On Sun, 02 Jul 2006 22:45:57 GMT, David
wrote:

Since everyone is top posting.

Having G2 driven hard is good for large signal handling as mixing
will occure with lesser levels. So that's why that needs to be done.
If driven hard enough bias has no effect.

I vaugely remember you saying your using two filters back to back...

If so you may have the interfilter coupling way off. Also the loss for
two 3db loss filters would be a bit higher than 6db if everything is
right.

Allison




I noted in We Hayward's book "Experimental Methods in RF Design" that he
suggests the gain of a Dual Gate Mosfet in a mixer circuit is about 1/4
of the gain of the same device in an RF amp circuit. If this is true
then the 6dB gain I now see in the mixer stage would be about right.

I cranked up the LO level and now have +/- 4V on G2. I tried biasing the
Gate up to 3 but it made no difference than when just 47K to ground
bias was used on G2.

The main issue I am having now is matching to the 4-pole filter.
I have tried several approaches and the performance is disgusting.

The current mixer to filter circuit is....

This "should" have matched down to 800R

100nH (Q=100) inductor to Vcc from Drain
6-30p trimmer to ground from drain
Split capacitor tap from drain to ground (220pF in series with 200pF)

1st filter, 4p7 to ground between 1st and second filter.

Output match to 50R from 800R for testing...
100nH to ground from filter output
82pF to ground
6-30p trimmer to ground
15pF in series to 50 Ohm load.

The loss through the filter is around 10dB instead of 3dB, the ripple is
around 6-8db instead of 1dB. The filter response shows double peaks with
dip between, either side of the peaks falls off extremely quickly at
around 2 kHz off (should be +/- 15kHz bandwidth).

I would appreciate any help I can get to determine what is happening and
to correctly match into this filter that requires 800R//3pF terminations
at 45 MHz.


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