The Best of Both Worlds
To those actively engaged in microwave and RF design and manufacture the relative merits of solid state versus vacuum tube technology is, remarkably, still open to lively discussion.
Some forty or so years ago anyone would be forgiven for assuming that the then new, ubiquitous solid state technology would rapidly make the ‘old and faithful’ vacuum tube well and truly redundant. Has that happened? Well, not quite! The fact is that although solid state technology in the microwave industry has made tremendous strides forward, the vacuum tube can still offer the designer some significant advantages.
Solid state would seem be the only route to take – with several benefits such as graceful degradation (no single point of failure - as can be experienced with a vacuum tube) and cost effectiveness being high on the list. But when considering products for operation at higher frequencies and higher power, a few cracks appear in the normally attractive solid state approach – with lower efficiency driving larger, heavier cooling solutions or leading to overheating and subsequent component failure among the potential problems. Tubes can still come up trumps and offer an attractive solution when operating in higher frequency or higher power systems when efficiency and total system weight are critical to the application. The use of multi-tube design, as just one example, is very effective when dealing with the single point failure issue.
Covering All Needs
The continuing on-going developments and advancements in both solid state and vacuum tube technology have reinforced preferences and commitments to adherents in both camps. As a world leader in the design and manufacture of professional microwave products for radar, EW and communications, TMD Technologies (TMD) is well equipped to recognise and evaluate the merits of both sides of the debate and is currently operating a very successful ‘sitting on the fence’ policy with regards to the microwave products it offers to its customers. We take the view that travelling wave tube (TWT) and solid state technology can live happily together, with each offering significant designer and user advantages and more choice for different applications – providing the best of both worlds!
In keeping with its philosophy of embracing both technologies, TMD has now introduced a new range of advanced TWT based microwave power modules (MPMs) and a new sophisticated GaN MMIC based solid state power amplifier (SSPA): designated the PTXM Series and the PTS6900 respectively.
TWT Based MPMs Offer Highest Power Density
The result of many years’ experience in the design and manufacture of high frequency, high power TWT based MPMs, the state of the art PTXM Series from TMD offers possibly the highest power density on the market.
By incorporating mini TWTs and employing extremely efficient packaging these ultra compact units, weighing typically only 1.7 kg, are ideal for airborne radar, EW and communications applications - particularly in UAVs, where low volume and weight are critical considerations. Moreover, the exceptionally low weight and small size (190 x 120 x 30 mm typically) has been achieved with no reduction in RF power or reliability! This high power-to-volume feature is the result of superior packaging techniques to efficiently use all available space.
To cover a range of application requirements, TMD is currently offering three PTXM models: namely the PTXM1000, PTXM1001 and PTXM1002, in total covering the frequency band 4.5 to 18 GHz and with output powers from 100 to 140 W.
Fully integrated and modular design
Because of TMD’s cost effective fully integrated, modular concept the PTXM units are easier to manufacture and test - offering higher performance, extreme reliability and safe and simple installation at a lower price.
Each PTXM unit integrates a mini TWT and an optimised high density switched mode power supply to produce a single ‘drop in’ microwave amplifier block. Included are a solid state pre-amplifier and/or lineariser, depending on model or application. Moreover, the system designer’s task is made easier by the integration of the TWT and its high voltage power supply – eliminating the TWT interconnect along with associated reliability and safety hazards. This integration also allows reduction in system size, simplifying installation.
All units feature excellent thermal management and high electrical efficiency, with minimum cooling required over a wide operating temperature range - significantly contributing to the exceptional reliability.
No user adjustments required
To optimise TWT performance the MPM adjustments are set at the factory. This not only eliminates the need for user adjustment, but also makes any field replacements easier and quicker.
The versatile configuration of the MPMs means that a range of different TWTs can be incorporated, allowing the customer to specify frequency and peak power. The range also covers models with a number of input power supply options including 28 V DC, 270 V DC, or 115 V AC.
A control interface provides remote operation and status monitoring, making available diagnostic outputs for BIT purposes.
Withstanding the harshest conditions
A fully encapsulated high voltage section allows the PTXM units to operate in the harshest of environments - at high altitudes and humidity, and surviving high levels of vibration and shock.
To sum up…
The aerospace and defence market continues to demand higher frequency and power at the lowest possible weight and volume. In high power, or high frequency applications tube based solutions still have the advantage. TMD continues to invest in engineering development to remain at the forefront of this technology.
However at lower frequency and for certain applications, solid state can be the better choice of technology:
Solid State MPM Employs GaN MMIC Technology for Top Performance and Reliability
The new PTS6900 MPM is a fine example of TMD’s growing solid state design and manufacturing expertise. Employing the latest advances in
0.25 µm GaN MMIC technology and low loss combiners the compact, low weight PTS6900 covers the 2 to 6 GHz frequency band at 150 W RF output power, and has a gain of 55 db adjustable from 50 to 60 dB. It has been optimised for use in EW/ECM systems, but is also suitable for other high power microwave applications where volume and weight are critical requirements.
The inherent high reliability of the PTS6900 offers a predicted MTBF of over 30,000 hours for an airborne uninhabited fighter environment, at the maximum allowable baseplate temperature. Moreover, the module will survive an infinite output load VSWR at the full rated RF output power.
Also, the new MPM is entirely ITAR free. Since most similar competitor-manufactured units are from the USA, this gives non-US users the significant benefit of an alternative European ITAR-free source.
Fast RF mute capability
As well as providing an instant start up, the PTS6900 allows the RF output to be muted in less than 1 µs - making it ideal for EW applications.
The PTS6900 can be supplied with a heatsink, but is not offered as standard so that customers have the option of using their own cooling fixture. Provided that the baseplate hot-spot temperature of +85 ºC is not exceeded the module will operate reliably in high ambient temperatures.
The unit offers good gain flatness without the need to incorporate a gain equaliser and is internally temperature compensated to maintain its performance across the whole operating temperature range.
Built-in testing
A comprehensive built-in test capability includes monitoring of the internal DC to DC converters and module baseplate over-temperature. An RF sample output is included to aid testing and integration of the module.
The PTS6900 is a very compact measuring just 325 x 200 x 50 mm at 4.5 kg. It operates from a power supply of + 28 V DC (optional + 270 V DC) as a standard primary source, with a maximum input power requirement of only 850 W. To ensure full compliance with the EMC requirements of MIL-STD-461F an internal filter is incorporated on the power supply input.
To sum up…
When comparing the solid state PTS6900 with a tube based solution in the same frequency range, it has the advantages of an instant on time; lower output noise; fast mute time; and will continue to function with inadvertent output mismatches (VSWR) while providing high reliability and a long service life.
