There was a time when every
broadcast engineer had an
oscilloscope for troubleshooting and
aligning equipment, from a
microphone input to the antenna. It
is true that the nature of our
plug-and-play world has reduced the
requirement for this kind of
instrument, but using one can be
very helpful in solving problems.
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On the bench. |
You can literally "see" what an
audio circuit is doing wrong on an
oscilloscope, while your ears tell
you the audio has a problem. My
favorite is the Tektronix model
2213A, which is no longer being
manufactured. They are sometimes
available at hamfests for around
$100. There are many new
oscilloscopes available on the
market for more money.
For those who do not understand
or use an oscilloscope regularly,
let�s look at the basics.
Follow the trace
An oscilloscope has a display
screen, usually a cathode ray tube,
with a dot that travels from the
left side to the right side and then
starts on the left again. This is
called the trace.
You can control the speed of this
dot with the sweep speed control.
There is at least one vertical input
to deflect this dot up or down by
how much voltage the oscilloscope
sees at that instant. My
oscilloscope has a 3-by-4-inch
screen and is just right for the
job.
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Just a little
clipping. |
Let's start with looking at a 1 kHz
audio sine wave. By setting the
sweep speed to correspond to that
frequency, we can see an audio wave
standing still on the screen as you
do in the photo of a complete
oscilloscope. At that point, you can
look for distortion and oscillations
riding on the wave. The screen has graticules to divide it into equal
parts left to right and up to down.
Most oscilloscopes are calibrated
so you can use these marks for
measuring peak-to-peak voltage on a
waveform. Sweep speed is calibrated,
too, and can be translated into
frequency. The higher the frequency,
the faster the dot needs to go to
follow the waveform.
When I work on an analog audio
console, I head straight for the
calibration controls. Often,
announcers hoping for better sound
quality on the air have accidentally
or mysteriously misadjusted them. I
put a 1 kHz tone into a microphone
input and adjust the input level
followed by the console program
amplifier gain until there is just a
tiny bit of clipping on the top and
bottom of the waveform at the
console output terminals.
This is the limit to how much
audio the console can produce. Then
I reduce the audio input by 20 dB
and set the console VU meters to
read 100 percent, just before the
red 0 to +3 dB portion of the meter
display. This gives the classic 20
dB of headroom from 0 VU before
clipping starts to occur.
Exact audio output level is
usually irrelevant, although it
usually comes out to be about +4 dBm
active balanced, which is 2 dBm
(1.74 Volts peak to peak) on each of
the positive and negative audio
outputs. With a dual-trace
oscilloscope you can see left and
right audio channels simultaneously.
This is good for setting equal
console gain on both channels.
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Sine wave on
Tektronix 2213A
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You can watch audio with sine waves
or with actual program audio. Follow
audio from section to section in an
amplifier or place to place in a
radio station. You might see the
left channel going in the positive
direction on the screen while the
right channel goes in the negative
direction. This is a clear
indication that there is a phase
reversal in the system that might
sound acceptable when listening in
stereo, but will sound low level and
terrible in monaural.
The key here is that you can
actually see what the audio is
doing.
AM use
One great use of use an
oscilloscope is to set modulation on
an AM transmitter. A photo shows
about 90 percent negative modulation
when voice program audio has
depressed the carrier to almost zero
at times. You can also see that
positive modulation at the top and
bottom of the display is being
limited or clipped.
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AM modulation
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Modulation monitors can and do go
out of calibration. The display on
an oscilloscope cannot lie. If an
oscilloscope shows 100 percent
negative modulation with the carrier
going to zero along the horizontal
centerline, you should believe it.
I employ an oscilloscope on the
service bench when adjusting an RF
generator for 100 percent AM
modulation and then use that RF to
calibrate modulation monitors. No
laboratory standards are required.
The method is simple and doesn�t get
any better than that.
In conclusion, an oscilloscope
always went with me on the road to
radio stations when doing updates
and repairs. It continues to be an
extremely important test instrument
on the service bench. I couldn�t do
the job without one.
Mark Persons, W0MH, is
certified as a professional
broadcast engineer by the Society of
Broadcast Engineers and has more
than 30 years experience. He has
written numerous articles for
industry publications over the
years. His website is
www.mwpersons.com. Read past
Tech Tips articles under the News &
Technology tab of
radioworld.com. |