5000 - 0.0003i 0.5000 - 0.0003i 0.5000 - 0.0003i 0.5000 - 0.0003i 0.5000 - 0.0003i 0.5000 - 0.0003i 0.5000 - 0.0003i 0.5000 - 0.0003i 0.5000 - 0.0003i 0.5000 - 0.0003i 0.5000 - 0.0004i 0.5000 - 0.0004i 0.5000 - 0.0004i 0.5000 - 0.0004i 0.5000 - 0.0004i 0.5000 - 0.0004i 0.5000 - 0.0004i 0.5000 - 0.0004i 0.5000 - 0.0004i 0.5000 - 0.0004i 0.5000 - 0.0004i 0.5000 - 0.0005i 0.5000 - 0.0005i 0.5000 - 0.0005i 0.5000 - 0.0005i 0.5000 - 0.0005i 0.5000 - 0.0005i 0.5000 - 0.0005i 0.5000 - 0.0005i 0.5000 - 0.0005i 0.5000 - 0.0006i 0.5000 - 0.0006i 0.5000 - 0.0006i 0.5000 - 0.0006i 0.5000 - 0.0006i 0.5000 - 0.0006i 0.5000 - 0.0006i 0.5000 - 0.0007i 0.5000 - 0.0007i 0.5000 - 0.0007i 0.5000 - 0.0007i 0.5000 - 0.0007i 0.5000 - 0.0007i 0.5000 - 0.0008i 0.5000 - 0.0008i 0.5000 - 0.0008i 0.5000 - 0.0008i 0.5000 - 0.0008i 0.5000 - 0.0008i 0.5000 - 0.0009i 0.5000 - 0.0009i 0.5000 - 0.0009i 0.5000 - 0.0009i 0.5000 - 0.0009i 0.5000 - 0.0010i 0.5000 - 0.0010i 0.5000 - 0.0010i 0.5000 - 0.0010i 0.5000 - 0.0011i 0.5000 - 0.0011i 0.5000 - 0.0011i 0.5000 - 0.0011i 0.5000 - 0.0012i 0.5000 - 0.0012i 0.5000 - 0.0012i 0.5000 - 0.0012i 0.5000 - 0.0013i 0.5000 - 0.0013i 0.5000 - 0.0013i 0.5000 - 0.0014i 0.5000 - 0.0014i 0.5000 - 0.0014i 0.5000 - 0.0015i 0.5000 - 0.0015i 0.5000 - 0.0015i 0.5000 - 0.0016i 0.5000 - 0.0016i 0.5000 - 0.0016i 0.5000 - 0.0017i 0.5000 - 0.0017i 0.5000 - 0.0018i 0.5000 - 0.0018i 0.5000 - 0.0018i 0.5000 - 0.0019i 0.5000 - 0.0019i 0.5000 - 0.0020i 0.5000 - 0.0020i 0.5000 - 0.0021i 0.5000 - 0.0021i 0.5000 - 0.0022i 0.5000 - 0.0022i 0.5000 - 0.0023i 0.5000 - 0.0023i 0.5000 - 0.0024i 0.5000 - 0.0024i 0.5000 - 0.0025i 0.5000 - 0.0025i 0.5000 - 0.0026i 0.5000 - 0.0027i 0.5000 - 0.0027i 0.5000 - 0.0028i 0.5000 - 0.0029i 0.5000 - 0.0029i 0.5000 - 0.0030i 0.5000 - 0.0031i 0.5000 - 0.0031i 0.5000 - 0.0032i 0.5000 - 0.0033i 0.5000 - 0.0034i 0.5000 - 0.0034i 0.5000 - 0.0035i 0.5000 - 0.0036i 0.5000 - 0.0037i 0.5000 - 0.0038i 0.5000 - 0.0039i 0.5000 - 0.0039i 0.5000 - 0.0040i 0.5000 - 0.0041i 0.5000 - 0.0042i 0.5000 - 0.0043i 0.5000 - 0.0044i 0.5000 - 0.0045i 0.5000 - 0.0046i 0.5000 - 0.0047i 0.5000 - 0.0049i 0.5000 - 0.0050i 0.4999 - 0.0051i 0.4999 - 0.0052i 0.4999 - 0.0053i 0.4999 - 0.0054i 0.4999 - 0.0056i 0.4999 - 0.0057i 0.4999 - 0.0058i 0.4999 - 0.0060i 0.4999 - 0.0061i 0.4999 - 0.0063i 0.4999 - 0.0064i 0.4999 - 0.0065i 0.4999 - 0.0067i 0.4999 - 0.0069i 0.4999 - 0.0070i 0.4999 - 0.0072i 0.4999 - 0.0073i 0.4999 - 0.0075i 0.4999 - 0.0077i 0.4999 - 0.0079i 0.4999 - 0.0081i 0.4999 - 0.0082i 0.4999 - 0.0084i 0.4999 - 0.0086i 0.4998 - 0.0088i 0.4998 - 0.0090i 0.4998 - 0.0092i 0.4998 - 0.0095i 0.4998 - 0.0097i 0.4998 - 0.0099i 0.4998 - 0.0101i 0.4998 - 0.0104i 0.4998 - 0.0106i 0.4998 - 0.0109i 0.4998 - 0.0111i 0.4997 - 0.0114i 0.4997 - 0.0116i 0.4997 - 0.0119i 0.4997 - 0.0122i 0.4997 - 0.0125i 0.4997 - 0.0128i 0.4997 - 0.0131i 0.4996 - 0.0134i 0.4996 - 0.0137i 0.4996 - 0.0140i 0.4996 - 0.0143i 0.4996 - 0.0146i 0.4996 - 0.0150i 0.4995 - 0.0153i 0.4995 - 0.0157i 