On Wed, 2006-05-03 at 13:38 -0400, Sinisa Hristov wrote:
> 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 receiverâs
> 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
> scopeâs 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 45Â 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. Youâre 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
> indeterminate 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
And I don't think its actually been done because this technique DOES NOT
CREATE A LINE on the scope with SSB. It creates a trapezoid for AM only
that you hope to have two straight lines as upper and lower boundaries.
To get the lines as those boundaries you need to have zero phase shift
vs frequency for all audio components through the entire transmitter.
With coupling and bypass capacitors and filters (analog or DSP) at audio
you get varying phase shifts and that line opens up into an ellipse,
every time.
We got a nice straight line at Collins when demonstrating the 821A-1
(250 KW AM) at least on the scope but that was comparing RF output to
the audio detected by the modulation monitor and so was the transfer
curve for the modulation monitor not the transmitter.
One can only compare audio in to RF out on SSB at a single audio
frequency by including a variable phase shift network to zero out the
phase shift and then with a SSB transmitter you get a single amplitude
carrier so you achieve NO INFORMATION from the test. You can get a
bowtie pattern from a SSB transmitter IF and only IF you apply two tones
and solve the audio phase shift problems.
For a line you have to detect the RF output and know that the detector
is linear.
Collins always used intermod components to check SSB transmitters by
running two tone audio and a narrow bandwidth spectrum analyzer. Driving
to gain compression or clipping drastically increases the 3rd and higher
order intermod products, which in most solid state PAs are already far
poorer than 6146A with RF feedback like used in the Collins S-line doing
-35 db 3rd order intermod. Solid state finals have a hard time doing -25
dB 3rd order and often have strong higher order intermod products, e.g.
splattering all across the band.
I heard the details of the KWM-1 PA design which preceded the S-Line
from Warren Amfahr, W0WL, who made it work after he got RCA to linearize
the 6146 which became the 6146A. He told me about that at the DMRAA
hamfest April 22, this year.
Details on bow tie displays were probably published in QST about 1955
and in the late 50s to early 60s ARRL SSB handbooks.
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|