From: w8jitom@postoffice.worldnet.att.net
Priority: normal
> From: "George T. Daughters" <gdaught6@pobox2.stanford.edu>
> Priority: normal
>
> hi tom,
>
> in your very helpful reply you wrote,
>
> > You can do IMD tests simply at home with a good selective receiver,
> > and attenuator pad, and TWO separate transmitters.
>
> (snip)
>
> > Feed the PA with two signals of equal level with a known spacing (say
> > two kHz) and tune the receiver to each of the two tones.
>
> how do you recommend this be done? a combiner/splitter? a resistive
> mixer? (high power resistors would be necessary) what's your method?
Hi George,
I use a magic T combiner. Any combiner with input port to port
isolation will work fine.
A magic T is a tightly coupled center tapped winding, with a resistor
twice the 3 dB port impedance value across it. The input is the
center tap.
The port to port isolation (outer ends of the windings) require
proper termination with the resistor and at the center tap.
You will need a 1:2 step up transformer at the center tap. It looks
like this: 100 ohm R
--WWW--
50 ohm source ^-))))))))))))-^ 50 ohm source
^ ct
^
25 ohm output
Of course the 25 ohm output needs to be stepped up to 50.
For a single band you could use a pair of 75 ohm 1/4 wl lines, and
the result would be a pair of 50 ohm inputs with a ~50 ohm output.
Remember a spectrum analyzer is just a sweep receiver with a
CRT S meter display. No need to run out and buy something that you
already have just to look at known frequencies one at a time.
Fair radio has surplus Selective Level meters for sale that would
work very well. They are usually cheap. I bought a good one at a
hamfest for $50. Compare that to a 141T.
73, Tom W8JI
73, Tom W8JI
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