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RE: Noise with MKU24



It is quite common for microwave preamps to oscillate with nothing connected
to
the input.  This will often result in an apparent decrease in noise, as real
oscillators
must exhibit a decrease in active device gain.  Conventional linear analysis
will not
handle this situation.

Ideally, preamps would not be sensitive to the source or load
impedance--microwave
designers go to great lengths to design "unconditionally stable" amplifiers.
Lots
of math and theory is available on this topic.  

With HEMT devices, it is common to add a series resistor at the drain of
FET.
However, this decreases the gain and increases the NF--some designers may
not
add enough resistance to obtain unconditional stability.  At C-Band, it is
common
to see isolators--non reciprocol devices to insure the FET sees a good
impedance
match over an octave or more.

The ARRL Lab does not measure amplifier stability, though it certainly would
nice
to have a 50GHz network analyzer to perform the measurements.  Measurements
need to be performed over a wide frequency range--a 2.4 GHz preamp may have 
a problem at 8, 18, or even 30 GHz, depending on the active device used.

--Zack W1VT
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