Simple question about function generator

[alba452]

I have a simple question about a function generator.
I am testing a Phase locked loop circuit on PCB using TLC2934.
A reference signal from the function generator is provided to the circuit.
The initial condition of the signal Was 3V of high level, 0V of low level, and 20Mhz.
To test PLL, the frequency is changing. However, as the frequency increases (up to 80Mhz), the Vpp decreases, and the offset voltage is increasing.
I have no idea why Vpp is varying with the increasing frequency.

 

[rhaynes]

You probably have a termination problem at that high of a frequency. It would be best to use 50 Ohm coax from the generator to the load and place a 50 Ohm resistor to ground at the load.

Ray

 

[thannara123]

Increasing the frequency may cause the reducing the resistance the of the output wire that's why the wire may effects a short circuit so the output VPP reduce