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Increasing use of wireless systems and their services is continuing to drive data rates and consume available bandwidths. In recognition of this, the Radiocommunications Sector of the International Telecommunication Union (ITU-R) has established wireless data rates to 100 Mb/s for high-mobility applications and 1 Gb/s for low-mobility applications1 through its IMT-Advanced standards. The ITU recognizes the need to support wireless data rates to 10 Gb/s and beyond.

These higher data rates will require greater spectrum resources and a push to higher frequencies, such as the use of the 3.5-GHz band for the Time-Division, Long-Term-Evolution (TD-LTE) system detailed in ref. 2. Therefore, lowering the complexity and cost of a wireless transceiver’s front end is vital in implementing mobile communication systems for these higher-frequency bands. As an example, ref. 3 details a receiver in the 6.1-GHz band with 100-MHz bandwidth designed to support 1-Gb/s wireless service. For systems operating beyond IMT bands, even wider channel bandwidths will be required.4,5

To support higher-frequency wireless applications, a wideband superheterodyne receiver was designed for X-band use at 9 GHz. The front end, which employs a commercial mixer and GaAs high-electron-mobility-transistor (HEMT) amplifiers, can be fabricated with standard printed-circuit-board (PCB) techniques and low-cost PCB laminates. It translates RF input signals to a 2-GHz intermediate-frequency (IF) band and provides a 200-MHz bandwidth at 9 GHz, with better than 3.5-dB noise figure and input return loss of better than 19 dB. The conversion gain is better than 35 dB, with gain ripple of a mere 1.24 dB. For high-speed mobile applications that require good error-vector-magnitude (EVM) performance, the front end also meets the EVM performance requirements of those systems.

Low-Cost Front End  Receives 9 GHz, Fig. 1

As shown in Fig. 1, the receiver front end is composed of six parts, including a low-noise amplifier (LNA) RF and IF filters, and a local-oscillator (LO) module. A commercial mixer is used for frequency downconversion (from 9 to 2 GHz), while a two-stage gain block is used as the IF amplifier to guarantee link gain.

According to system requirements, the overall conversion gain of the receiver front end should be greater than 30 dB and the input 1-dB compression point higher than -30 dBm. With the help of the Advanced Design System (ADS) simulation software from Agilent Technologies, the link budget of the receiver front end was simulated and optimized. Table 1 shows the final gain distribution of the RF and IF modules.

Low-Cost Front End  Receives 9 GHz, Table 1Compared to GaAs metal-epitaxial-semiconductor field-effect-transistor (MESFET) semiconductor technology, GaAs HEMT technology has advantages in noise, gain, and linearity.6 In this work, a super-low-noise HEMT tube is used for the design of a low-noise amplifier (LNA). Figure 2(a) shows the structure of the two-stage LNA, with the two-stage (rather than single-stage) configuration chosen to provide sufficient gain. To improve input VSWR, a balanced structure is used in the first stage, with a 3-dB directional coupler employed. Since the noise figure of the two-stage LNA depends mainly on the first stage, the first-stage amplifier (LNA1 and LNA2) employs minimum noise figure matching, while the second stage (LNA3) employs maximum gain matching.

Low-Cost Front End  Receives 9 GHz, Fig. 2

Figure 2(b) shows the circuit topology of a single amplifier (LNA1, LNA2, and LNA3). Since the HEMT tube is conditionally stable over the frequency band of interest, a small resistance is added behind it to improve stability. A bandstop filter with quarter-wavelength microstrip line and open-circuit sector stub is designed in a bias network to choke off RF transmission over its stopband, all the while maintaining optimum transmission characteristics for direct current.7,8

In simulations of the two-stage LNA using ADS simulation software, the LNA design yielded a simulated noise figure of less than 0.7 dB, with |S21| of greater than 27 dB, and |S11| and |S22| of less than -20 dB from 8.8 to 9.2 GHz.

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