[AMRadio] Use of an oscilloscope to monitor am audio


Jim candela jcandela at prodigy.net
Sat Mar 18 13:15:25 EST 2006


Jim,

   It should work so long as the sweep circuitry has an X:Y position. I use
a Tek 465, and it has that option. Now sampling the RF and audio with these
scopes needs a little discussion.

For RF, I like to make a voltage divider to say take my highest power at
WD5JKO at 50 ohms (input to tuner) and divide that voltage to about 10 volts
peak (could be 1, or 5). Make the lower divider resistor 50 ohms. Some usage
of ohms law, and power formulas are required, but nothing tough here. Then
use coax to your scope, BNC-BNC. If the coax is very long, and you worry
about line SWR, then terminate the scope input with a 50 ohm BNC Female to
BNC male feed through termination adapter. As an alternative, if your SWR
will remain low, and the power is below 500 watts, then simply use a coax
"T" and a scope 10X probe. Stick a banana plug into the "T" with a little
piece of wire (1/2 inch) affixed to it so the scope probe can clip on. Use
the scope ground clip to the coax cable shield (a small worm drive hose
clamp fits nicely on the PL-259 knirled end, and the aligator clip clips to
the hanging clamp tang). Another option for some with a Johnson Matchbox as
they have a pick-up already included that you can tie your scope into.

For the audio, I like to divide that down to about the same 10 volts peak
level (could be 1, or 5). For low level modulation all you need is to tap
into the audio chain as close to the modulated element as possible, remove
any DC, and scale level such that the scope input attenuator can easily
handle it. For plate modulation where the plate voltage might be pretty darn
high, you need to remove the DC with a suitable capacitor, and divide the
audio level way down to the proper level. This might take a bunch of series
resistors (limit voltage across each resistor to maybe 500 volts peak), and
consider power dissipation per resistor to no more than 50% of the rating.
With a high ratio divider you will see phase shift (R-C) increase with
increasing audio frequency, and this will corrupt the presented trapezoid.
To get around this minimize the capacitive load on the divider, or
compensate the divider like is done on a 10X scope probe.

Hope this helps,

Jim



-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net]On Behalf Of Jim Miller WB5OXQ
Sent: Saturday, March 18, 2006 11:29 AM
To: Discussion of AM Radio
Subject: [AMRadio] Use of an oscilloscope to monitor am audio


I have a Tektronix model 453a scope and I would like to know how it can be
used to monitor transmitted signals.  Do I need extra hardware or an
interface of some type to obtain the trapezoid pattern?  I know some scopes
designed for amateur radio have this feature built in so I wonder if my
scope can be used for this?
WB5OXQ.


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