LMH6518SQEVK: LMH6518SQEVK: Query related to amplification of signal

Morning Vishal, 

Well PAM3 is a relatively low frequency signal and you are using a 900MHz DVGA. You might be just seeing excess integrated noise. You might try adding an output filter, perhaps in the 35MHz cutoff region (about 5harmonics I think on the PAM3) - can you provide the details of the LMH6518 output interface? 

I suspect PAM3 has no DC component? If so, can also AC couple the output filter at say about 1MHz. 

Hi Michael,

I am using PAM3 signal of automotive Ethernet as an input which is configured for 100Mbps.

Related to AC coupling, i have already done rework on EVM board & added 0.1uF capacitor in series of traces which are going to SMA output connectors.

Regards,

Vishal Dhanani

So 100Mbps is not the analog bandwidth which will be less, just trying to provide noise bw limiting as much as possible while passing the pulse waveform acceptably. Do you need the DVGA function? 

And actually, one quick way to find the required BP filter would be see the broadband spectrum of your PAM signal 

I suspect those blocking caps are setting your HP corner way too low passing more noise than necessary. 

Hi Michael,

I have seen FFT response of PAM3 signal & it looks like it contains frequency of approx. 50MHz.

After that i have tried to limit bandwidth of amplifier (which is configurable through software) but there is no improvement on output of amplifier.

Regards,

Vishal Dhanani