Advantages of Suspended Substrate Stripline Microwave Filters

Introduction

Filters are key components in any RF/microwave system. From mobile networks to the most sophisticated military RADAR, every single device needs at some point to select or reject specific frequencies of the RF spectrum. The different technologies used to implement this functionality have evolved along with RF technology. Several technologies are available today to implement different types of filters, each offering advantages in terms of insertion loss, selectivity, size, fractional bandwidth, power handling, and more.

Description

Air is a good choice for implementing a filter: it features almost no dielectric loss and a stable dielectric constant over temperature. Suspended Substrate Stripline (SSS) consists of transmission lines printed onto both sides of a thin substrate, which is suspended in air between two ground planes.

This 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 compared to other planar technologies such as microstrip or dielectric stripline. SSS filters are more stable over temperature since the critical sections are realized in air, so the dependence of the substrate material on temperature has a negligible effect, resulting in very small temperature dependence for the device.

From a realization standpoint, the range of impedances is wider than either microstrip or dielectric stripline, allowing both lower-loss (wider) lines and higher fractional bandwidths. Lower band-edge insertion loss is also obtained for the same skirt selectivity thanks to the use of the generalized Chebyshev prototype, which allows high rejection levels closer to the passband. Thanks to the shielded structure, RF leakage is minimized and hermetic designs are assured.

SSS filters are well suited to military environments, having been successfully tested under shock and vibration conditions. From a packaging perspective, they can be directly integrated into a system or shipped as stand-alone connectorized modules for flexibility. Finally, the performance of SSS filters is highly repeatable.

Main advantages of SSS filters

• Selectivity
• Size
• Low loss
• Temperature stability
• Repeatability

Performance

Figure 3 shows the approximate range of center frequency and fractional bandwidth (FBW) for different filter technologies, such as waveguide or dielectric resonators. This is indicative for illustrative purposes and not strict limits. As shown, SSS offers a competitive solution for both narrow and broadband designs at frequencies ranging from 0.5 to 50 GHz, covering areas traditionally reserved for waveguide and dielectric resonators.

Broadband Filters

Three examples of broadband filters are presented below: two lowpass filters with cut-off frequencies of 10 and 18 GHz, and a highpass filter from 18 to 40 GHz. All feature a multi-octave flat response, very low insertion loss, and sharp band edges providing rejection above 80 dB. Each has been compared with the most performant market alternative found matching the frequency response and cut-off frequency.

Lowpass Filter from 0 to 10 GHz

The first example is ERZ-LPF-0000-0100-1.5.

The filter features low insertion loss together with high selectivity (70 dB rejection at 0.85 GHz above the cut-off frequency), maintained from 11 to 14 GHz. The closest alternative found on the market is based on tubular (coaxial) technology. Both share the same cut-off frequency and comparable return loss; the alternative has a slight edge on insertion loss and a more compact size, though similar length. However, the SSS filter delivers unbeatable selectivity, showing 50 dB rejection at just 0.7 GHz above the cut-off frequency.

Lowpass Filter from 0 to 18 GHz

The second example of a wideband filter is ERZ-LPF-0000-1800-2.4.

Both low insertion loss and return loss can be observed, along with a sharp band edge. The performance of a commercial planar filter with a similar cut-off frequency is shown for comparison; both filters are similar in size. Again, the selectivity of the SSS filter is a clear advantage.

Highpass Filter from 17.5 to 41 GHz

A third example is ERZ-HPF-1750-4100-2.0.

The SSS filter achieves 34 dB rejection at 1.5 GHz below the cut-off frequency, while the alternative (a cavity filter) requires an additional 1.5 GHz to meet the same rejection level. It was notably difficult to find a comparable filter for such a wide bandwidth, with a cavity filter being the only result found.

Conclusions for Wideband Filters

Three wideband SSS filters were compared against alternative technologies. At lower operating frequencies, the filters show similar performance and size, with the SSS filter clearly superior in selectivity. At higher operating frequencies up to 40 GHz, no waveguide counterparts were found — only a cavity filter — and again the SSS filter delivered better selectivity.

Narrowband Filters

Two examples of narrowband SSS filters are presented below and compared with counterparts implemented in other technologies, exploring the lower end of the SSS frequency range.

Bandpass Filter from 3.5 to 3.8 GHz

The first example is a C-band bandpass filter (ERZ-BPF-0350-0380-2.4), centered at 3.65 GHz with a fractional bandwidth of 8.2%.

The alternative cavity filter features slightly better insertion loss but worse selectivity. Both options are similar in size.

Bandpass Filter from 1.9 to 2.1 GHz

The second example is an L-band filter.

Conclusions for Narrowband Filters

Two narrowband SSS filters were compared with alternative counterparts. Similar selectivity was achieved at a much smaller size compared to the waveguide alternative, and higher selectivity at a similar size compared to the cavity alternative.

Performance of SSS Filters Over Temperature

Temperature stability is crucial for reliable units used in extreme environments or heavy-duty applications such as RADAR, EW, and telecommunications. This section presents the performance of a lowpass filter from 0 to 2.5 GHz (ERZ-BPF-0000-0250-1.3) at +25°C, +85°C, and -40°C.

There is no significant variation in performance at the extreme temperature values. A small frequency shift can be observed: +10 MHz at +85°C and -20 MHz at -40°C. This demonstrates the thermal stability of SSS designs and can be extrapolated to all the examples presented above. The temperature variation of alternative technologies is much wider and highly technology-dependent.

Conclusion

A general description of SSS microwave filters has been presented, along with their main advantages and features. Five different SSS filters were compared to alternatives based on different technologies.

For wide frequency bands, SSS filters offer unbeatable selectivity while maintaining similar size and insertion loss compared to cavity and coaxial alternatives. At higher frequencies, other wideband alternatives exist but with less selectivity. Narrowband SSS filters are a strong option in terms of insertion loss and selectivity compared to cavity filters, though waveguide filters remain slightly better at the expense of significantly larger size.

Power handling and detailed comparative temperature stability were not evaluated in depth and may be the subject of future work. The power handling of the SSS filters presented is in the range of 10–15 W CW.

Next Steps

The datasheets of the filters used in this brief:

• ERZ-LPF-0000-1000-1.5
• ERZ-LPF-0000-1800-2.4
• ERZ-HPF-1750-4100-2
• ERZ-BPF-0350-0380-2.4
• ERZ-BPF-0190-0210-1.8
• ERZ-LPF-0000-0250-1.3

ERZIA catalogue of filters — Suspended Substrate RF/Microwave Filters

ERZIA is under constant review of the state of the art to design new reliable and performant amplifiers. If you don't find what you're looking for in the catalogue, contact us at sales@erzia.com.

References

1. J.E. Dean, "Suspended substrate stripline filters for ESM applications," IEE Proceedings F (Communications, Radar and Signal Processing), Vol. 132, Issue 4, July 1985, pp. 257–266, DOI: 10.1049/ip-f-1.1985.0059.
2. N. Lioutas, "Design of generalised Chebyshev suspended substrate stripline filters," April 1986.