Sinisa,
What do you recommend then as a method for picking off a sample
for the 'scope? Is there a relatively inexpensive method that gives
reliable results?
73, Duane
On Wed, 03 May 2006 13:38:46 -0400 Sinisa Hristov <shristov@ptt.yu>
writes:
> Quoting from page 7:
>
> Amplifier linearity check
>
> Since you have made the transmitter adjustments at the full
> power level appropriate for your class of entry, the monitor
> receiver?s
> audio has included any impairment introduced by the
> power amplifier (if used).
>
> An oscilloscope can provide a quick check for linear operation
> of an amplifier. Wind a length of hook-up wire around the coax
> line connected to the amplifier input and another length around
> the output coax. Increasing the number of turns will increase
> the
> signal picked off the coax; I find this simple approach picks up
> enough signal to deflect the scope beam. Connect one wire to the
> scope?s X input via a probe, and the other wire similarly to the
> Y
> input. Adjust the X and Y gain controls so that, while
> transmitting
> the recorded message, a diagonal line of around 45° appears on
> the scope.
>
> Examine this line, focusing particularly on modulation peaks. If
> the diagonal line remains straight and the audio quality is good
> in
> the monitor receiver, the amplifier is operating linearly
> (good!). If
> the tip of the diagonal line starts to deviate away from
> straight,
> even just a little amount, then the amplifier has moved into
> nonlinear
> operation. You?re definitely transmitting some garbage on
> the bands then!
>
>
> It's difficult to imagine a more unreliable way of sampling
> amplifier's input and output signals.
>
> A wire wound around coax is a capacitive probe picking an
> undeterminate mixture of coax leakage and common mode voltage.
>
> The later is likely to dominate, even on the "input" probe,
> invalidating the test because it originates mostly from the
> amplifier output (i.e. coax - antenna junction).
>
>
> 73,
>
> Sinisa YT1NT, VE3EA
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>
Duane Calvin, AC5AA
Austin, Texas
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