[AMPS] SSB interference

[Tom Rauch]

Hi Verner,

> Would a pulsed two-tone modulation reveal something ???

Not a pulse, but a low frequency sine wave of a few Hertz would be 
good as a third tone.

Two tone tests (and noise notch tests), despite popular rumor, do 
not show most problems caused by poor supply dynamics.

The reason is quite simple. If you inject two constant tones, the 
loading on the supplies is constant except for a high frequency 
ripple that is equal to the tone spacing. 

Say for example you inject a 14.000 MHz and a 14.005 MHz signal 
into an amplifier.

The power supplies and bias supplies are loaded with a steady dc 
load except for the difference between 14.000 and 14.005, or a 5 
kHz tone. Because of the constant load, poor dynamic regulation 
simply won't show up as IMD. The filter and bypass capacitors 
prevent the 5 kHz tone from affecting the supplies in many cases.

If you vary the level of both tones at a few Hz rate that is not 
synchronized to either of the two main tones, poor supply 
dynamics will show as IMD. You do have to use a peak indicating 
IMD measurement device.

There is a very large difference between a three tone test and a two 
tone test when testing PA's or exciters with poor dynamic supply 
regulation.

My IC-751A, with internal switching supply, was almost 8 dB worse 
on IMD using a three tone test. It cleaned up with a external supply.


73, Tom W8JI
w8ji@contesting.com

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