Frequency Response damping with external DC Bias

[Giovienni]

Hello,


I have designed PCB Traces with a Tee connection. One input is for AC signal input (with blocking capacitor) and another is for DC signal input (with blocking inductor). Both AC and DC signal combines and enters my DUT. The problem i am facing is the frequency response is damping. I do not get smooth/flat response. I have simulated the circuit prior of designing and i do not find any damping problem. May i know what is the possible reasons this occur and any method to troubleshoot this problem?

 

[chuckey]

The problem is that the input impedance of the DUT is not 50 ohms, so some of your input RF is being reflected so as to cancel part of the input level. If we consider that the first cancellation occurs at 180 X10^6 MHz. Then this is frequency at which the path from your input to the DUT is 1/4 wavelength. So reflections will occur at N X this frequency, where N is odd. Which gives the effect of your trace. What you have to do is to match the input impedance of the DUT, via the PCB line to the correct input impedance (50 ohms?)
Frank