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The transceiver’s transmitter consists of several amplifiers, an IF/RF filter, a mixer, digital attenuators, and switches. Because the signal from baseband remains constant, the transmitter’s output power is changed by means of two digital attenuators which work in conjunction with several small-signal amplifiers and one power amplifier to achieve desired transmit power levels. These components should be carefully designed to achieve the required transmit performance parameters, including output power, linearity, and spurious suppression. For example, an LTE signal must have high peak-to-average power ratio (PAPR), especially with complex modulated signals reaching 8 dB or higher. As a result, the power amplifier should be designed with such requirements in mind.

The receiver circuitry—with its low-noise amplifier (LNA) IF/RF filter, mixer, digital attenuators, switches, and small-signal amplifiers—is somewhat similar in architecture to the transmitter circuitry, differing in its dynamic range. The receiver circuitry should not only provide enough gain for small signals, but also avoid amplifier saturation for large signals. As a result, gain control is an important function for the receiver circuitry.

The gain control is achieved by means of several gain blocks and two digital attenuators for 64-dB total gain control with 0.5-dB gain control per step. The passive mixer is used to convert RF signals to the IF stage. As with the transmitter, the layout and device selection for the receiver circuitry should be carefully considered to achieve desired linearity and spurious suppression.

Transceiver Supports 8 × 8 MIMO Systems, Fig. 3

Each channel of the transceiver was designed on a four-layer FR-4-based printed-circuit board (PCB) with relative dielectric constant (εr) of 4.6 to achieve small size and low cost. System-level transceiver simulation was performed with the ADS 2009 Advanced Design System (ADS) simulation software from Agilent Technologies. To avoid possible interference between function modules, each RF board was designed and assembled with several metal compartments for isolation. This metal framework with covers was indispensable for achieving good electromagnetic-interference (EMI) shielding. Figure 3 shows one of the transceiver channels.

For general-purpose use, all of the transceiver’s control signals are realized by a single-chip complex programmable logic device (CPLD), and all eight transceiver channels can be controlled and synchronized separately through the backboard. The system requires a frequency synthesizer control signal and reference clock for the synthesizer’s phase lock loop (PLL). Since the quality of the reference oscillator is closely tied to the oscillator phase noise, the amplitude, waveform quality, and jitter of the reference oscillator are important parameters in the design of the overall system.

The transceiver system’s power board provides DC-to-DC conversion from +48 VDC to +6 VDC for the system. The control board acts as an interface between the baseband control signal and the RF board’s control operation. It includes power and gain control for each transmitter and receiver, respectively, along with other functions like reference clock calibration.

Transceiver Supports 8 × 8 MIMO Systems, Fig. 4

The reference board provides an accurate 10-MHz reference clock which is generated by an oven-controlled crystal oscillator (OCXO). A PLL circuit is carefully designed to lock and track a 30.72-MHz Global-Positioning-System (GPS) reference clock from baseband quickly, even if the OCXO has a very slight change due to an external disturbance.

The full transceiver system includes eight RF boards, one backboard, one power board, one control board, and one reference board (Fig. 4). The performance of the transceiver was evaluated with commercial test equipment from some of the top suppliers. A model N5767A power supply from Agilent, for example, provides the full system’s +48 VDC voltage, while models E4438C and N5182 signal generators and a model N9030 signal analyzer from the firm were also used in the testing.

A model FSUP50 signal source analyzer from Rohde & Schwarz, with available models to 26.5 GHz, was also used as part of the test system to check LO phase noise. The LO module is designed to provide low-phase-noise signals, providing IF LO signals at 965 MHz and RF LO signals at 1.45 GHz.

Transceiver Supports 8 × 8 MIMO Systems, Fig. 5Figure 5 shows the phase-noise performance of the LO module, revealing that the phase noise of the IF LO is slightly better than the phase noise of the RF LO. According to the test results, the phase noise of the RF LO is better than -93 dBc/Hz offset 10 kHz from the carrier, while the phase noise of the IF LO is better than -96 dBc/Hz offset 10 kHz from the carrier. The low phase noise is due to the careful design of LO’s power supply, where an appropriate decoupling capacitor is essential. In addition, the choice of PLL bandwidth also plays a role in minimizing phase noise, with the PLL bandwidth of the two LOs chosen at 20 kHz.

Transceiver Supports 8 × 8 MIMO Systems, Fig. 6

For each transmit channel, output power, linearity, and gain flatness are key performance parameters. Across the full 100-MHz bandwidth of the transmit channel, gain fluctuation is less than 1 dB. This is due to careful design and tuning to achieve optimum impedance matches between difference devices and circuit layouts, especially for the impedance match of the amplifier and filter. The maximum output power of each channels is about +23 dBm, with third-order intermodulation (IMD3) held as low as -53 dBc (tested with a two-tone signal with 1-MHz spacing). As Fig. 6 shows, each transmitter channel achieves good linearity.

Transceiver Supports 8 × 8 MIMO Systems, Fig. 7

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