[TenTec] IM figures

[W8JI@contesting.com (Tom Rauch)]

> The PSK crowd really gets upset when I tell them I'm using a 500hz
> filter. They say stuff like...."You need to keep the filters open wide
> so you can see all the stations on your waterfall." Weird Science!

This goes back to the same problems receivers in general have.

We often look at data taken for two pure tones, and assume the 
dynamic range for two tones is the dynamic range for multiple 
tones. 

The accumulated power of many signals within the roofing filter 
reduces the dynamic range from what we might expect from two-
tone test values. In other words because a receiver might have 
50dB DR range (typical for anything filtered way back in the IF or 
audio system with wide front end filters) with two pure single tone 
signals, it doesn't mean you won't have problems when everyone in 
the passband is less than 50dB stronger than the noise floor signal.

A similar problem exists in testing transmitters and amplifiers for 
IMD products. Testers use IM taken with two steady tones, or 
steady white noise and a notch, which does not show IM 
deterioration for syllabic power supply loading changes. The two-
tone amplifier or transmitter test, because it misses power supply 
and bypassing problems, often provides inflated IM performance 
figures.
73, Tom W8JI
W8JI@contesting.com