Fixed Frequency Analog Filters

MEMS Filters

Microelectromechanical systems (MEMS) technology was developed for switching elements in the microwave range and is used for switches, tunable capacitors and inductors with dimensions on the order of of 10s of nanometers. MEMS technology allows implementation of small-size microstrip devices with switching or tunable parameters stable to the influence of the environment. RF MEMS resonators offer the potential of on-chip integration of high-Q resonators and low-loss bandpass filters. The Q-factor of RF MEMS resonators is in the order of 100 to 1000. With the help of MEMS technology chains of coupled resonators are constructed to form bandpass or band-reject filters. The capability of electrical position control of the microelectromechanical cantilever is used for smooth or discrete tuning of the central frequency or bandwidth.

Sandia National Laboratories has developed an aluminum nitride process for fabricating RF MEMS microresonators at frequencies ranging from 1 MHz to 3 GHz. The resonant frequency is determined lithographically. This process uses the same equipment and materials that were developed to fabricate film bulk acoustic resonators that are widely used to implement cellular phone duplexers and filters at 1.9 GHz. The piezoelectric transduction mechanism of these resonators allows the realization of low insertion loss filters. It is this technology that allows the scaling of MEMS resonators without introducing spurious modes or reduction in quality factor, and with acceptable power handling for both the transmit and receive paths in full-duplex radios. This technology is most suited for realizing resonators with the quality factors approaching 5000 and impedances less than 300 Ω.

Memtronics, for example, produces filters based on RF MEMS technology providing the ability to create low-loss, high-linearity tuning while simultaneously reducing volume, weight, and parts count (see Figure 11). A four-pole filter centered at 11.8 GHz with a 5.1 percent bandwidth (11.5 to 12.1 GHz), exhibits a nominal 1.1 dB insertion loss over the passband. Spurious rejection is greater than 35 dB. The unloaded quality factor Q is greater than 450. Performance over temperature is stable, with Δf ~ 3.5 MHz from 25°C to 125°C. The small weight, waveguide filter chip is 12 mm × 35 mm × 0.8 mm and weighs 0.73 g.

RF MEMS switches, switched capacitors, and varactor technology offer the tunable filter designer a compelling trade-off between insertion loss, linearity, power consumption, power handling, size and switching time.

HARMONIC FILTERS

To satisfy RF transmitter electromagnetic compatability requirements in accordance with MIL-STD-469, it is necessary to reduce the spurious radiation of frequencies that are multiples of the fundamental. Harmonics, which are integer multiples of the fundamental (e.g. second and third harmonics) are the most troublesome. To suppress them, we can use band-reject and notch filters (BRF); however, harmonic filters in the form of LPFs, in which the cutoff frequency is 30 to 50 percent higher than the fundamental frequency, are simpler and more compact. One should consider the frequency response of such a filter over a wide frequency band exceeding the operating frequency and to take into account the handling power of the fundamental. Figure 12 shows the attenuation curve of a typical harmonic filter at the fundamental and its harmonics. Some serial harmonic filters produced for specific values of carrier frequency also include notch (rejection) filters tuned to increase attenuation at specific harmonics.

Harmonic LPFs are intended for suppressing emission of the higher harmonics of broadcast transmitters, VHF and UHF TV transmitters, and base stations for cellular communication (see Table 7). They are characterized by low passband loss, high suppression at the second and third harmonics, and high power handling.

When selecting the type of LPF harmonic filter, it is necessary to check high-frequency interference suppression up to the centimeter wave range. With increasing frequency and at increased rated capacitance of the noise-suppression filter, intrinsic resonance and reduced dielectric permeability become issues. Waveguide implementation (see Figure 13) provides a high level of attenuation at higher harmonics with high power handling capability.

FREQUENCY MULTIPLEXERS AND DUPLEXERS

Multiplexer

The frequency multiplexer is a frequency-separating device with n outputs having different frequency passbands and stopbands. The number of output channels distinguishes it as a diplexer (n = 2), a triplexer (n = 3), a quadriplexer at (n = 4), a quantuplexer (n = 5) and so forth (see Table 8). In most cases, frequency multiplexers possess the property of reciprocity (i.e., they may be used both for signal frequency separation and for combining of two or more signals).

To improve isolation of input and output ports and to equalize reflection coefficients over the wide frequency range of input signals, an LPF and HPF with appropriate adjustments are included in the diplexer structure. Bridge-balanced circuits and polarization solutions are used to improve mutual channel isolation.