0.4995 - 0.0161i 0.4995 - 0.0164i 0.4994 - 0.0168i 0.4994 - 0.0172i 0.4994 - 0.0176i 0.4994 - 0.0180i 0.4993 - 0.0184i 0.4993 - 0.0189i 0.4993 - 0.0193i 0.4992 - 0.0197i 0.4992 - 0.0202i 0.4991 - 0.0207i 0.4991 - 0.0212i 0.4991 - 0.0216i 0.4990 - 0.0221i 0.4990 - 0.0227i 0.4989 - 0.0232i 0.4989 - 0.0237i 0.4988 - 0.0243i 0.4988 - 0.0248i 0.4987 - 0.0254i 0.4986 - 0.0260i 0.4986 - 0.0266i 0.4985 - 0.0272i 0.4984 - 0.0278i 0.4984 - 0.0285i 0.4983 - 0.0291i 0.4982 - 0.0298i 0.4981 - 0.0305i 0.4980 - 0.0312i 0.4980 - 0.0319i 0.4979 - 0.0327i 0.4978 - 0.0334i 0.4976 - 0.0342i 0.4975 - 0.0350i 0.4974 - 0.0358i 0.4973 - 0.0366i 0.4972 - 0.0375i 0.4970 - 0.0383i 0.4969 - 0.0392i 0.4968 - 0.0401i 0.4966 - 0.0410i 0.4965 - 0.0420i 0.4963 - 0.0429i 0.4961 - 0.0439i 0.4959 - 0.0449i 0.4957 - 0.0460i 0.4955 - 0.0470i 0.4953 - 0.0481i 0.4951 - 0.0492i 0.4949 - 0.0503i 0.4946 - 0.0515i 0.4944 - 0.0526i 0.4941 - 0.0538i 0.4939 - 0.0550i 0.4936 - 0.0563i 0.4933 - 0.0576i 0.4930 - 0.0589i 0.4926 - 0.0602i 0.4923 - 0.0616i 0.4919 - 0.0630i 0.4916 - 0.0644i 0.4912 - 0.0658i 0.4908 - 0.0673i 0.4903 - 0.0688i 0.4899 - 0.0703i 0.4894 - 0.0719i 0.4889 - 0.0735i 0.4884 - 0.0752i 0.4879 - 0.0768i 0.4873 - 0.0785i 0.4868 - 0.0803i 0.4862 - 0.0820i 0.4855 - 0.0838i 0.4849 - 0.0857i 0.4842 - 0.0875i 0.4835 - 0.0894i 0.4827 - 0.0914i 0.4819 - 0.0934i 0.4811 - 0.0954i 0.4802 - 0.0974i 0.4793 - 0.0995i 0.4784 - 0.1016i 0.4774 - 0.1038i 0.4764 - 0.1060i 0.4754 - 0.1082i 0.4743 - 0.1105i 0.4731 - 0.1128i 0.4719 - 0.1151i 0.4707 - 0.1175i 0.4694 - 0.1199i 0.4680 - 0.1223i 0.4666 - 0.1248i 0.4652 - 0.1273i 0.4637 - 0.1298i 0.4621 - 0.1324i 0.4604 - 0.1350i 0.4587 - 0.1376i 0.4569 - 0.1403i 0.4551 - 0.1430i 0.4532 - 0.1457i 0.4512 - 0.1484i 0.4491 - 0.1512i 0.4470 - 0.1540i 0.4447 - 0.1568i 0.4424 - 0.1596i 0.4400 - 0.1624i 0.4376 - 0.1653i 0.4350 - 0.1681i 0.4324 - 0.1710i 0.4296 - 0.1739i 0.4268 - 0.1768i 0.4239 - 0.1796i 0.4209 - 0.1825i 0.4177 - 0.1854i 0.4145 - 0.1882i 0.4112 - 0.1911i 0.4078 - 0.1939i 0.4043 - 0.1967i 0.4007 - 0.1995i 0.3970 - 0.2022i 0.3931 - 0.2050i 0.3892 - 0.2077i 0.3852 - 0.2103i 0.3811 - 0.2129i 0.3768 - 0.2154i 0.3725 - 0.2179i 0.3681 - 0.2203i 0.3636 - 0.2227i 0.3590 - 0.2250i 0.3542 - 0.2272i 0.3494 - 0.2294i 0.3446 - 0.2314i 0.3396 - 0.2334i 0.3345 - 0.2353i 0.3294 - 0.2371i 0.3242 - 0.2387i 0.3189 - 0.2403i 0.3135 - 0.2418i 0.3081 - 0.2431i 0.3026 - 0.2444i 0.2971 - 0.2455i 0.2915 - 0.2465i 0.2859 - 0.2474i 0.2803 - 0.2482i 0.2746 - 0.2488i 0.2689 - 0.2493i 0.2631 - 0.2497i 0.2574 - 0.2499i 0.2516 - 0.2500i 0.2459 - 0.2500i 0.2401 - 0.2498i 0.2344 - 0.2495i 0.2287 - 0.2491i 0.2230 - 0.2485i 0.2173 - 0.2479i 0.2117 - 0.2470i 0.2061 - 0.2461i 0.2005 - 0.2450i 0.1950 - 0.2439i 0.1895 - 0.2426i 0.1842 - 0.2412i 0.1788 - 0.2397i 0.1736 - 0.2380i 0.1684 - 0.2363i 0.1633 - 0.2345i 0.1583 - 0.2326i 0.1533 - 0.2306i 0.1485 - 0.2285i 0.1437 - 0.2263i 0.1390 - 0.2240i 0.1345 - 0.2217i 0.1300 - 0.2193i 0.1256 - 0.2169i 0.1213 - 0.2143i 0.1171 - 0.2118i 0.1131 - 0.2092i 0.1091 - 0.2065i 0.1052 - 0.2038i 0.1014 - 0.2011i 0.0978 - 0.1983i 0.0942 - 0.1955i 0.0907 - 0.1927i 0.0874 - 0.1899i 0.0841 - 0.1870i 0.0809 - 0.1841i 0.0778 - 0.1813i 0.0749 - 0.1784i 0.0720 - 0.1755i 0.0692 - 0.1726i 0.0665 - 0.1698i 0.0639 - 0.1669i 0.0614 - 0.1640i 0.0589 - 0.1612i 0.0566 - 0.1584i 0.0543 - 0.1556i 0.0521 - 0.1528i 0.0500 - 0.1500i 0.0480 - 0.1472i 0.0460 - 0.1445i
