CN0320: An IQ Demodulator-Based IF-to-Baseband Receiver with IF and Baseband Variable Gain and Programmable Baseband Filtering, and Dual ADC

CIRCUIT FUNCTION AND BENEFITS
This circuit is a flexible, frequency agile, direct conversion IF-to-baseband receiver. A fixed conversion gain of 5 dB reduces the cascaded noise figure. Variable baseband gain is used to adjust the signal level. The baseband ADC driver also includes a programmable low-pass filter that eliminates out-of-channel blockers and noise.

The bandwidth of this filter can be dynamically adjusted as the bandwidth of the input signal changes. This ensures that the available dynamic range of the ADC that this circuit drives is fully used.

The core of the circuit is an integrated IQ demodulator with fractional-N PLL and VCO. With just one (variable) reference frequency, the PLL/VCO can provide and a local oscillator (LO) between 750 MHz and 1150 MHz. Precise quadrature balance and low output dc offsets ensure that there is minimal degradation of the error vector magnitude (EVM).

The interfaces between all of the components in this circuit are fully differential. Where dc coupling is required between stages, the bias levels of the adjacent stages are compatible with each other.

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