Glass is good for more than just windows: in recent years, the material has attracted attention for its dielectric properties and cost-effectiveness as a high-frequency-circuit substrate. It features low circuit losses over wide frequency and temperature ranges and is suitable for circuit fabrication using standard integrated-circuit (IC) planar manufacturing methods.

When evaluated for several high-frequency filter topologies, the performance of glass-based components has been shown on a par with other traditional microwave-integrated-circuit (MIC) substrate materials. Using commercial glass substrates, the authors designed and fabricated two filter topologies: a single parallel-coupled-line topology operating at C-band frequencies and a wide-bandwidth bandpass filter designed with anti-parallel coupled lines.

Although the concept of multichip module (MCM) technology makes sense for a great many systems and products because of its potential for high performance and high packaging density, the technology still has limitations. For example, single-chip solutions for RF and microwave front-ends still do not support complete system integration. External components are still required for impedance matching to the system characteristic impedance (usually 50 Ohms) as well as for power switching in phased-array antennas, and for forming resonant circuits for bandpass and other types of filters as well as for signal attenuation and processing at high signal frequencies.

Furthermore, the problems of substrate coupling (manifested as crosstalk or as a noise coupling especially in mixed-signal circuits) become more apparent and troublesome with increased integration. It is often preferable to integrate discrete components with high-quality-factor (Q) characteristics, such as waveguides and dielectric resonators (DRs), into a component package instead of on-chip passive components because of the improvements in performance possible with those discrete components. And in compact packaged multifunction components, glass and quartz substrates are viable materials for attaining high performance levels within a small package while also being extremely cost-effective.

Glass substrates have captured the imagination of high-frequency circuit designers for their low loss over wide operating frequency and temperature ranges. A number of different RF integrated passive devices (RFIPDs) have been fabricated on glass substrates for applications in wireless communications. ¹ In addition, glass substrates have been used in the fabrication of multifunction integrated circuits (ICs) that combine both analog and digital functions, and for high-frequency sensors, and for microelectromechanical- systems (MEMS) designs. Glass substrates support the micromachining of structures with well-defined features for use in passive components such as filters at high frequencies. Due to its low dielectric constant, glass substrates offer ease of fabrication of high-frequency devices and circuits at RF and microwave frequencies. Glass features excellent dimensional stability with temperature compared to traditional dielectric materials along with tightly controlled sheet flatness for high circuit and device yields even at high frequencies. Signal losses associated with glass substrates are 12 dB/cm lower than those of CMOS-grade silicon substrate materials. As a result, it is possible to fabricate a 4-b phase shifter on glass substrate that exhibits average loss of only 2.7 dB at 78 GHz.²

To demonstrate the capabilities of commercially available glass substrates for high-frequency designs, two different C-band filter topologies were realized on glass substrates. The two different filters yielded better than 2.5-dB insertion loss and better than 15-dB return loss at C-band. Of the different bandpass filter topologies considered for fabrication on glass substrate, the parallel-coupled topology with half-wave- length line resonators is the simplest.³ For realizing a single-pole filter at X-band, key parameters were determined by means of the insertion-loss calculation method and standard equations. The line length for this filter was determined to be one- quarter wavelength (?_g/4) at the frequency of interest. A line length optimization performed for open-ended discontinuities reduced the line length further by an amount equal to ?L as given in Eq. 1,⁴

where
L = the filter line length (in mm),
C₀ = capacitance of the line section,
Z_oo = the odd-mode impedance of the line section,
Z_oe = the even-model impedance of the line section,
e_e = the effective dielectric constant of the substrate material, and
e₀ = the measured dielectric constant of the substrate material.

The coupled-line section shown in Fig. 1 can be represented by the impedance inverter circuit shown in Fig. 2, where
? = the phase,
J = the impedance inversion function, and
Z₀ = the characteristic impedance.

The even- and odd-mode impedances can be found by solving first for Eqs. 2 and 3,

where f_bw = the fractional bandwidth, _Jn,n+1 = the characteristic admittance of the J-inverter, and Y0 = the characteristic admittance of the terminating lines, and then applying Eqs. 4 and 5.

The even and odd mode impedances work out to be 53 Ohms and 48 Ohms, respectively, for a single coupled section having 0.2 dB ripple. Using these values, the corresponding microstrip width (W) and gap (g) can be computed for the first bandpass filter topology on glass substrate.

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