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LTCC Leads To Tiny 90-deg. Splitter

This multilayer circuit-fabrication technology allows these traditionally large high-frequency passive components to be made in miniature, low-cost formats.

Low-temperature-cofired-ceramic (LTCC) technology makes possible multilayer circuits with active and passive components occupying a fraction of the volume of conventional planar high-frequency circuits. Mini-Circuits' engineers applied their Blue Cell™ LTCC fabrication techniques to a pair of 90-deg. splitter designs to develop low-cost packaged components measuring just 0.15 × 0.15 × 0.027 in. (0.381 × 0.381 × 0.69cm) but with high isolation and low insertion loss from 1200 to 2500 MHz.

The two new 90-deg. splitters are the models QCC-20 (Fig. 1) and QCC-22, with frequency ranges of 1200 to 2200 MHz and 1500 to 2500 MHz, respectively. These components are also known as quadrature couplers due to the 90-deg. difference in phase between the two output ports. This type of splitter is generally characterized by means of several parameters, including isolation between ports, insertion loss, phase unbalance between output ports, amplitude unbalance between output ports, VSWR or return loss, and power-handling capability.

The isolation between a two-way power splitter's output ports is evaluated by measuring the attenuation between the two ports when the common (input) port is terminated in 50 Ω (or the characteristic impedance of the system, which can be 75 Ω in video setups). The QCC-20 power splitter, for example, achieves at least 25 dB minimum midband (1400 to 1800 MHz) isolation of 25 dB (see table), with typical midband isolation of 35 dB (Fig. 2). The QCC-22 boasts typical isolation of 23 dB from 2200 to 2500 MHz, with typical isolation of 25 dB from 1500 to 1700 MHz and 28 dB from 1700 to 2200 MHz. The minimum isolation is 17 dB from 2200 to 2500 MHz, 20 dB from 1500 to 1700 MHz, and 23 dB from 1700 to 2200 MHz.

The insertion loss in a two-way power splitter is actually a measure of the amount of loss above the theoretical value due to power division. In an ideal two-way splitter, for example, the two output signals would be one-half the value (3 dB lower) of the applied input signal. The actual insertion loss in a two-way power splitter is the amount of signal lost (due to resistive losses, circuit impedance mismatches, and other factors) in addition to the theoretical 3-dB division loss. For example, the maximum midband insertion loss (the average of the two output powers minus the 3-dB theoretical division loss) for the QCC-20 is 0.7 dB while the typical midband insertion loss for this splitter is 0.4 dB. At lower and higher frequencies, the insertion loss is slightly lower and higher, respectively (Fig. 3).

The QCC-22 offers the small size of the QCC-20 power splitter for applications requiring slightly higher frequencies, from 1500 to 2500 MHz. The QCC-22 features typical isolation of 25 dB from 1500 to 1700 MHz, with minimum isolation of 20 dB across that band. The typical isolation is 23 dB across the full frequency range, with minimum fallband isolation of 17 dB. The typical insertion loss is 0.4 dB from 1500 to 2200 MHz, while the typical insertion loss is a mere 0.5 dB from 2200 to 2500 MHz. The maximum insertion loss is 0.8 dB across the full frequency range of the QCC-22 power splitter.

These power splitters make full use of the company's Blue Cell™ multilayer LTCC technology to achieve full-sized performance in a fraction of the size of traditional microstrip and stripline circuits. LTCC circuits are fabricated by laminating unfired ceramic tapes with printed conductor lines as well as passive circuit elements, such as inductors and capacitors, and then firing the tapes at a temperature between +850 to +875°C to form miniature circuits. Although shrinkage in the ceramic tape occurs in the x, y, and z axes during the firing process, newer LTCC materials minimize and make more predictable the amount of shrinkage, thus increasing final yields of fabricated LTCC products. By reducing the circuit area and tightly controlling their LTCC process, the engineers at Mini-Circuits have developed this pair of chip-sized power splitters at a fraction of the cost of conventional power splitters.

The basic packaging concept for the power splitters had already been developed for a line of LTCC filters,1 and showed good port match for frequencies through 6 GHz. By modifying that basic package design, a four-port version of the package was created for couplers and power splitters. One additional requirement was a top-side ground to reduce sensitivity to the external electrical environment (Fig. 4).

Because of their small size, the new power splitters offer outstanding phase and amplitude balance between their output ports. The QCC-20 and the QCC-22 manage to keep the unbalances at the highest frequencies to no worse than 4 deg. For example, the phase unbalance of the QCC-20 is typically 1 deg. and no worse than 3 deg. from 1200 to 1800 MHz, and typically 1 deg. and no worse than 4 deg. from 1200 to 2200 MHz (Fig. 5). The phase unbalance of the QCC-22 is typically 2 deg. from 1500 to 2500 MHz with worst-case phase unbalance of 4 deg. across that frequency range.

The amplitude unbalance for the QCC-20 is typically 0.5 dB from 1200 to 2200 MHz, with maximum amplitude unbalance of 1 dB. The amplitude unbalance for the QCC-22 is typically 0.6 dB or better from 1500 to 2500 MHz.

These first-generation LTCC power-splitter designs are available for use from 1200 to 2500 MHz, with soon-to-be-announced models available for lower and higher frequency ranges. The power splitters, which are designed to handle input power levels as high as 5 W (+37 dBm), are rated for operating temperatures from −55 to +100°C. Mini-Circuits, P.O. Box 350166, Brooklyn, NY 11235; (718) 934-4500, FAX: (718) 332-4661, Internet:

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