Feature: RF
measured AM-AM distortion matches the simulation quite closely; however, the simulation indicates there is no AM-PM distortion, which is not correct. Tis is because the device model and dataset only contain small-signal phase information, or S21. While the simulator can use OP1dB and _Delta data from the device’s model to
PSAT
estimate AM-AM distortion, it does not have any large signal S-parameter data to work with. Tis is an instance where a more elaborate model such as the X-parameter format would be more appropriate, since X-parameter models have built-in level- dependent S-parameters. Figure 4 shows a power sweep at
3.2GHz for a 10W GaN RF amplifier, HMC1114LP5DE. GaN RF amplifiers tend to have a much soſter compression characteristic than GaAs devices, which necessitates an adjustment of the PSAT
_
Delta; in this case the delta has been set to 7dB, based on observed measurements. While the simulator will in some cases generate a warning because of the large delta, it will still simulate correctly and produce a result that closely matches measured performance.
Figure 1: Screenshot from Keysight Genesys showing a typical sys-parameter model
ACLR simulation As we move away from continuous waveform (CW) measurements (where CW is an electromagnetic wave of constant amplitude and frequency) and to simulations of modulated signal behaviour, the value of a sys-parameter dataset increases. Whilst information on device gain, compression, IP3 (third- order intercept point, which is a specific
figure of merit associated with the more general third-order intermodulation distortion) and noise figure are readily available in device data sheets, plots showing performance with modulated signals are unlikely to be found for devices designed for general-purpose use. In addition, metrics such as ACLR and EVM are not easily determined without simulation or measurement.
Figure 2: Measured and simulated power sweep of a gallium arsenide (GaAs) RF amplifier
www.electronicsworld.co.uk April 2022 33
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