Sizes = [size(T2.FreqHz); size(Rsp)]
Sizes = 2×2
401 1 401 1
sysfr = idfrd(Rsp,T2.FreqHz,0,'FrequencyUnit','Hz') % Create System Response Data Object
sysfr = IDFRD model. Contains Frequency Response Data for 1 output(s) and 1 input(s). Response data is available at 401 frequency points, ranging from 10 Hz to 1e+05 Hz. Status: Created by direct construction or transformation. Not estimated.
sys_ss = ssest(sysfr, 1) % State Space Realisation
sys_ss = Continuous-time identified state-space model: dx/dt = A x(t) + B u(t) + K e(t) y(t) = C x(t) + D u(t) + e(t) A = x1 x1 -2e+05 B = u1 x1 256 C = x1 y1 390.6 D = u1 y1 0 K = y1 x1 0 Parameterization: FREE form (all coefficients in A, B, C free). Feedthrough: none Disturbance component: none Number of free coefficients: 3 Use "idssdata", "getpvec", "getcov" for parameters and their uncertainties. Status: Estimated using SSEST on frequency response data "sysfr". Fit to estimation data: 100% FPE: 1.13e-31, MSE: 1.119e-31
figure
compare(sysfr, sys_ss)
sys_tf = tfest(sysfr, 1,1) % Transfer Function Realisation
sys_tf = 1e05 -------- s + 2e05 Continuous-time identified transfer function. Parameterization: Number of poles: 1 Number of zeros: 1 Number of free coefficients: 3 Use "tfdata", "getpvec", "getcov" for parameters and their uncertainties. Status: Estimated using TFEST on frequency response data "sysfr". Fit to estimation data: 100% FPE: 1.58e-30, MSE: 1.557e-30
figure
compare(sysfr, sys_tf)
figure
pzmap(sys_ss)
grid on
format long E
Poles = pole(sys_ss)
Poles =
-2.000000000000001e+05
Zeros = zero(sys_ss)
Zeros = 0×1 empty double column vector
figure
subplot(2,1,1)
semilogx(T2.FreqHz, mag2db(abs(Rsp))) % Changed
title('Amplitude (dB)')
grid
subplot(2,1,2)
semilogx(T2.FreqHz, angle(Rsp)) % Changed
title('Phase (°)')
grid
xlabel('Frequency')
syms s
H = (s + 1e05 ) / (s + 200000)
H = 
G = H*100/(1 - H)
G = 
G = simplifyFraction(G)
G = 
GdB = vpa(20*log10(abs(G)))
GdB = 
figure
hfp = fplot(G, [0 1E+5]);
Xv = hfp.XData
Xv = 1×45
1.0e+00 * 0 2.085524378664551e+03 4.545454545454545e+03 6.924930780053428e+03 9.090909090909090e+03 1.132082961640785e+04 1.363636363636364e+04 1.614407746540481e+04 1.818181818181818e+04 2.071481972960689e+04 2.272727272727273e+04 2.491983619176126e+04 2.727272727272727e+04 2.945752839701479e+04 3.181818181818182e+04 3.435343593193532e+04 3.636363636363636e+04 3.880114675676139e+04 4.090909090909091e+04 4.330127399050895e+04 4.545454545454546e+04 4.770619139409764e+04 5.000000000000000e+04 5.227332318762936e+04 5.454545454545454e+04 5.690852009058446e+04 5.909090909090909e+04 6.154579464415575e+04 6.363636363636364e+04 6.587009057391543e+04
