Direct conversion RF MIxer

Hi, I am DEsigning an RF mixer (Gilbert Mixer) using ADS of Agilent but the problem is that in ADS the IF output can't be 0Hz (DC output) and an error message is appearing.
What is the solution to Do Direct conversion by ADS?
Thanks

make a very samll difference between 2 , abot 1 Khz or even less , and in this case they are very close

khouly

Foufou
at DC there is no information.
Consider the bandwidth of your received signal, let's say BW at center freq CF
After the RF downmixer you will have CF= 0 Hz and the signal bandwidth will be [-BW/2,BW/2]. So all your signal analysis (ac, noise, linearity...) have to be performed in [0,BW/2] bandwidth (we are neglecting negative frequencies because IQ signals are helping us!).
I hope it will help.
Mazz

yes Mazz
I know, but my problem is in ADS and it Dc output, I think I will try the solution of khouly.
And I have an other question. And always using ADS, when we define the correct DC biais values of the RF and the LO port to have transistors working in saturation for exemple Vdclo=1.2V and VdcRF=1.5V, when I finish the hole design, how could I replace the DCs tensions used ? using Vdd tension of my circuits
thinks

Added after 26 minutes:

Iknow that to bias the DC comonent we sould add the transistors like in figure attached,
but the question is how could I manipulate W/L to have the Desired tensions
Help and exemple
thanks[/img][/GVideo]

W/L ratio is depended in technology,desired Vgs, current etc.
There isn't enough information here..

You need to design a bias for your circuit.
Bias can be a voltage or a current and usually stays some chapter BEFORE the design of amplifier and some chapter BEFORE RF design. OK, I'm joking, but it is true.
You should always consider the equations that keep MOS in saturation and properly fix some variable; for example, if you fix current and W/L you will obtain a certain Vgs (this is a very simplified explanation).
I hope it can help.
Mazz