Forum: GNU Radio Transfer function

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unknown (Guest)
on 2007-05-21 15:00
(Received via mailing list)
Hi, i'm an Italian student.

I'm using an Oscilloscope with Basic Tx and finding a strange behaviour
sending
a costant value of 20000. I see a sinusoidal wave generated from Nco
(fine
modulator) + coarse modulator in this way :

32 MHz -> only coarse -> 670 mVpp
16 MHz -> only coarse -> 730 mVpp
....
0.5 MHz -> only fine -> 680 mVpp
0.1 MHz -> only fine -> 570 mVpp
After this frequency, Tension gets down:
12.5 KHz -> only fine -> 148 mVpp
1 KHz -> only fine -> 13.3 mVpp

The transfer function on the datasheet shows that with interpolation
4xAD9862
(I'm using Interpolation value of 124 -> 31 (FPGA) * 4 (AD9862) ) it
should not
be multiplied by the sinc function, so I can't explain the Tension drop
after
100kHz.
Thanks a lot.
Giuseppe Pilla
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