I have a Heathkit monitor that provides that trapazoid pattern - not
sure if it has a detector in it on the input, but it certainly provides
a decent trapazoid that gives a pretty good indication of linearity.
Gonna take a look at the schematic this afternoon (if I can find it :?}
Since it shows rf on the vertical, I suspect it's detecting the input
and feeding audio to the horizontal deflection plates - in fact I know
it is since you can control the horizontal gain, and I doubt you could
do this with such a simple scope at 14mhz!
Dave - K5WNV
-----Original Message-----
From: tentec-bounces@contesting.com
[mailto:tentec-bounces@contesting.com] On Behalf Of Sinisa Hristov
Sent: Wednesday, May 03, 2006 12:39 PM
To: Discussion of Ten-Tec Equipment
Subject: Re: [TenTec] Orion - K3NA transmit audio chain article now
onTen-Tecwebsite
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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