Yv = hfp.YData
Yv = 1×45
1.000000000000000e+02 1.020855243786645e+02 1.045454545454545e+02 1.069249307800534e+02 1.090909090909091e+02 1.113208296164078e+02 1.136363636363636e+02 1.161440774654048e+02 1.181818181818182e+02 1.207148197296069e+02 1.227272727272727e+02 1.249198361917613e+02 1.272727272727273e+02 1.294575283970148e+02 1.318181818181818e+02 1.343534359319353e+02 1.363636363636364e+02 1.388011467567614e+02 1.409090909090909e+02 1.433012739905089e+02 1.454545454545455e+02 1.477061913940976e+02 1.500000000000000e+02 1.522733231876294e+02 1.545454545454546e+02 1.569085200905845e+02 1.590909090909091e+02 1.615457946441558e+02 1.636363636363636e+02 1.658700905739154e+02
grid
xlabel('Frequency (Hz)')
ylabel('Absolute Amplitude')
figure
semilogx(Xv, Yv, 'LineWidth',2)
grid
xlabel('Frequency (Hz)')
ylabel('Amplitude (dB)')
.

4 Comments
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francesco baldi
francesco baldi on 26 Dec 2021
isnt' there an easier way to do it? I mean, i only need to compute that operation, i'm not interested in any plot. I'd just like to know if there's a way to take a .txt file from Spice with the values of my transfer function and use it directly to calculate another parameter. The fact is that i'd like to operate in the frequency domain, without using the Laplace variable.
Star Strider
Star Strider on 26 Dec 2021
The Laplace domeain is the frequency domain, since .
The Fourier domain is the Laplace domain confined to the imaginary axis . Calculations are generally easier in the Laplace domain, the reason I prefer it.
If I remember correctly, ‘G’ is formed by a feedback of a second transfer function (actually the same one here), so the feedback function may also be appropriate.
If the plot is not necessary, then there is no requirement to use it. I included it to give a visual interpretation of the result of that calculation, since that usually helps make it easier to understand.
.
francesco baldi
francesco baldi on 26 Dec 2021
and once i calculate G in the Laplace domain, is there a way to put the sigma variable of Laplace equal to zero? because i need to separate G between real and imaginary part, and i don't know how to do it in the Laplace domain.
Star Strider
Star Strider on 26 Dec 2021
Ran in:
Ran in:
I sould do something like this —
s = tf('s');
G = s/1000 + 100;
[mag,phs,wout] = bode(G);
magv = squeeze(mag)
magv = 55×1
100.0050 100.0200 100.0273 100.0374 100.0511 100.0699 100.0956 100.1307 100.1787 100.2443
phsv = squeeze(phs)
phsv = 55×1
0.5729 1.1458 1.3398 1.5666 1.8318 2.1418 2.5042 2.9277 3.4225 4.0005
Gv = magv .* exp(1j*phsv);
Gre = real(Gv)
Gre = 55×1
84.0354 41.2434 22.9033 0.4184 -25.8194 -54.0882 -80.4422 -97.8490 -96.2513 -65.4845
Gim = imag(Gv)
Gim = 55×1
54.2131 91.1207 97.3700 100.0365 96.6622 84.1930 59.5667 21.2539 -27.7752 -75.8992
figure
bode(G)
grid
Experiment!
.

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