Circuit Note CN-0245 is a reference circuit designed to solve analog, mixed-signal, and RF design challenges. It includes devices such as ADF4350 wideband synthesizer, ADL5387, and ADL5380 quadrature demodulators.
The document presents a reference circuit connecting the ADF4350 wideband synthesizer with either the ADL5380 or ADL5387 quadrature demodulators. This configuration utilizes differential signals to reduce common-mode noise and cancel even-order LO harmonics, eliminating the need for an external LO buffer in many cases.
The text explains that filtering requirements depend on the specific architecture of the demodulator used. The ADL5380 requires a low-pass filter to attenuate odd-order harmonics due to its polyphase filter-based phase splitter. Conversely, the ADL5387 uses a 2x LO architecture with digital circuitry and generally does not require filtering below 1 GHz, though it may be needed at higher frequencies.
Filtering is not required at frequencies below 1 GHz, but should be used above 1 GHz to attenuate even-order harmonics.
The ADF4350 covers a frequency range from 137.5 MHz to 4400 MHz.