Ben,

The series C17 coupling cap is working against the net reactance of the
C19-TC2-L2 circuit to form a resonant impedance matching network. Although
it's not obvious, the tank network is tuned on the inductive side of
resonance in order to produce exactly enough net inductive reactance to
resonate C17. At this setting, C17 and the net inductive reactance of the
C19-TC2-L2 network form a series resonant circuit. Because the whole network
is resonant, the junction of C17 and the C19-TC2-L2 network is a higher
impedance point than the pin 8 output of U2. Assuming lossless components,
energy must be conserved, and so the signal voltage at this high impedance
point is higher than at pin 8 - yes, voltage gain in a passive network! The
unmarked coupling cap feeding into U4A is probably just a large value "DC
block", since the U4A stage design will have a fairly high input impedance
and so will not substantially load the tuned impedance step-up network.

Most people think that couping caps are just used to drop signal voltage
sort of like inserting a resistor, but if designed carefully as part of a
resonant interface as I described, they transfer energy with little or no
power loss, and can actually produce voltage gain. This is a very important
RF design concept that many folks don't understand.

Joe
W3JDR


"Ben Jackson" wrote in message
...
Most of the time I see RF stages coupled with .1u caps, which are
basically transparent at RF. Sometimes I see very small values
used, like 3-10p.

A couple examples are in:

http://www.amqrp.org/kits/38spcl/schematic.html

C17 (near the middle top, connecting U2 to a tank) and then the output
of that tank to U4A (unmarked, but also 5p).

It's clearly important to the circuit (empirically) but I don't understand
what they're doing. I can sort of see C20 (the unmarked second one)
acting
as a high-pass RC filter.

--
Ben Jackson AD7GD

http://www.ben.com/