On 2 Jul 2006 10:27:37 -0700, "AndyS" wrote:


Andy comments:

I haven't used that particular dual gate Mosfet, but I will tell you
how I have used 3n140, 3n141, and others:

G2 was used as the LO and was bias at around 3 volts DC with
resistors. The LO was cap coupled in at a leveL of about 2.5 Vrms.
It is useD as a switching signal to drive the MOSFET into and out
of conduction. It takes very little power from the LO, but the
2.5 Vrms swing was needed...

G1 was used as the RF port. G1 and the source were biased
pretty much as you described.

The drain was similar to your design. While using a 1K resistor
gives
a great source impedance to the xtal filter, it also absorbs most of
the signal.... If I remember, the output Z is 9 or 10 k for the devices
I used. I would use maybe a 7.5 K resistor and then impedance
match in the tuned circuit to get the source right. This usually took
some experimentation. Since the drain output was tuned to a different
frequency than the LO or the RFin, I never had a stability problem.
Power gains of 15 to 20 db were very common... ( Note , I said
POWER gain, and not voltage gain , which could be anything... )



Maybe my experience is outdated with the newer Mosfets, but I
have never seen, or used a circuit that put the LO on G!.... It was
always on G2.. I used dual gate mosfets to build receiver front
ends and mixer from about 1967 to about 1990. Then I started
using doubly balanced stuff like SBLs and MDs ..... I followed them
with a nice amplifer and good termination.....It costs a little more,
but the designs were always more stable and gave better
performance.....

I hope this helps. Perhaps others here have used your particular
device and can give better guidance...

Andy W4OAH

Your experience is like mine and holds well for current generation
DGfets or even a pair of casoded Jfets.

Allison