TSW38J84EVM: TRF3722 parazitic AM modulation

The same situation for TRF3705 on TSW38J84

Andrey,

I am sorry, I am not exactly sure how to interpret your result and also interpret the problem. If you can highlight the problem with indicators and arrows (perhaps in a power point or word document attachment) to explain your concerns, I would greatly appreciate it. 

My brief understanding (with lots of assumptions) is that your low level signal at I/Q result to some amplitude in modulator output. I would say this is due to the nature of quadrature mixing where there could be some LO leakage at around -40dBc of default suppression. You can perhaps look at the spectrum analyzer to see if you observe some sort of LO leakage directly at the LO frequency. Then, correlate this result to the time domain plot.

-Kang

Then I send to modulator only I or Q (another quadrant is 0) the picture 1 is same (amplitude RF signal is changed in symblol time). 

Hi,

I think this has primarily has to do with the quadrature mixing of the TRF3705 and TRF3722. Keep in mind the I is modulated with Cosine(2*pi*LO) while Q is modulated with Sine(2*pi*LO). The sine/cosine has exactly 90degree phase shift, which explains that the waveform is different 1/2 the time, this is basically the cosine/sine transition happens.

If both I+Q = full-scale at all 1s, you will get 3dB more the amplitude. You can give it a try, or use Matlab model to help you analyze the time domain expected waveform.

Matlab Code

close all;

clear all;

%% definitions

% the following section defines the sampling frequency and fft size for the

% entire transceiver design.

% define sampling frequency

fs = 1000;

% define FFT size

N = 1000;

% define number of samples

n = 1:N;

% complex bin size = n/fs instead of n/(2fs)

fft_bin_size = fs/N

% fft x-axis

bin = -fs/2:fft_bin_size:fs/2-fft_bin_size;

%% baseband model.

% defines the baseband frequency and introduce baseband analog higher order

% distortion gain in both quadrature and real paths.

fb = 100; %define baseband frequency

flo = 300; %define LO frequency

i_in = cos(2*pi*fb/fs*n);

q_in = sin(2*pi*fb/fs*n);

bb_in = a1*i_in + a1*j*q_in;

%calculate the energy of baseband signal for normalization.

energy_bb_in = bb_in*bb_in';

% convert time sample into frequency samples via Matlab's FFT function

BB_IN = fft(bb_in);

% plotting the ideal baseband signal after modulation and demodulation

% note: the operation of BB_IN/sqrt(N) is to ensure energy calculation of

% time domain and frequency domain are conserved. I.e. Parseval identity.

% 10*log10(energy_bb_in) is used to normalize the result to 0dB.