In the previous article, we learnt how microwave filters are used in military around the world in the form of electronic warfare. Today, lets look at how microwave filters are used in satellites.
The emergence of satellite technologies in the 1970s has further driven the needs for microwave filters after the increase in importance during WW2. Unlike its usage in electronic warfare, the specification and requirement for in satellite communication are mainly to tackle the need for long distance broadcast. This need had further driven the innovation in filter theories and realizations in comparison to during WW2, where most of the filter realizations are based on relatively straightforward circuit theories. One example is an advanced theory based on forming coupling matrix when designing filters. In another word, the theory revolves around manipulating the interactions between the many stubs and bars found in a filter for a performance more suitable for satellite applications.
Amplitude Approximation Vs Group Delay
Signal takes time to travel from point to point. Usually it happens so fast that the delay is negligible. However, when coming to long distance satellite communication with signals needing to travel through multiple layers of atmosphere, the signal delay factor become more significant. This means that by improving the signal delay performance as well as the linear phase of the signal will directly improve the quality for long distance communication in satellites. Due to this requirement, it poses another challenge on filter design in which phase and group delay performance are to be considered besides the amplitude response. Before this, most filters were realized based on amplitude approximations such as Chebyshev, Elliptic or Generalized Chebyshev functions as described in previous articles. These approximations were limited to achieve desired selectivity and stopband rejection level however it did not address the phase and group delay issues.
It is also important to note that selectivity performance contradicts with the phase linearity performance. Hence a trade off must be achieved in order to satisfy both requirements. The solution to this is to introduce transmission zeros at imaginary frequencies. To keep it simple, the filter structure will be much more complicated and therefore the simple circuit synthesis theory is no longer sufficient. It was then the first coupling matrix synthesis was introduced that resulted in a new class of waveguide filters being introduced. Nowadays, waveguide filters are designed and implemented in every satellite payload all around the world.
Another important aspect is the size and weight of satellite payload. The increase in size would also mean an increase in cost definitely! Besides, there is only so much space you can have in a satellite. This means the big number of filters attached to to handle the tremendous amount of throughput and channel would have to be as light weight as possible. This pushes for miniaturization when coming to filters in satellite application, especially on its resonators. Again, the waveguide perform well in miniaturization. The waveguide cavity can exhibit 2-3 resonant modes which gives a size reduction of more than 50%.
To summarize it all, with the emergence of satellites in communication system, filter technology continues to evolve to suits its requirements. These requirements help push for time delay performance as well as miniaturization technology in filters.
I will discuss more applications for the 80’s onward in next article and how microwave filters again played an important role in our lives. Stay tuned.