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Using X-Parameters
PathWave RF Synthesis (Genesys) and
PathWave System Design (SystemVue)
You’re convinced (as we are) that X-parameters provide the very best kind of nonlinear
model for use in an RF System simulation, so two questions arise:
1. Where do they come from?
2. How do I use them?
There are only two ways to generate an X-parameter model. One is to measure a
device yourself using a nonlinear vector network analyzer (NVNA). If you’re lucky
enough to have such a measurement system, you’re all set. But there’s another way
to get them: PathWave Advanced Design System (ADS) can turn a circuit schematic
into an X-Parameter file that can be consumed in PathWave RF Synthesis
(Genesys) or PathWave System Design (SystemVue). That process has been
outlined in another Technical Overview.
Once you obtain a model from either of these two methods, using it is simple. In
fact, if you already know how to incorporate S-data into a simulation, the process is
exactly the same. Take the schematic in Figure 1.
Figure1. Schematic
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Wait a second, there’s no X-Param part on that schematic… so what gives? The model is hidden inside
the Circuit_Link, which is always a good idea. Inside a Circuit_Link network, you can place multiple
nonlinear devices with additional passive components in a single nonlinear analysis. As the parts interact
according to their frequencies, load pull impedances, DC bias, signal power, etc., the overall result is
presented to the RF System simulator with greater accuracy than system-level behavioral models would
have provided by themselves into static loads.
Pushing into the Circuit_Link, we see
the X-Parameter model in Figure 2. The
IPROBE is there in case we want to
calculate power added efficiency or a
similar measurement.
The XPARAMS part points to an
amplifier’s X-parameter file. This device
has nominal Gain = 20 dB and TOI =
+25 dBm. An XPARAMS part functions
just like an SPARAMS part does for S-
data.
X-Parameters support multi-port
devices, mixers, and any other nonlinear
RF device. Figure 2. X-Parameter model
The header of the file is as show in Figure 3. It has important information about the conditions when the
model was extracted.
Figure 3. Header of the file
This is telling us that the model has 3 ports, 2 of which are RF ports and 1 DC port. The model was
extracted with two RF tones, 1.0 GHz and 1.2 GHz. Input power was swept from -30 dBm to +10 dBm;
and the DC pin had +10 V on it. Figure 4 shows the results of a single-tone power sweep of the circuit.