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MLVD20XEVM: Cannot get board to transmit

Glenn Marquis
Prodigy 20 points
Part Number: MLVD20XEVM

Hi, I am trying to use this evaluation board for the first time. I can't get an output on any of the boards I have using a Keysight U2761A function generator. 

What I am trying to do at this point is to get an output from the LVD205 Via J6 by jumpering W1 through W4 to VCC, I have VCC and VCC 01 jumpered together, as well as both grounds. As far as I can tell, it should enable the transmit and disable the receive. Pin 4 of the 205 has been verified as high, as is the power to the chip.

At this point the input at J2 is a square wave, 3MHz, +2V/-1V voltage levels. VCC = 3.3VDC. 

The outputs at P2 are 0.5VDC and 1.3VDC, respectively. 

I even tried tying the input to VCC and then GND to see if the output would change states but that did not show any change in output state either.  

The boards draw 32mA when idle, seems normal to me. 

Any idea what I might be missing here? Seems the operation should be pretty straightforward. I think I have tried everything. Why is the transmit not enabling?

Thanks.

  • 0
    TI__Guru 64326 points

    Hi,

    Can you try using +2V/0V voltage levels instead? 

  • 0 in reply to Malik Barton57
    Prodigy 20 points

    I would if I could. I am also using a 50-ohm 5-way splitter about 8" upstream from the eval board which seems to annihilate my ground reference, so I get a signal symmetrical to Gnd no matter what offset I put to it. 

    So what I have discovered is that I have to bypass the splitter and crank the function generator waaay up to get my voltage level over the minimum voltage to change the output state. There is nothing wrong with the board. 

    I put the splitter back in and changed an input termination resistor from 50 to 121 ohms to see if I could get the amplitude up to where it needs to be. I got closer but not enough to reach the threshold. 

    So if you have any comments towards my changing an input termination resistor I would like to hear them. I am not an RF guy but am being paid to pretend that I am one. 

  • 0 in reply to Glenn Marquis
    TI__Guru 64326 points

    Hi,

    It seems your splitter may be AC coupled or buffered? In either case changing R3 will increase the signal level but create a unbalanced transmission line that could cause distortions to your signal/reflections assuming it has a 50 ohm output termination. What kind of test are you trying to perform? If only functional, do you have a AWG on hand to use?