Tom...
IMD in almost all linear amplifiers follows the 3:1 rule...for a 1 dB
increase in power output, the _absolute_ IMD levels increase by 3 dB.
This rule is so reliable that it forms the basis of the IP3 intercept
specification...That's the imaginary point where the spur levels rise to
equal the amplifier output...but that one number allows calculation of
all the other intermod numbers. In _relative_ IMD terms, spur levels
improve by 2 dB for each 1 dB reduction in power level. This effect
occurs without regard to bias level, so there's no need to rebias for
Class A.
There's no special maximum power output level in linear amplifier...it
just depends on what IMD's you can tolerate. The DownEast transverter
doesn't have a design problem, it just has a specification problem
(depending you your definition of problem). For example, it's -26 dB
and you want -40, that 14 dB more performance. Simply run 7 dB less
power and it will work.
Another example: small signal amplifiers generally have an IP3 about 15
dB higher than the 1 dB compression level. This means that PEP is -15
dB(IP3) and intermod therefore run about -45 dB(IP2)...for an IMD level
of -30 dBc (relative to carrier). To maintain -40, each stage needs to
run more than 5 dB below PEP...especically considering that all the
intermod products add up along the way. Generally designers run the low
level stages at lower power and let the last few higher power stages
generate all the IMD.
The usual way that a linear amplifier runs worse than this rule involves
bad bias voltages stability. If the power bus or base/gate bias varies
with modulation, it can really screw up the IMD performance.
I really like the FET/Tube cascode design idea. But, the tube biasing
gets a bit weird. Normal drive pushes the tube into grid current, so
instead of a positive cathode zener to generate the idling current, the
bottom of the the FET need to connect to a negative voltage (effectively
allowing the grid to swing positive.
hope this helps...
jeff, wa1hco
Tom Cathey wrote:
Hi Marv,
Your idea about using a 4CX350J IPA is interesting. I suppose it would have
to be VERY physically close to the 8877 or the RF negative feedback will
have phase shift problems. ie, It will not work in a separate cabinet with a
short coaxial lead supplying feedback, I would imagine. The "J" [linear
version] might be an expensive tube too.
Also, I like the idea about putting an MRF-151 in the cathode of the 8877
as a "gain booster", but also wonder if it would degrade the IMD of the 8877
significantly. Anyone have ideas about this?
Another interesting idea was about using a ~200W triode and letting it idle
in class A1, giving maybe 60W output without worrying about neg feedback.
Maybe that would approach -40 db, or perhaps the device itself has
limitations - like using a triode connected 813, etc, dunno.
The bad news: I just got the 3rd order IMD figures back from DownEast on
their 50-28 transverter. At 14 watts output it is NO WORSE than -26db.
Typically -30db. She said at a lower power it would be better. [sometimes
true, sometime not]. So it appears that I will have to either look into
idling those devices higher or modifying somehow, or tapping off the low
level area which should be cleaner and amplifying from 100MW? up to 60W with
something cleaner - as suggested above... [sigh]
Anyone have ideas for cleaning up the 50-28 final amplifier stages? I wonder
what the rest of the world has been doing about this or just living with -30
db IMD on 6M?
73,
Tom, K1JJ
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