A unique single sideband upconversion architecture
24 April 2002
Telecoms, Datacoms, Wireless, IoT
Information from Parsec Components
In traditional transmit applications, a two-step upconversion occurs in which a baseband signal is modulated by one carrier to an IF then modulated a second time to the transmit frequency. The single-sideband upconversion architecture eases the filtering requirements by suppressing one of the unused sidebands. Coupling this approach with a direct RF analog modulator eliminates one mixing stage and can realise significant cost savings while maintaining exceptional performance.
Analog Devices is the only chip manufacturer that offers a two-chip solution to realise this architecture, with its AD9777 family of TxDAC+ converters and AD8345/AD8346 Direct RF analog modulators.
AD9777 block diagram
How it works
The idea behind image rejection or sideband suppression is to cancel out energy created in the sampling and mixing process.
Figure A
The signal starts as a baseband I and Q component signal with a finite bandwidth. This baseband I/Q signal is mixed in the AD977x TxDAC+ at a fraction of the sampling clock, placing the carrier at a low IF. Without doing a complex mix, the DAC output spectrum would look like that shown in Figure B.
Figure B
By engaging the complex mixer in the AD977x DAC, the orthogonal opposite sideband is produced in the selected channel (real or imaginary). When the complex signal is then mixed in an analog quadrature modulator, one of the sidebands is cancelled and the desired band is increased in power by 3 dB. The LO leakage can be minimised by calibrating the I/Q channel offset matching and filtering using a single SAW filter stage.
Figure C
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