[Elecraft] K3 Transmit IMD

[Geoffrey Mackenzie-Kennedy]

 Julian G4ILO wrote:

> In any event, ALC
> should not have an effect on a laboratory IMD test since the two tones are
> not varying in amplitude, unlike a real SSB or PSK signal. So the 
> laboratory
> IMD test bears little relationship to the real world.

Perhaps. In the case of two constant level test tones driving the
transmitter, the level of intermod products seen at the output can change
when the ALC activated. The usual cause of this problem is that the
linearity of the low level stage or stages whose gain is controlled by the
ALC gets "worse" when the ALC starts to throttle them back. How much of an
effect occurs depends on the PA's  natural IMD behaviour, the exciter's IMD
and the IMD behaviour of the controlled stage(s) vs applied ALC.

> This suggests to me that complex characteristics
> of the ALC will have more effect on signal width in real-world SSB 
> operation
> than small differences in PA linearity that are reflected in the IMD
> measurements that only relate to the transmission of two constant 
> amplitude
> tones. Too slow ALC and the PA could clip, causing severe splatter. Too 
> fast
> and it will operate like compression creating many IMD products.

The design of ALC systems is not a casual matter especially when dealing
with human speech, and the points which you raise have to be addressed by
the designer if the ALC might be used improperly. It would appear that ALC
systems are too often used as speech processors rather than the protective
system as intended.

As a design tool and as a means for specifying transmitter odd order IMD
performance, the two tone test at least removes the variables of human
speech. When I was involved in writing Canadian Type Approval specs for
commercial SSB / ISB equipment, transmitter IMD performance was measured
with the test tones applied to the transmitter's input interface from
external generators, which was and still is the accepted practice in the
comms field. Common practice at work was to design a PA and driver stage
whose odd order IMD products relative to PEP were comfortably better than
spec at various power output levels, with both 50 ohm and "mismatched"
loads. ALC was then added later on in the design process as a protective
system for the PA.

There is an inherent problem with the type of ALC systems, when misused,
which sample the PA's output to control the gain of low level stages, which
is that splatter and other crud resulting from overdriving have been
transmitted before the ALC can reduce drive. Those ALC systems which sample
further back in the exciter chain and control the gain of a following stage
can remove these problems. The mirror image is a receiver's AGC system.

73,
Geoff
GM4ESD