Quadrature Downconverter Board Modifications

• Added 1 pF capacitor: U13 Pin 3 to ground on all 16 downconverter boards (from MAX4217 op-amp input to ground).
  
  This is to prevent some boards from small oscillation seen on output, at ~480 MHz, probably due to board layout problem. 1 pF is adequate to cut the 480 MHz spur way down, yet it doesn't degrade image rejection very much.
  
  
• Added 2.2Kohm size 803 surface mount resistor on back of jumper post SW1: Pin 2 to Pin 3.
  
  This is to keep the 16 channel DAC gain control board from occasional oscillation--The DAC chip cannot drive a heavy capacitive load, and there is a bypass cap on the gain control voltage input on the downconverter board. The solution is to send the DAC signal through a 2.2Kohm resistor rather than directly to the gain control on the AD8348 and bypass capacitor.
  
  NOTE: Do not install a shorting jumper on the three position SW1 jumper post, otherwise the 2.2 Kohm resistor will be shorted and the modification will be defeated.