Suspended Substrate Stripline Filters Offer Optimal Selectivity vs. Microstrip, Dielectric Stripline or Cavity Filters
In a recent Tech Brief published by ERZIA, we explore the various advantages offered to microwave circuit and assembly designers by air cavity based, suspended substrate stripline filters (SSSFs). In this blog we provide the highlights of our Tech Brief as an introduction to the technology and how it has been proven to be the most optimal when frequency selectivity is a critical design/performance factor, such as in high-performance duplexers used in antenna systems and RADAR arrays, as examples.
RF/microwave filters are key components in nearly every microwave system design from mobile networks to the most sophisticated military radar. In these applications, a necessity arises to routinely select and pass through a desired signal while rejecting specific unwanted frequencies. Each filter technology can offer an advantage in terms of insertion loss, selectivity, size, fractional bandwidth, temperature stability, power handling and repeatability. For the purpose of comparison, we’ll call the technologies being compared SSSF, microstrip, dielectric stripline and cavity. (Comparison to waveguide is also included in the Tech Brief.)
Various Microwave Filter Technologies Overview
SSSFs consist of transmission lines printed onto both sides of a thin substrate, which is, in-turn, suspended in air between two ground planes. The air dielectric is the key to this technology’s selectivity performance, durability and application versatility.
Microstrip technology is a category of electrical transmission line which utilizes the printed circuit board technology for its fabrication process, through which the microwave signals are communicated. It is often used to design and fabricate RF and microwave components such as directional coupler, power divider/combiner, filter and antennas.
Stripline is another type of transmission line that can be easily built on a circuit board. It is identical to microstrip, but with ground planes both above and below the trace. Striplines are most often used for either high- or low-level RF signals requiring isolation from surrounding circuitry.
A cavity filter is a resonator inside a conducting “box” with coupling loops at the input and output. Cavities are usually constructed as cylinders, with an axial tuning capacitor. They’re typically packaged with microwave coaxial connectors.
Why an SSSF?
The SSSF configuration constitutes an air stripline in which most of the electric fields propagate through air rather than the supporting substrate, enabling higher unloaded quality factors as compared to other planar technologies, such as microstrip or dielectric stripline. SSS filters are therefore more stable over temperature since the critical sections are realized in air, so the substrate material’s temperature has a negligible effect. The result is a wide available operating temperature for these devices. The range of impedances is also wider than either microstrip or dielectric stripline, which allows both lower-loss (wider) lines and higher fractional bandwidths. Moreover, lower band-edge insertion loss is obtained for a same skirt selectivity thanks to the use of the generalized Chebyshev prototype. This response has the additional advantage of allowing high rejection levels closer to the passband. Plus, thanks to the shielded structure, RF leakage is minimized.
ERZIA SSS filters are suited for military environments as these components have been successfully tested under shock and vibration conditions. SSS filters can be directly integrated in a system, or shipped as stand-alone connectorized modules.
The main advantages of SSS filters are summarized as follows:
• Low loss
• Temperature stability
Overview of testing/comparison vs. alternative microwave filter technologies
Broadband (Wideband) Filters
Three examples of broadband filters were tested and observed. Two of them were lowpass filters with a cut-off frequency of 10 and 18 GHz, respectively and the third one was a highpass filter from 18 to 40 GHz. All of them featured a multi-octave flat response, very low insertion loss and sharp band edges, providing a rejection above 80 dB.
One lowpass filter featured low insertion loss together with high selectivity (70 dB rejection at 0.85 GHz above the cut-off frequency). The 70 dB minimum rejection level is demonstrated to be maintained along the stopband (from 11 to 14 GHz). The closest filter found in the market (not SSS) is based on coaxial cavity technology. Both show the same cut-off frequency and offer comparable return loss. The alternative features a slight advantage on insertion loss (IL) and has a more compact size, although the length is similar. However, the SSS filter features an unbeatable selectivity with a sharp band edge showing 50 dB rejection at just 0.7 GHz above the cut-off frequency.
A 17.5 to 41 GHz high pass SSS filter, demonstrated a 34 dB rejection at 1.5 GHz below the cut-off frequency, while the alternative (cavity) takes another extra 1.5 GHz to meet this rejection level.
Two examples of narrowband SSS filters were tested next and compared with counter-parts implemented in other technologies. This time the frequency of operation was relatively low in contrast to the wideband filters presented in previous section, in order to explore the other extreme of the SSS range of operation.
The first example was a C-Band bandpass filter (ERZ-BPF-0350-0380-2.4), centered at 3.65 GHz with a fractional bandwidth of 8.2 percent. The filter provides sharp band edges with exceptional selectivity. The closest alternative found was a waveguide filter. It is important to remark that the center frequency and passband have been normalized for comparison. Actual data of the alternative filter can be gathered from the Tech Brief. (Notably, the waveguide filter gives better response in terms of selectivity but at the expense of a much bigger size and weight.)
The second example was an L-Band Bandpass filter from 1.9 to 2.1 GHz. In this case, the alternative was a cavity filter, which featured slightly better IL but worse selectivity. Both options were similar size.
Performance of SSS filters over temperature
Temperature stability is crucial for reliable units which might be used in extreme environments or hi-rel applications such as radar, EW and satcoms. The performance of a lowpass filter from 0 to 2.5 GHz (ERZ-BPF-0000-0250-1.3) over temperature was presented. There was no significant variation in the performance at the extreme temperature values. A small frequency shift was observed: +10 MHz at +85°C and -20 MHz at -40°C. This example shows the thermal stability of SSS designs and can be extrapolated to all the examples presented before.
The temperature variation of alternative technologies is comparatively very wide and technology dependent.
Other characteristics like power handling or comparative temperature stability responses were not evaluated in detail and might be the subject of another tech brief and blog.