ADVERTORIAL TECHNOLOGY IN ACTION A world-first in microwave tuning
Achieving higher stability over time and extremes of temperature has been made easier with new developments in microwave tuning screws used in Space and other harsh environments, explains Roger Tall, product Specialist at Charcroft Electronics
here is a limit beyond which is it impossible to push the physical capability of any material. For brass, that limit has been reached with the Ka, Ku and Q-band working frequencies and the increasingly miniaturised cavity filters used in the latest Space applications.
T
This was the challenge addressed by Exxelia Temex when the company began to research the possibility of manufacturing tuning screws in 100% Invar which is known for its exceptionally low coefficient of thermal expansion (CTE or ) and stability under extreme temperature conditions.
Invar in high-frequency Space applications
The development of 100% Invar high-frequency tuning screws began with the selection of a very specific alloy of 64% iron (Fe) and 36% nickel (Ni), known as Invar-36. At this ratio, Invar produces the lowest coefficient of thermal expansion and the highest stability of 1.1 ppm K–1 between 0°C and 100°C, as shown in Figure 1. This makes Invar-36 around 17 times more stable than brass.
A breakthrough in brass
However, whilst brass is soft and easy to machine, Invar-36 is extremely tough and has always been considered impossible to machine, especially at the level of miniaturisation demanded by high-frequency Space applications.
So each aspect of the conventional manufacturing processes used for brass screws was assessed and re-considered to create an Invar tuning screw. After significant research it was discovered that, instead of the single machining process required for a brass tuning screw, an Invar screw needed to be taken through up to seven separate machining processes. This not only enabled the screw to be produced in 100% Invar, as shown in Figure 2, but guaranteed a perfect thread quality. It also met the significantly higher precision and stability demanded for Space-grade systems, including the option of gold plating.
The use of Invar is not the only breakthrough in high-frequency tuning screws: even conventional brass screws have evolved to provide higher levels precision for setting cavity waveguides in applications such as aeronautics, defence, rail and telecommunications. The introduction of a new technique to solder the rod of the tuning element inside the rotor has helped to increase the stability of these commonly used screws. Although a relatively simple concept, the implementation proved to be highly complex and required the development of specialised manufacturing tools.
In the new process, the rotor is inserted into the bushing and the rod is then soldered into the rotor after the gold-plating has been applied to the surface of the screw. The rotor, rod and bushing of a brass tuning screw are shown in Figure 3.
New materials & processes deliver new benchmarks Fig 1
Whether they are manufactured in Invar-36 for high-frequency Space applications, or in gold-plated brass for less demanding applications, tuning screws are now able to offer a lower thermal coefficient of expansion and higher stability. Inserted into microwave cavities, these screws provide the capacitance, resistance and inductance characteristics which can be used in conjunction with dielectric resonators, or in hybrid screw/resonator assemblies, to achieve stable filter regulation.
www.charcroft.com/ExxeliaTemex
Fig 2
Fig 3 Self-locking screws increase stability
Having created the world’s first 100% Invar tuning screws, the development process was extended to introduce a self-locking mechanism to further increase the stability of the screw.
The self-locking mechanism is created using two threaded segments separated by two parallel slots. After cutting both parallel slots, the rotor is compressed lengthwise to create a plastic deformation in the screw which causes the two threaded segments to be off-set. This generates a constant tensile stress in the rotor from the moment threaded segments are screwed.
The spring effect created by the self-locking system delivers controlled torque which eliminates the need for an additional locking nut, in addition to ensuring a high electrical contact. The self-locking mechanism also enables a reduction in size and increases precision by minimising slack while tuning. The addition of a brass bushing, machined to provide an exact fit with the Invar rotor, can be used to ensure the best setting drift to allow for tight control of the screwing torque.
Charcroft Electronics Ltd. 01591 610408
www.charcroft.com
34
OCTOBER 2016 | ELECTRONICS
/ ELECTRONICS
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64
