Can't get better interpolation at some points

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Azam
Azam 2024-8-15
回答： nick 2024-8-16
Ran in:
Hi all,
I would be grateful if someone could help with the following matter:
I have a set of complex data called [a2norm_real and a2norm_imag (as per attached)], and another set of complex data was generated as shown in the code. I tried to achieve better interpolation for the Gain, but it fails at certain points (as per attached). No matter how many points I generate, it fails at the same place each time. I have tried different 1D and 2D interpolation methods (including the one below), but this was the best one I could achieve. Thanks!
Here is the code:
data2 = readtable('System Gain.xlsx', 'VariableNamingRule', 'preserve');
% Process the second file
a2norm_real = data2{:, 5};
a2norm_imag = data2{:, 6};
a2norm_complx = complex(a2norm_real,a2norm_imag);
Gain = complex(data2{:, 1}, data2{:, 2});
a21_mag = linspace(min(abs(a2norm_complx)),max(abs(a2norm_complx)),31).';
a21_ph = linspace(0,0,31).';
a21_complx = a21_mag.*exp(1i*pi/180.*a21_ph);
% Perform the interpolation
Interp = griddata(a2norm_real, a2norm_imag, Gain, real(a21_complx), imag(a21_complx), 'nearest');
figure;
plot(abs(a2norm_complx), Gain, 'o', 'DisplayName', 'System gain');
Warning: Imaginary parts of complex X and/or Y arguments ignored.
hold on;
plot(abs(a21_complx), Gain, ':*', 'DisplayName', 'System\_Gain\_Selected');
Warning: Imaginary parts of complex X and/or Y arguments ignored.
legend('show'); % Displays the legend with the DisplayName labels
title('(Default) Linear Interpolation');
grid on;
legend('Location', 'best')
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Azam
Azam 2024-8-15
移动：John D'Errico 2024-8-15
Sorry for this oversight, the code has been updated.
Walter Roberson
Walter Roberson 2024-8-15
Gain = complex(data2{:, 1}, data2{:, 2});
Inherently complex-valued.
plot(abs(a2norm_complx), Gain, 'o', 'DisplayName', 'System gain');
You are trying to plot the inherently complex value.

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回答（1 个）

nick
nick 2024-8-16
Hello Azam,
I understand that you want to interpolate the 'Gain' variable.
As @Walter Roberson mentioned, the 'plot' function is incorrectly used with the complex valued variable 'Gain'. For interpolating the 'Gain', you can use the function 'interp1' for 1-D data interpolation, as shown below :
data2 = readtable('System Gain.xlsx', 'VariableNamingRule', 'preserve');
% Process the second file
a2norm_real = data2{:, 5};
a2norm_imag = data2{:, 6};
a2norm_complx = complex(a2norm_real,a2norm_imag);
Gain = complex(data2{:, 1}, data2{:, 2});
a21_mag = linspace(min(abs(a2norm_complx)),max(abs(a2norm_complx)),31).';
a21_ph = linspace(0,0,31).';
a21_complx = a21_mag.*exp(1i*pi/180.*a21_ph);
% Perform the interpolation
InterpolatedData = interp1(abs(a2norm_complx),abs(Gain),a21_mag);
figure;
plot(abs(a2norm_complx), abs(Gain), 'o', 'DisplayName', 'System gain');
hold on;
plot(abs(a21_complx), InterpolatedData, ':*', 'DisplayName', 'System\_Gain\_Selected');
legend('show'); % Displays the legend with the DisplayName labels
title('(Default) Linear Interpolation');
grid on;
legend('Location', 'best')
It fits the 'Gain' well, as can be seen in the plot :
You may refer to the following documentation of 'interp1' function to learn more about it :
https://www.mathworks.com/help/matlab/ref/interp1.html
Hope this helps!

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