to break into parasitic oscillations. R9 provides a low impedance load at the beginning of the "serpentine"
and helps to stabilize IC1. It also provides a load for the bandpass filter if that is used. C12 prevents the DC
bias from being grounded through the resistor. The 100 ohm resistors at the input to each of IC2 through IC7
decouple the amplifier from the long traces and are primarily responsible for stabilizing the system.
R9 was originally 51 ohms, but increasing that to 270 ohms allows somewhat higher output signal levels,
and makes a significant difference in output if the optional bandpass filter is used. Increasing the value of
this resistor does not seem to have an impact on stability, but in the event of parasitic oscillation, reverting to
a lower value may help.
IC2-7 are configured as unity gain followers. Their outputs are DC isolated, fed through a “back
termination” into an isolation transformer. As on the input, the connector is grounded through a capacitor to
further reduce the possibility of ground loops,. The back termination is a 50 ohm series resistor designed
both to match the coax cable impedance, and to ensure amplifier stability even when driving a capacitve load
(such as a long coax cable). Because the back termination and the load impedance form a voltage divider, the
output voltage into a 50 ohm load will be half the unloaded voltage. The adjustable gain of IC1 compensates
for this loss (within reason).
The power supply uses a standard LM7809 regulator. JP5 and JP6 are provided for board-stacking purposes.
F1 is a 0.5 amp pico-fuse. C39 and R27 form an input ripple filter. D1 provides reverse voltage protection;
if the power leads are reversed, D1 will conduct and blow F1. To help stability and minimize noise, lots of
bypass caps are used -- each MAX477 has a 0.1 and 0.001 cap at its supply input, and additional 1uF
tantalum caps are spaced along the power bus.