A frequency diplexer (see Figure 14) is the simplest form of a multiplexer, which can split signals from one common port into many different paths. The incoming signals must be offset in frequency by an appreciable percentage, so that filters can do their job sorting them out. A diplexer could be used to route signals to two different receivers, based on frequency, or it could be used to create a matched filter that is nonreflective outside of the intended passband. It could also be used as a bias tee, to feed an active device with DC power.

Duplexer

A duplexer is used with wideband antenna for simultaneous transmission and reception in frequency-shifted bands (i.e. duplex communication). As such, it is necessary to separate the transmit signal applied to an antenna from the receive channel, protecting the receiver input circuit (see Figure 15). Along with nonreciprocal or directing devices such as directional couplers, ferrite isolators and circulators, filters are used to prevent the passage of transmitter power to the input of the low-noise amplifier.

Many companies offer duplexers with different frequency ranges (see Table IX) to connect a common antenna with a receiver and a transmitter. To improve isolation between the receiver input signal and the more powerful signal from the  transmitter in a closely located frequency band, duplexers have several sections (as many as eight).

FIXED FREQUENCY FILTER ASSEMBLIES

Complex integrated microwave filter assemblies are designed for numerous and varied applications. Among them, for example, are multiband frequency multiplexers, filter banks, filters for automatic recognition of a frequency band, filters for the extraction of a frequency band with narrowband rejection of a frequency zone inside of it. Many, which are classified by the manufacturers as frequency assemblies, may include parameter control (e.g. digitally tuning, auto tuning, switching, tracking agility).

Related Publications:

1. H. Campanella, Acoustic Wave and Electromechanical Resonators: Concept to Key Applications, Artech House, Norwood, MA, 2010.

2. K. Hashimoto, RF Bulk Acoustic Wave Filters for Communications, Artech House, Norwood, MA, 2007.

3. S. I. Baskakov, Radio Engineering Circuits and Signals, 3rd ed., Vysshaya Shcola, Moscow, 2000 (in Russian).

4. W. Siebert, Circuits, Signals and Systems, McGraw-Hill, 1986.

5. J. -S. Hong, Microstrip Filters for RF/Microwave Applications, John Wiley & Sons, Hoboken, NJ, 2011.

6. Pozar, D. M., Microwave Engineering, 3rd ed., John Wiley & Sons, Hoboken, NJ 2005.

7. G. M. Rebeiz, RF MEMS Theory, Design, and Technology, John Wiley & Sons, Hoboken, NJ 2003.

8. T. C. N. Clark, “MEMS Technology for Timing and Frequency Control,” IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, Vol. 54, No. 2, February 2007, pp. 251–270.

9. A. Abramovicz, “Modeling of Wide Band Combline and Interdigital Filters,” Microwave Review, July 2010, pp. 15-22.

10. A. Tombak, J. P. Maria, F. T. Ayguavives, Z. Jin, G. T. Stauf, A. I. Kingon and A. Mortazawi, “Voltage-Controlled RF Filters Employing Thin-Film Barium-Strontium-Titanate Tunable Capacitors,” IEEE Transactions on Microwave Theory and Techniques, Vol. 51, No. 2, February 2003, pp. 462-467.

11. J. Brank, J. Yao, M. Eberly, A. Malczewski, K. Varian and C. Goldsmith, “RF MEMS-Based Tunable Filters,” International Journal of RF and Microwave Computer-Aided Engineering, Vol. 11, No. 5, September 2001, pp. 276–284.

12. K. Entesari and G. M. Rebeiz, “A 12–18 GHz Three-Pole RF MEMS Tunable Filter,” IEEE Transactions on Microwave Theory and Techniques, Vol. 53, No. 8, August 2005, p. 2566-2571.

13. D. Scarbrough, C. Goldsmith, J. Papapolymerou and Y. Li, “Miniature Microwave RF MEMS Tunable Waveguide Filter,” European Microwave Conference, September 2009, pp. 1860–1863.

14. M. S. Aftanasar, P. R. Young and I. D. Robertson, “Rectangular Waveguide Filters Using Photoimageable Thick-Film Processing,” 32^nd European Microwave Conference, September 2002.

15. K. M. Lakin, “A Review of Thin-Film Resonator Technology,” IEEE Microwave Magazine, Vol. 4, No. 4, December 2003, pp. 61-67.

16. J. S. Kim, K. B. Lee, J. Y. Lee and H. Shin, “A Broadband Suspended Substrate Stripline Filter Using Dual-Mode Resonator,” Microwave and Optical Technology Letters, Vol. 53, No. 7, April 22, 2011.