Integration to reduce system size, while still maintaining performance levels achieved by discrete solutions has been a main focus across multiple industries. Recently, Analog Devices developed a series of highly integrated IQ demodulators with a fractional-N phase locked loop (PLL), a voltage-controlled oscillator (VCO), and multiple low drop-out regulators (LDO) into a compact 40-lead 6 × 6 mm LFCSP package. The RFICs leverage SiGe BiCMOS technology to achieve this small size without sacrificing electrical performance.
The ADRF680X family uses a high performance mixer core that results in an exceptional input IP3 and input P1dB, with a very low output noise floor for excellent dynamic range, along with a low noise VCO, combined to achieve an excellent error vector magnitude (EVM). The three devices in the ADRF680X family support LO frequency ranges from 50 to 1150 MHz to cover a wide range of IF frequencies used in QAM/QPSK receivers as well as supporting common cellular standards, such as W-CDMA/CDMA2000/LTE, and also microwave point-to-point and point-to-multi-point radio architectures. The ADRF680X family also features multiple programmable functions through its SPI port. This allows the user to control the fractional-N PLL, the demodulator LO divider, multiple optimization options, low power mode, as well as allowing for an externally applied LO, or to generate a divided-down VCO signal for external use. The ADRF680X family is the only known quadrature demodulators to combine three RF functions into a single device, thereby simplifying design and reducing board space and bill of materials cost.
Figure 1 ADRF6806 block diagram and pin outs.
The newest member of the ADRF680X family is the ADRF6806, Figure 1. It uses a differential RF input and operates over an LO frequency range of 50 to 525 MHz. The differential I and Q output paths have excellent quadrature performance with a phase accuracy of < 0.5° and amplitude accuracy of < 0.1 dB and can handle baseband signaling or complex IF up to 120 MHz. The ADRF6806 has an input P1dB of 12.2 dBm, an input IP3 of 28.5 dBm, a noise figure (DSB) of 12.2 dB, a voltage conversion gain of 1 dB, and a wide 3 dB baseband demodulation bandwidth of 170 MHz. When the part is run in low power mode to reduce current consumption, the ADRF6806 has an input P1dB of 10.6 dBm, an input IP3 of 25.2 dBm, a noise figure (DSB) of 11.4 dB, a voltage conversion gain of 4.2 dB, and a 3 dB baseband demodulation bandwidth of 135 MHz.
The next member of the ADRF680X family is the higher frequency ADRF6807. It also uses a differential RF input and operates over an LO frequency range from 700 to 1050 MHz. The differential I and Q output paths have excellent quadrature performance with a phase accuracy of < 0.5° and amplitude accuracy of < 0.1 dB and can handle baseband signaling or complex IF up to 120 MHz. The ADRF6807 has an input P1dB of 12.8 dBm, an input IP3 of 26.7 dBm, a noise figure (DSB) of 13.1 dB, a voltage conversion gain of 1 dB, and a wide 3 dB baseband demodulation bandwidth of 170 MHz. When the part is run in low power mode to reduce current consumption, the ADRF6807 has an input P1dB of 11.7 dBm, an input IP3 of 24 dBm, a noise figure (DSB) of 12.4 dB, a voltage conversion gain of 4.3 dB, and a 3 dB baseband demodulation bandwidth of 135 MHz.
Next is the single-ended 50 Ω input ADRF6801, that operates over an LO frequency range of 750 to 1150 MHz. The differential I and Q output paths have excellent quadrature performance with a phase accuracy of 0.3° and amplitude accuracy of 0.05 dB and can handle baseband signaling or complex IF up to 120 MHz. The ADRF6801 has an input P1dB of 12.5 dBm, an input IP3 of 25 dBm, a noise figure (DSB) of 14.3 dB, a voltage conversion gain of 5.1 dB, and a wide 3 dB baseband demodulation bandwidth of 275 MHz.
To evaluate overall demodulator performance, the EVM versus RF input power was analyzed. EVM is a measurement used to quantify the performance of a digital radio transmitter or receiver. A signal received by a receiver has all constellation points at their ideal locations. However, various imperfections in the receiver signal chain, such as magnitude imbalance, noise floor, and phase imbalance cause the actual constellation points to deviate from their ideal locations.
Figure 2 EVM performance of ADRF6806 for a 16 QAM modulated signal.
In general, a demodulator exhibits three distinct EVM limitations versus received input signal power. As signal power increases, the distortion components increase. At large enough signal levels, where the distortion components due to the harmonic non-linearities in the device are falling in-band, EVM degrades as signal levels increase. At medium signal levels, where the demodulator behaves in a linear manner and the signal is well above any notable noise contributions, the EVM has a tendency to reach an optimal level determined dominantly by either quadrature accuracy and I/Q gain match of the demodulator or the precision of the test equipment. As signal levels decrease, such that the noise is a major contribution, the EVM performance versus the signal level exhibits a decibel-for-decibel degradation with decreasing signal level. At lower signal levels, where noise proves to be the dominant limitation, the decibel EVM proves to be directly proportional to the SNR.
A 140 MHz modulated signal was used to test the EVM of the ADRF6806 on its evaluation board, and the ADRF6806 shows excellent EVM performance for various modulation schemes. Figure 2 shows the EVM of the ADRF6806 being better than −45 dB over a wide RF input range of about +35 dB for a 16 QAM modulated signal at a 5 MHz symbol rate with a baseband IF of 5 MHz. EVM was tested for both power modes: normal power mode, LPEN 0, and low power mode, LPEN = 1. When low power mode is enabled, the EVM is better at lower RF input signal levels due to the parts' lower noise. While in normal power mode the EVM remains lower at higher RF input signal levels.
Figure 3 Peak EVM of ADRF6806 for a 256 QAM modulated signal.
Figure 3 shows the peak EVM of the ADRF6806 for a 256 QAM modulated signal that is not degraded with respect to the previously shown 16 QAM signal results. The symbol rate was also 5 MHz with a baseband IF of 5 MHz. Again, EVM was tested for both power modes and is better than −45 dB over a wide RF input power range of about +35 dB.
The ADRF680X family provides very high levels of integration and performance by incorporating a high dynamic range mixer core, a versatile fractional-N PLL, a low noise VCO, and multiple LDOs. Packaged in a compact 40-lead 6 × 6 mm LFCSP, it delivers exceptional dynamic range and EVM performance required of today's demanding receiver architectures.
Analog Devices,
Norwood, MA
(800) 262-5645,
www.analog.com