"Eamon Skelton" wrote in message
...
On Fri, 30 Jun 2006 03:36:28 +0000, David wrote:

Thanks for the feedback (no pun intended I appreciate it.

I will add a shield between input and output circuit. I do remember
seeing this in other designs. In my prototype construction I do have the
input and output coils fairly close and in the same orientation so I
imagine there very well could be coupling between them.


I had a similar problem with a 2M RF amp. I cured it by
reversing the winding direction of one of the inductors.


Can anyone suggest a reasonably simple method to determine noise figure
of this stage ?

I'm a bit reluctant to suggest a method that I haven't tried
mysef, but you should be able to calculate NF by measuring the
receiver noise output with a hot and cold resistor connected
to the input. The usual method involves boiling water and
melting ice. http://tinyurl.com/psdfo

73, Ed. EI9GQ.


David,
I don't know your technical level, but here goes. I thought the dual
gate is a type of cascode arrangement and should be stable owing to the
isolation of the "top" device... Anyway...

You are very correct about trying to "band-aid" it with de-Q'ing resistors.
You need to find the root cause, not hope a patch works. You are well aware
of and trying to solve a possible proximity feedback issue, so I won't go
there.

It is not clear what your term "sprogging" may mean. There was a term
"Squeeging" which referred to oscillation or what is also called
regen(regeneration) which is typically used for dirty (sideband-type),
rather than clean (single frequency) oscillation.

Is the oscillation a clean, oscillator signal (pure carrier), or does it
have sidebands?
If sidebands, are they clear and distinct or is it more of a wideband
grunge/garbage?
Are the sidebands mirrored on both sides or are they lopsided?
These are all clues.

If you have regular sidebands or grunge, you have a low frequency
oscillation component and need to also look at bypassing. 33-200 ohms or a
ferrite bead in series with a bypass can help diagnose it.
If the sidebands are unequal, there are most likely both AM and FM
components to the oscillation. Putting a regular scope on the supply or
other places which are supposed to be bypassed might see the signal, helping
to ID the cause.

Be aware that two bypasses in parallel, of different size, can become a
tuned circuit and therefore a high impedance at frequencies where the larger
one is above self resonance and appears inductive.

Pay extreemly close attention to the ground currents for the input and
output circuitry. You want to have an absolute minimum of common path for
these. I didn't see anything in your circuit description about the FET
source. If it is self biased with a resistor & cap combination or what, but
from the source you should have two distinct paths in the ground you have to
the respectice components tith no length of this "ground current path" in
common - where both currents share a common path. you can even cut the PCB
so there are two distinct ground planes which only meet at the FET source or
its resistor/bypass cap.
Watch the power supply side pretty much the same. Where does the Drain
bypass connect to the plane?

Hope this helps.
73, Steve, K9DCI