18 Nov 2014
Design of 15 GHz GaN MMIC Power Amplifiers (Page 2 of 3)
Simulating RF performance
Small signal s-parameters of the amplifier are plotted in Figure 9. The gain is 22.1dB ± 0.4dB from 14GHz to 16GHz, providing a healthy guard band on the 14.5 to 15.35GHz operating band. The input return loss is better than 15dB from 14.5 to 16.2GHz and the output return loss better than 14dB to 15.6GHz. Improved output return loss is straightforward to achieve, but results in degraded large-signal performance.
Figure 9 15GHz GaN PA small signal s-parameters
The simulated output power at 3dB compression shown in Figure 10 is better than 38dBm across the band, and the corresponding power added efficiency is better than 36% across the band. Further increases in both output power and PAE are possible if a higher level of compression can be tolerated.
Figure 10 15GHz GaN PA output power at 3dB compression and corresponding power added efficiency
The OIP3 versus output tone power shown in Figure 11 was simulated at 15GHz with two tones 10MHz apart, and indicates an OIP3 of 46dBm at 22dBm per output tone. The simulations indicate that the target performance requirements of the amplifier have been met. However, investigations were undertaken to look at options for improving the OIP3. This is most conveniently achieved by increasing the quiescent bias current.
Figure 11 OIP3 versus output tone power at 15GHz
Figure 12 shows the improved OIP3 performance when the quiescent current is increased from 130mA/mm (red trace) to 160mA/mm (blue trace) and 200mA/mm (pink trace), while paying attention to ensure adequate thermal performance.
Figure 12: OIP3 versus output tone power at 15GHz for 3 different quiescent bias currents
Higher order modulation simulations
The modulation schemes used in point-to-point microwave radio links are chosen to be spectrally efficient, so a large amount of data can be passed through a single channel. This is because such links are typically used in high data rate applications, such as cellular backhaul, with required data rates of the order of hundreds of megabits, or even gigabits, per second.
Figure 13 shows the constellation diagram of four such schemes - QAM 64, 128, 256 and 512 — where each data symbol represents 6, 7, 8 and 9 bits respectively.
Figure 13: Constellation diagram of higher order modulation schemes. Top to bottom: QAM 64, 128, 256 & 512.
The corresponding eye diagrams in the I (in-phase) plane for these schemes (Figure 14) show the point in the received transmission where the signal has to be sampled to correctly determine its identity. The corresponding eye diagram for the Q (quadrature) plane is not shown. The receiver symbol timing recovery loop must ensure the receiver samples the signal at the correct sampling point — the middle of the eye.
Figure 14 Eye diagram of higher order modulation schemes. Top to bottom: QAM 64, 128, 256 & 512.
All four modulation schemes occupy almost the same amount of spectrum, but the linearity required to preserve modulation fidelity increases with the order of the modulation.
The occupied spectrum of QAM 64, 128, 256 & 512 modulation schemes are all virtually identical, and an example of an undistorted QAM 256 spectrum referenced to a 1Hz bandwidth is given in Figure 15. The almost rectangular spectrum demonstrates that it will be well confined in frequency to an allocated channel, which is a result of root-raised cosine filtering of the digital modulation. In this case, the application is for a 15GHz Class 4H transmission with a 56MHz channel spacing and a symbol rate of 46MBd.
The class 4H transmission is one of a set of transmission standards applied to point-to-point links as specified in ETSI standard EN 302217. This standard specifies a transmit mask within which the modulated spectrum must lie, which is also depicted in Figure 15.
Figure 15 QAM 256 modulation spectrum and associated transmit spectral mask for point-to-point links using EN 302217 Class 4H transmissions.
About the author
Stuart Glynn began his career at Matra Marconi Space in 1997 after graduating from UMIST. Mooving to Sony he later moved on to Nanotech Semiconductor, later joining Plextek,in 2009 becoming a member of the RF Integration Group, which became a separate line of business in November 2012.
Tony Richards graduated from Loughborough University of Technology in 1977 and joined Pye Telecommunications Ltd, later moving on to Philips Research Labs in Eindhoven later returning to Philips in the UK. In 1999 he joined Plextek, becoming a member of the RF Integration Group, which became a separate line of business in November 2012.
Liam Devlin graduated from Leeds University in 1988. After working for Philips and GEC, he joined Plextek, in 1996 becoming a member of the RF Integration Group, which became a separate line of business in November 2012. He has been involved in the design of over 80 custom ICs on a range of GaAs, GaN and Si processes at frequencies from baseband to 90GHz. He has published over 40 technical papers in peer reviewed journals and conferences. Liam is also a non-executive director of Interlligent UK.
Plextek RF Integration is a UK-based design house specialising in the design and development of RFICs, MMICs and microwave/mm-wave modules. We have designed over 80 custom ICs at frequencies ranging from baseband to 100GHz and are a third party design house for Cree, GCS, TriQuint and WIN. Our designs are used in a wide range of applications from test instrumentation to infrastructure equipment and very high volume consumer wireless devices. We have in-house test facilities for both bare die (RFOW) and SMT packaged components. Our microwave and mm-wave module development activity encompasses a wide range of technologies including conventional SMT on laminate substrates, High Density Interconnect (HDI), chip and wire, thin film, thick film and LTCC. Plextek RF Integration is part of the Plextek Group.
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