strange simulation problem

Hello everyone,

I am runing a simulation following the Cadence's SpectrRF tutorial, and I have the circuit's schematic as follow. Please pay attention to the small circuit at upper , which is composed of a "port" and "res".... During the simulation, I found that with or without this part, there are different waveform for the input of first Opamp. Can anybody tell me why?
B.T.W The function of this circuit is to simulate QPSK modulation and demodulation.

Thanks

Not sure what you meant, but be aware of this when using 'port' in cadence. When you specify the voltage using 'port', twice the specified voltage will be put out by the generator. When you specify dBm, it will calculate the voltage required from the generator using the res specified in 'port'. So when you have a mismatched connection to 'port' such as a high impedance amplifier, unless the res you specified in 'port' is the same impedance, the voltage you probe out of the 'port' will not be what you had specified.

Hi Frank

Thanks for your reply!
What I means is that the "port" circuit, which site at the upper of my circuit, doesn't contact to any other place, but it does affect the circuit's performancs. I want to know why?

Regards,