High Dynamic Range RF Transmitter Signal Chain using Single External Frequency Reference for DAC Sample Clock and IQ Modulator LO Generation (CN0243)
The combination of theADRF6702 IQ modulator and the AD9122 16-bit dual 1.2 GSPS TxDAC has the dynamic range necessary for a modern high level QAM or OFDM based wireless transmitter as shown in Figure 1. The dynamic range of this circuit is good enough to enable both ZIF (zero IF/baseband) and CIF (complex IF up to 200 MHz to 300 MHz). The AD9122 has the option of up to 8× interpolation, as well as a 32-bit NCO for very fine IF frequency selectivity.
Overall performance of a transmitter is highly dependent on the dynamic range of the components directly in the signal chain. In a mixed-signal transmitter using a DAC and IQ modulator, the noise floor and distortion characteristics of these components define the overall dynamic range of the signal chain. However, the noise floor of the DAC can also be degraded by sample clock jitter, and the IQ modulator performance is dependent on the noise and spur characteristics of its local oscillator (LO). Using high performance components for sample clock and LO generation is, therefore, key to a high performance transmitter.
In addition, generating these signals physically close to the DAC and modulator on the PCB and using a single external reference can make the design much simpler. Generating the sample clock and LO (LO is very often a multi-GHz signal) separately and at some distance from the DAC and IQ modulator requires great care in the PCB layout. Subtle layout errors can cause coupling to and from these critical signals and degrade overall signal chain performance.
The ADRF6702 includes an on-board fractional PLL for LO generation so that a low frequency reference (typically less than 100 MHz) is all that is necessary to synthesize the IQ modulator LO. Using the PLL in the AD9516 clock generator allows a single reference to generate both the DAC sample clock and the PLL reference for the ADRF6702.
The circuit in Figure 1 was built using the AD9516-0, but other members of the AD9516 family could be used depending on the desired internal VCO frequency.
Figure 1. AD9122, ADRF6702, and AD9516 Used in a High Dynamic Range Transmitter
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