ADS example 1: two-state devices

Updated December 30, 2009

Click here to learn about multi-state phase shifter simulations in ADS

Click here to go to our main page on computer-aided design (CAD)

Click here to go to our page on switch-FETs

Click here to go to our page on S-parameters

This page will show you how to create a two-state device for use in a larger schematic, using measure S2P data. It is useful for any design that might be switched from one state to another, such an RF switch, or a digital phase shifter or attenuator.

The trick we employ is to add a parameter to the schematic, which we call "ON", to which we will later assign either a value of 0, or a value of 1. We use this parameter to connect the input and output of a two-port design to one of two different S2P files (but we could just as well use it to connect to two different circuit models). We assume that the reader has some knowledge of ADS and what a "parameter" means in a schematic.

In the schematic below, four resistors are used to connect port 1 and port 2 to either "FET_ON.s2p" or "FET_OFF.s2p". The top two resistors are controlled by R1, the bottom two resistors are controlled by R2. When R1 is set to zero and R2 is set to infinity, the circuit will behave like a switch-FET that is ON. When R1 is infinity and R2 is zero, it will behave like a switch-FET that is off.

The resistors need to be either open or short circuits for the circuit to behave as intended, but the parameter "ON" is either 0, or 1. How do we do make this work? In the "Var Eqn" block we apply equations to "ON" to toggle the resistors from a very small number (close to zero) to a very high number (approaching infinity), with "ON" as the dependent variable:

R1=(1.000000001-ON)/(0.000000001+ON)

R2=1/R1

When ON=0, R1 will be very high (actually, 1,000,000,001) which is close enough to an open circuit to serve our purpose. When ON=1, R1 is very close to zero (0.000,000,001), close enough to a short circuit. Because the value of R2 must be opposite of R1, we merely take the reciprocal of R1 to arrive at R2.

Now let's place the two-port circuit into a new schematic, and set ON=1, and analyze the response:

Pulling up the S-parameters in a separate window, here we see the ON state of the FET, where S21 is very close to zero dB at DC (a direct connection), because the FET looks like a small-value resistor.

OOPS, graph is missing!

Now let's look at the OFF state by setting ON=0:

Analyzing the circuit, we see the OFF state S-parameters below. In this case S21 is far from zero dB at DC because the FET looks like a series capacitor:

How about if you want to compare both states on the same graph? Create a four-port circuit, with the two-state schematic entered twice, once with ON=0, and once with ON=1.

Now we can display ON and OFF state transmission coefficients at the same time, as S21 and S43.

Here's an example of a two state device, in this case it's a switch FET where we use an equivalent circuit model, and add parallel inductor to resonate the FET's off capacitance.

We'll post more ADS examples from time to time. Send us an email if you have any suggested topics!