Quadrature Downconverter Board Modifications
9 August 2004, by Dan Werthimer:
- 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.