A very broadband Ka-band vector modulator was developed using a commercial foundry to produce a high reliability MMIC for space applications. TriQuint's 0.15 [micro]m PHEMT GaAs process was chosen for the fabrication because of its high performance, high reliability and low cost prototype service, along with the company's ability to provide space level qualification services. This MMIC was developed at the Johns Hopkins University, Applied Physics Laboratory (JHU/APL) for the Mars Technology Program under sponsorship from NASA's Jet Propulsion Laboratory (JPL). The task's goal is to produce GaAs MMICs for spacecraft communications requiring high performance, high level function integration, low power consumption, and extremely low mass and size.
Traditional microwave transmitter systems can be simplified by replacing up-converting mixers and associated filters with a vector modulator that directly modulates data onto a carrier signal using orthogonal I and Q control inputs for simultaneous amplitude and phase control. The versatility of the vector modulator allows unlimited modulation control and methods, from simple BPSK or QPSK phase modulation schemes, to dual independent data rate modulations on the non-interfering I and Q channels, to Gaussian minimum shift keying schemes that fully utilize simultaneous amplitude and phase control. This article describes the architecture, design, control, layout and measured performance (24 to 44 GHz) of a versatile, broadband, high performance and high reliability Ka-band vector modulator MMIC targeted for space applications.
Depending on the frequency of operation, performance criteria and technology of choice, there are various ways to implement the vector modulator and its components. Before describing the design tradeoffs and optimization of the Ka-band MMIC, a review of the basic function of a vector modulator is appropriate. Figure 1 shows a typical vector modulator architecture with a 90[degrees] splitter, two attenuators and an in-phase combiner. A hybrid splitter divides the input signal into in-phase (I, 0[degrees]) and quadrature (Q, 90[degrees]) components. The attenuators control the amplitude and sign ([+ or -]) of each I and Q path; ideally, the amplitude is between -1 and +1. The orthogonal vectors I and Q are combined by an in-phase summer to create a vector of any phase and amplitude. Direct vector control of the carrier allows simultaneous phase, amplitude and frequency modulations.
Since a quarter-wavelength line at Ka-band is relatively short in a GaAs MMIC (approximately 0.8 mm), simple distributed elements, such as Lange or Wilkinson couplers, can be used as components in the design. The input 90[degrees] splitter is easily implemented as a single Lange coupler. Likewise, the output summer can be implemented with a single Wilkinson combiner. The critical part of the design involves the two identical (I/Q) attenuators.
A simple reflective attenuator consists of two identical variable resistors and a Lange coupler (see Figure 2). With two identical PHEMTs used as variable resistors and connected to the through and coupled ports of the Lange coupler, the input and output matches will be good since the reflections from the resistors will cancel. The normal isolated port of the coupler becomes the through path receiving...