Feature: RF
RF circuit simulation L
equipment designers typically turn to the widely-available S-parameter device models. Nonlinear simulation has traditionally been more challenging due to lack of available data in digital form (for example, IP3, P1dB and noise), along with a historical absence of frequency-variant model structures in popular RF simulators. RF circuit designers resort to using
homemade spreadsheets that calculate cascaded noise and distortion. These spreadsheets, however, struggle to simulate system-level characteristics, such as error vector magnitude (EVM) and adjacent channel leakage ratio (ACLR), which become relevant when the signal chain is driven by modulated signals.
32 April 2022
www.electronicsworld.co.uk
By Eamon Nash, Applications Director, RF amplifiers and beamformers, Analog Devices S-parameters S-parameter datasets are in a standardised tabular format, consisting of input return loss, gain, reverse isolation and output return loss vs. frequency, all in vector format. Te data is generally collected under small-signal conditions with drive signals well removed from the signal compression points. S-parameters are typically used to
inear and nonlinear RF circuit simulations have traditionally occupied different domains. To simulate cascaded small- signal gain and loss, RF
_Delta), where Psat
simulate cascaded gain to design input and output matching networks and to assess stability. However, they contain no information about a device’s noise, compression or distortion characteristics. Analog Devices maintains an extensive
RF amplifier and mixer sys-parameter library, which is available for download and is also included with Keysight Genesys and SystemVue installations; see Figure 1. Te library is readily accessible via the Part Selector. Figure 2 shows measured and simulated
results for a power sweep at 10GHz of a 10MHz-10GHz HMC788A RF gain
is 2dB. Tis results in a PSAT
block. We can see that the simulated power sweep matches measured data quite closely. Te simulator is using the device’s
gain and OP1dB data (an important gain compression parameter, the power input resulting in a 1dB compression), along with the difference between the 1dB compression point and the saturation point (PSAT
is the
saturated output power of the amplifier, to generate the plot shown; here PSAT
_Delta value that is
2dB above the OP1dB level, which is a typical default for GaAs RF amplifiers.
Distortion To take a closer look at the simulated compression characteristics, we can look at gain offset (compression or expansion) – also called “AM-AM distortion”; and phase distortion, also known as “AM- PM distortion”. Measured and simulated results are shown in Figure 3 for ADI’s RF amplifier HMC930A. We can see that the
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