CN0144: Broadband Low Error Vector Magnitude (EVM) Direct Conversion Transmitter Using LO Divide-by-2 Modulator

CIRCUIT FUNCTION AND BENEFITS
This circuit is a complete implementation of the analog portion of a broadband direct conversion transmitter (analog baseband in, RF out). RF frequencies from 68.75 MHz to 2.2 GHz are supported through the use of a PLL with a broadband integrated voltage controlled oscillator (VCO). Unlike modulators that use a divide-by-1 LO stage (as described in CN-0134), harmonic filtering of the LO is not required.

To achieve optimum performance, the only requirement is that the LO inputs of the modulator be driven differentially. The ADF4350 provides differential RF outputs and is, therefore, an excellent match. This PLL-to-modulator interface is applicable to all I/Q modulators and I/Q demodulators that contain a 2XLO-based phase splitter. Low noise LDOs ensure that the power management scheme has no adverse impact on phase noise and error vector magnitude (EVM). This combination of components represents industry-leading direct conversion transmitter performance over a frequency range of 68.75 MHz to 2.2 GHz. For frequencies above 2.2 GHz, it is recommended to use a divide-by-1 modulator, as described in CN-0134.

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