Of course, maybe you don't need such a high Q, Paul. Qu of 30 is
quite reasonable for small SMT RF inductors, at least the type I use.
In the following list, "series" means in series from the gate output
to the next gate input, in order, and "shunt" means shunt to ground at
that point. Anyway, try this (build it or SPICE it or RFSim99
it...add resistors to any simulation to account for the Qu. I'd
suggest 3 ohms series and 12k ohms shunt for each 1.8uH.)

47pF series
1.8uH series
470pF shunt
45pF series
1.8uH shunt
3.3pF series
40pF shunt
1.8uH shunt
DC blocking cap series
high-value DC bias resistors, and the gate input (I assumed to be
about 4k net resistance to ground at the gate input, including the
bias resistors).

It should give you enough voltage gain at 18MHz to drive the second
gate at the fifth harmonic, and should attenuate the third at least
50dB if you build it properly, even with low-ish Qu inductors. This
is rather a "hack" circuit, but works. The premise is that it's
easier to get three inductors all the same value than muck about
tuning the inductors. Make the 47pF, 45pF and 40pF caps variable and
you can peak up the response at your desired frequency. Your
simulation should show a reasonably flat bandpass characteristic,
centered at about 18MHz.

Paul Burridge wrote in message . ..
Hi guys,

ISTR that one can improve Q in resonant tanks by having a low L-C
ratio. Or was it high L-C ratio. I can't remember but need to know.
Can any kind soul help me out? Thanks.

p.