Frequency Shifting Confusion

Started by cogwsn 6 years ago10 replieslatest reply 6 years ago175 views


It might be a very silly question. I have a baseband zigbee dump (zigbee._baseband.mat attached). I wish to shift it to -7 MHz, -2 MHz to the left and also 3 MHz, 8 MHz to the right. But when I multiply with corresponding carries and plot the fft, I see just opposite. What I am missing here? 


Here is the script 


close all

load('zigbee_baseband.mat') % Baseband 

fc2430 = -7e6; 

fc2435 = -2e6; 

fc2440 = 3e6; 

fc2445 = 8e6; 

fs = 20e6;

t = (0:1/fs:((1/fs)*(length(interference)))-1/fs);

carrier2430 = exp(1i*2*pi*fc2430*t)'; % shift to -7 MHz

carrier2435 = exp(1i*2*pi*fc2435*t)'; % shift to -2 MHz

carrier2440 = exp(1i*2*pi*fc2440*t)'; % shift to 3 MHz

carrier2445 = exp(1i*2*pi*fc2445*t)'; % shift to 8 MHz

interference2430 = interference.*carrier2430;

interference2435 = interference.*carrier2435;

interference2440 = interference.*carrier2440;

interference2445 = interference.*carrier2445;











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Reply by jimsiaf91February 9, 2018

Do you mean shift by -7 Mhz or shift to -7 Mhz? Becuase the plot isn't clear enough.

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Reply by cogwsnFebruary 9, 2018

As the baseband is it 0 MHz, either is applicable i.e. shifting to -7 MHz or shifting by -7 MHz.

I have attached the picture now for downloading. untitled.png

Link also https://ibb.co/exxvLc

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Reply by jimsiaf91February 9, 2018

What it seems to be happening is that the spectrum is reversed, probably due to aliasing.

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Reply by shivt1190February 9, 2018

What is "interference"? Could you plot without multiplying by it?

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Reply by cogwsnFebruary 9, 2018

Interference is zigbee. Yes when I plot it without multiplying with carrier, it lands on 0 MHz. The very first plot in the subplot. 

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Reply by kazFebruary 9, 2018

Why is there minus at -1/fs

t = (0:1/fs:((1/fs)*(zb_packet_len))-1/fs)
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Reply by cogwsnFebruary 9, 2018

So that the length of time array is equal to the length of interference. Apology these was a confusion in naming. 

load('zigbee_baseband.mat') % Baseband  loads 'interference' variable to workspace. This is actually zigbee baseband oversampled at 20 MHz. 

I just corrected in the post also. 

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Reply by kazFebruary 9, 2018

I notice the signal shape at baseband is slightly different when frequency shifted. This may give a clue

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Reply by cogwsnFebruary 9, 2018

Yes indeed. I found the mistake. I was supposed to transpose the carrier array by using ' operator but since carrier is complex, so ' operator was doing a conjugate transpose. 

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Reply by Tim WescottFebruary 9, 2018

I hate it when I do that.  Having the ' operator do a conjugate transpose is exactly correct, but forgetting that is a pain. .' to the rescue.