Very few peoples noted this characteristic regarding
the IMD to output power dependence.
For tetrodes, the IMD versus output power graph have a more
complicate, non linear dependence (sometimes like a "N" shaped form).
This may be seen from the 4CX1500B data also.
For this very linear tetrode, the 3rd IMD may vary few dB, but for
the 5th order IMD the variation may be more than 20 dB for
the different output levels.
May worth to note that the best 3rd and 5th IMD levels are
obtained for different output levels, these are not occurring for the
same output (the 3rd and 5th order IMD minimum do not coincide).
This is also true for many RF power transistors, but in this case the
IMD versus output graph have a "U" or "V" shaped form, the best
IMD occurring for mid output levels. Actually, the specified output
for most of the SSB solid state devices is limited by the IMD and
not because of the dissipated power. Unfortunately this is exploited
by some of the manufacturers of cheap SS amps which are pushing
the devices over the specified output, even if the IMD are going up.
So, using 5W out from a 100W trcvr for exciting a high gain very
linear tetrode amp may be not the best way for exciting that linear?!
73s,
Traian
COLIN LAMB wrote:
> If you will note on some curves, the imd changes with output. Often, the
> best imd is realized when output is at normal levels. I do not have the
> information handy, but you will note that on many tubes that the imd will
> go up if you are running it at significantly less than full power output.
> Imd levels is not a straight line for any particular tube.
>
> Colin K7FM
>
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