How to get argument of complex exponential?

Question

Louis Tomczyk 2022-5-29

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1729525-how-to-get-argument-of-complex-exponential

评论： Louis Tomczyk 2022-5-30

采纳的回答： Voss

在 MATLAB Online 中打开

Dear all,

I feel a bit ashamed to ask such question but I am quite upset by the outcome of

, where z is a chosen complex, which does not give what I expect.

Let's be more precise:

% axis parameters
dt      = 6.25e-3*1e-9;          % [s]    sampling time
Nsamp   = 1024*16;              % []     number of samples
t       = 0:dt:(Nsamp-1)*dt;    % [s]    array of time values
% signal construction
nu      = 193.41e12;            % [Hz]   frequency
CFO     = exp(1i*2*pi*nu.*t);   % []     Carrier Frequency Offset signal, so "my z"
% getting the angle
ARG     = unwrap(angle(CFO));   % []     it should give an ARRAY of size (1;16,384): 2*pi*nu*t
p       = polyfit(t,ARG,1)      % [rad/s]it should return [2*pi*nu,0]  
p = 1×2
1.0e+11 *

   -1.8850    0.0000
2*pi*nu
ans = 1.2152e+15

But it does not give the pulsation I used to construct my signal.

What did I miss?

Does anyone have an idea?

Thanks a lot,

Best,

louis

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Answer 1

Voss 2022-5-29

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1729525-how-to-get-argument-of-complex-exponential#answer_973955

在 MATLAB Online 中打开

The "sampling time" is not small enough. It must be less than 1/(2*nu) - i.e., sampling rate greater than 2*nu - to avoid aliasing.

nu = 193.41e12; % [Hz]   frequency
p0 = 2*pi*nu % target "pulsation"
p0 = 1.2152e+15
fs = 2.01*nu; % sufficient sampling rate
p1 = get_pulsation(nu,fs) % matches target
p1 = 1.2152e+15
fs = 1/(6.25e-3*1e-9); % original sampling rate
p2 = get_pulsation(nu,fs) % does not match
p2 = -1.8850e+11
fs = 1.99*nu; % *nearly* sufficient sampling rate
p3 = get_pulsation(nu,fs) % does not match
p3 = -1.2031e+15
function out = get_pulsation(nu,fs)
% signal construction
dt      = 1/fs;                 % [s]    sampling time
Nsamp   = 1024*16;              % []     number of samples
t       = 0:dt:(Nsamp-1)*dt;    % [s]    array of time values
CFO     = exp(1i*2*pi*nu.*t);   % []     Carrier Frequency Offset signal, so "my z"
% getting the angle
ARG     = unwrap(angle(CFO));   % []     it should give an ARRAY of size (1;16,384): 2*pi*nu*t
p       = polyfit(t,ARG,1);      % [rad/s]it should return [2*pi*nu,0]  
out = p(1);
end