Martin AA6E wrote:
> You could check the boxes being sold by www.cleanrf.com as described in
> the latest CQ. They let you look at "linearity" on a scope. My take is
> that you will only catch gross non-linearity (overdrive), but you
> wouldn't get quantitative IMD numbers with this approach.
Single Sideband Principles and Circuits includes a section on
calculating IMD from measurements of input vs output voltage. I reckon
this will include any effects from power supply sag.
For amplifiers where the transfer characteristic is linear, then going
into gain compression it follows through to graphs of IMD against % flat
topping on a two tone test. You can look at this by sampling the input
and output and comparing on a X-Y display, or make two identical
detectors and adjust the coupling so that the voltages detected on the
input and output are identical at low level. A meter showing the
difference will show you when non-linearity sets in. It won't be
automatic readout of IMD to 5 digits, but if you learn how to use it, it
will tell you most of what you need to know with voice or test modulation.
As a very rough rule of thumb, don't go beyond the point where Po/Pi has
dropped by 20% from its low level value. Don't be surprised if this less
output than you think you should run.
Steve
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