I'm testing my application with different attenuation values and I have problem with evaluated level values from IQ data.
It seems like the attenuation step is 1,13dB and not 1dB
AD6676 attenuator should be precise according to datasheet so I believe I´m doing something wrong (I have tested another AD6676)
I have attached matlab script what computes fft from sampled IQ data for different attenuation.
AD6676 parameters: Fif = 88,3MHz, BW = 20MHz, Fadc = 2949MHz F_iq= 245.75MSPS, 16384 samples
Testing signal: -6dBm / 90MHz
Attenuation: 0dB, 10dB, 20dB, 27dB
Testing signal is down converted around 1,7MHz.
Frequency axis is normalized to real frequency values in MHz.
Spectral lines at frequencies 92MHz and 157MHz are spur from signal generator and DC component.
If you compare level values you will see what I described.
Can someone please help me? Do I make bad calculations in Matlab or should I focus on AD6676 setting?