Other Parts Discussed in Thread: TRF3705,
Hi TI Experts,
I am experiencing a noise floor issue with DAC3482 + TRF3705. This combination works fine at most frequencies, but the noise floor was pretty high near 2313.5MHz.
On baseband board, DAC3482 is working at 16bit mode, PLL and NCO disabled, DACClock = 1228.8M, X4 or X2 mode. Baseband waveform data is stroed in DDR, where FPGA reads data then send to DAC. During the test, baseband waveform I/Q band width = 100MHz.
ON RF board, there is a modulator powered by LDO, no other components. DAC output connected to RF board, via 900MHz LPF. Both DAC and modulator are 50Ohm system.
Test: Change LO frequency, record spectrum as below
LO = 2260M
LO = 2280M
LO = 2300M
LO = 2320M
LO = 2340M
Lo = 2360M
LO=2380M
I tried chaning DAC Clock, Baseband Waveform I/Q bandwidth, DAC interplation x2 and x4; and found a 'rule'. It seems there is a frequency 'window' , centered near 2313.5M, with = I/Q BW. If LO frquency is set in this 'window', then noise floor increases. And noise increases much higher when Lo closer to 2313.5M. I also tried using a sine-wace as baseband waveform, noise floor did not increase because a sine-wave spectrum occupied bandwidth = 0, seems still obey the rule.
I don't know why this happens, and very appreciated if any ideas on how to improve this.
Thanks in advance!
T.L.
