Miniaturized low cost 30 GHz monolithic balanced BPSK and vector modulators

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From: Microwave Journal(Vol. 42, Issue 3)
Publisher: Horizon House Publications, Inc.
Document Type: Article
Length: 3,025 words

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Over the past decade, monolithic circuits have been increasingly used in telecommunication systems where they are employed in transceivers for satellite and digital microwave radio communications. The monolithic technology offers many advantages over hybrid circuits, including the possibility of symmetrical design and lower parasitics. The technology also allows broadband design so that the same circuits can be used for many different channels and complex circuits can be designed at a lower cost with a significant reduction in size. Because of the increasing need to improve performance at minimal extra cost, new techniques in microwave electronic subsystems are always being sought.

In 1990, Kumar(1) proposed a new concept for realizing inexpensive microwave subsystems for future very small aperture terminals (VSAT). His detailed cost examination showed that the bulk of the cost is concentrated mainly in the microwave circuits and that this expense can be greatly reduced by employing new modulation and demodulation techniques. For single-channel data communication systems, his alternative solution to frequency conversion was to use a baseband digital waveform to directly modulate the microwave carrier - a technique referred to as direct carrier modulation.

If a conventional microwave transmitter is to be employed, the modulator operates at an IF such as 70 MHz. To upconvert the modulated signal to a transmission frequency (for example, 30 GHz), the conventional method requires a complex chain of mixers, filters and amplifiers, resulting in a series of circuits that must operate from megahertz to gigahertz frequencies. It is very difficult to integrate all of the components onto a single MMIC substrate. However, if the modulator is designed to operate directly at the microwave carrier frequency, the RF hardware requirements are minimal. Thus, for mm-wave applications where size and cost restrict the widespread use of wireless systems, direct carrier modulation is potentially very attractive. This new approach has been used successfully in low cost VSATs to reduce the cost of their transmitters by 60 percent.(2) A conventional microwave transmitter and the direct carrier modulation approach are shown in Figures 1 and 2, respectively.

Simple modulators for schemes such as amplitude-shift keying (ASK), binary phase-shift keying (BPSK) and frequency-shift keying (FSK) can be designed using a wide range of techniques. Typically, ASK modulators employ switches, BPSK modulators use 180 [degrees] phase shifters and FSK modulators use either a switched VCO or one of several available small-shift frequency translation techniques. To improve the bandwidth efficiency, quadrature modulation schemes, such as QPSK or QAM, may be generated by combining in-phase and quadrature-phase channels in a vector modulator.[3,4]

A common technique for realizing direct BPSK microwave modulators is to employ reflection topology using Lange couplers with PIN diodes or FETs connected to the direct and coupled ports acting as switches, as shown in Figure 3. For an ideal case, the directional coupler is assumed to be loss-less, perfectly matched with infinite isolation. As can be seen in the diagram, a portion of the input voltage wave at port 1 travels along the direct line to port 4 where...

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Gale Document Number: GALE|A54336860