[ARC5] IMD performance

[Ian Wilson]

The topic of hollow-state receiver performance is a very
interesting one. Unfortunately most of the modern approach
to IMD testing, etc, postdates the times when tube receiver
design was at its peak.

One small exception appears to be the EF183/EF184 aka
6EJ7/6EH7, designed for low IMD use in color TV IF chains.
Their data sheet has a small section showing IMD performance.
Also, if I recall correctly, the RF input impedance versus frequency
is discussed to some extent.

Another contender for very good intermod performance would
appear to be the beam deflection tubes. To me, differential plate
current versus beam deflection voltage appears to be very linear,
but eyeballing these things is not very scientific.

There must be other published information and/or measurements
on this area of tube performance, surely? Anyone seen something
like this?

Chris Trask made comparisons between tubes of different
linearity but I don't know whether he had a way of doing this
quantitatively from data sheets, or whether it was subjective.

My latest tube receiver effort has a 6EJ7 RF amplifier mostly
to act as a buffer between two stages of preselection. This
at least gets rid of lots of interfering junk that is some way out
from the desired signal while giving reasonable IMD performance.

One of these years I guess I should read the literature on how to
make IMD/IP3 measurements and see how well these glass front
ends hold up against published figures for solid state gear.

My basic thought behind all this is that a figure of merit for tubes
may not be worse than that for transistors: solid state devices
have much higher gain-BW products but much more nonlinear
characteristics in general.

73, ian K3IMW