AD9874 LO input impedance

Hello,

An equivalent circuit of the AD9874 LO buffer is shown in figure 6 of the datasheet.   The SGA2463Z is an RF 20 dB gain block thus would not represent the best choice for a VCO buffer given its high gain which may result in overdriving the AD9874's LO input and its low input impedance (if directly connected to the VCO tank)..   A simpler approach for 41.85 MHz LO buffer would be to use a simple npn transistor configured as a buffer (i.e. common collector having gain slightly less than 1).  Alternatively, you could use a dedicated IC like the MAX2470/MAX2471 series.

*Note, most VCO off-the-shelf components include a buffer that isolates the tank from the load thus reducing frequency pulling to load changes.   

Hello,

An equivalent circuit of the AD9874 LO buffer is shown in figure 6 of the datasheet.   The SGA2463Z is an RF 20 dB gain block thus would not represent the best choice for a VCO buffer given its high gain which may result in overdriving the AD9874's LO input and its low input impedance (if directly connected to the VCO tank)..   A simpler approach for 41.85 MHz LO buffer would be to use a simple npn transistor configured as a buffer (i.e. common collector having gain slightly less than 1).  Alternatively, you could use a dedicated IC like the MAX2470/MAX2471 series.

*Note, most VCO off-the-shelf components include a buffer that isolates the tank from the load thus reducing frequency pulling to load changes.