Differential RF Input-Cadence

[hrkhari]

cadence differential simlation

In the process of feeding input port to a fully balanced Gilbert cell mixer, I had applied a single LO port at the switching quad, made it differential through two parallel vcvs source with a gain magnitude of 0.5 and -0.5 respectively. The input to the transconductance stage is applied in the same manner. I could measure some of the performance parameter such as IIP3 and THD in Spectre-RF through this method. However the measurement of S-parameter could not be obtained. Can anyone verify whether this method of single to differential conversion using voltage controlled voltage source (vcvs) is reliable in this situation?. Please do suggest other alternative?.

Rgds

 

[BigBoss]

why differential rf port

Why you use VCVS to transform single ended to balanced?? In Cadence all ports are already balanced !! Connect them as is...

If you intend to obtain 90 degree phase difference, simply put 90 degree in Phase specs. in Port definition.

Of course you may not obtain s-parameters because VCVS's don't have equivalent circuits and therefore s-parameter simulation won't work.VCVS's are only mathemetical definitions in all simulations.They can be valid in transient simulation but for others you don't need them...

I have used as I told you in my double-double IQ Modulator and there was no problem.

 

[hrkhari]

rf input

Hi BigBoss:

Did you use a single port as an input for the differential stage or two different port with an 180 degrees of phase shift difference.

Rgds

 

[BigBoss]

differential source cadence

Hi BigBoss:

Did you use a single port as an input for the differential stage or two different port with an 180 degrees of phase shift difference.

Rgds

Yes, of course..
Normally when you connect only single pin of a port, this pin will have 0 degree phase compare to other one. This is single ended operation.

But when you connect these pins differential, one pin will have 0 degree, other will have 180 degree automatically...

It's not so difficult...Try and see.

This phenomena is also valid in ADS and in other simulators except Ansoft. In Ansoft designer, Microwave port by default one pin is referenced to ground.In this case n a internal port should be used with whichever marked reference ...

When you need two independent port with 180 degree, you should fill "phase of sinusoid" box with 180 degree compare the other. So, while one port drives 0 degree sinus, other port will prodcue 180 degree compare to each other...

so simple..

 

[hrkhari]

measuring differential s parameters cadence

Thx BigBoss:

Got it, another question, the differential output of this mixer is to be terminated with 50 Ohms as well, is it possible to use a single port to terminated this differential terminal or it has to be two independant port. If it is two different port is it sufficient to probe only one of the output terminal to obtain the s-parameter plot.

Rgds

 

[BigBoss]

4 port s param measurement in cadence

Thx BigBoss:

Got it, another question, the differential output of this mixer is to be terminated with 50 Ohms as well, is it possible to use a single port to terminated this differential terminal or it has to be two independant port. If it is two different port is it sufficient to probe only one of the output terminal to obtain the s-parameter plot.

Rgds

No,

I doesn't has to be two independent port. You can connect only single port in differential mode to obtain either s-parameters and other PSS simulations.But if you would like to get 3Port s-parameter, in this case you have to terminate "half of the output termination" , thus for instance 25 ohm or something else...

In fact for a mixer, most probably you will do PSS ( or QPSS )analysis and therefore you have to terminate 50 or different termination at the output.In this case you don't need s-parameters because s-parameters won't work with nonlinear mixer devices.The mixers are highly nonlinear devices and generally s-parameters don't work.Instead, PAC or PSS(PXF) analysis will be more suitable.

Good Luck..