Miniaturized low cost 30 GHz monolithic balanced BPSK and vector modulators

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From: Microwave Journal(Vol. 42, Issue 2)
Publisher: Horizon House Publications, Inc.
Document Type: Article
Length: 2,725 words

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The balanced modulator topology requires a total of four couplers, one each for the input and output and one within each single-stage branch. Generally, in monolithic designs, a 3 dB power split is achieved by employing Lange couplers. However, Lange couplers (folded) require a total of five fingers and the overall length of the coupler is determined by the quarter-wavelength-long coupled fingers.[1] For an MMIC chip it is essential to minimize the expensive chip area to reduce the cost, and Lange couplers in particular tend to use most of the area due to the quarter-wavelength sections. Thus, for a balanced modulator that requires a total of four Lange couplers, the coupler layout must be made as compact as possible and have the ability to be meandered to minimize the layout area.

A microstrip transmission line has an inhomogeneous dielectric, that is, the dielectric is part dielectric substrate and part air. It is important to note that microstrip parallel-line couplers have an odd- and even-mode property, which always results in [Z.sub.oe] and [Z.sub.oo]. Hence, the unequal odd and even modes excited in the coupled region exhibit different phase velocities and, consequently, different wavelengths. This inequality shows itself in the coupler's poor directivity. The other disadvantage of a parallel-coupled microstrip coupler is that a 3 dB power split cannot be obtained because it requires the two transmission lines to be very close to each other - on the order of 1 [[micro]meter] or less. However, such small separation is not possible due to fabrication limitations.

To avoid these problems, a number of publications[2,3] have demonstrated that it is possible to equalize the different phase velocities over quite a broad band and achieve 3 dB coupling by introducing capacitors at the edges of the coupled region between the two lines. The new proposed multilayer directional coupler consists of two parallel-coupled microstrip transmission lines (compared to five fingers) and provides a 3 dB power split with a coupled length that is shorter than the standard quarter-wavelength.

The two parallel-coupled microstrip transmission lines are separated by 5 [[micro]meter]. Simulations using the Sonnet em[R] software package were performed on 95-[[micro]meter]-long subsections and included the various dielectric layers. This detail was necessary to account for the multiple dielectrics used in the foundry process. The simulated sections were then cascaded on HP EEsof[TM] Series 4 software with polyimide metal-insulator-metal (MIM) capacitors and included the remainder of the 50 [Omega] feed lines and RF on-wafer probe pads to obtain the overall response. The overall circuit is then optimized for the center frequency of 30 GHz. The top metal layer M3 and bottom layer M2 are used to form the plates of the capacitor and are separated with 1.4-[[micro]meter]-thick polyimide.

Figure 1 shows a comparison between the two layouts of a standard Lange coupler and the new miniature coupler at 30 GHz center frequency. It can be seen that the overall length of the miniature coupler is almost half and the width is less than a third of the standard...

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Gale Document Number: GALE|A54577353