Bode ploting

[Guida]

I am a Portuguese science student and although I am a little ashamed
to be doing this question, I really need to do it as I have a
practical exame this week.

One of the tasks we have to do is to make a frequency analysis of a
circuit in order to get a bode plot. I know how to do it making manual
increments of frequency and registering the amplitude. However, I have
to do it automatically and without using multisim. Somewhere in my
notes I have written that it has something to do with the SWP mode of
the function generator but I still don't know how to do this kind of
plot automatically using only a function generator and an
oscilloscope.

I wonder if you could help me.

Sorry for the trouble.

AV

 

[Bob Masta]

On Mon, 24 May 2010 04:31:37 -0700 (PDT), Guida
<ana__valente@hotmail.com> wrote:

I am a Portuguese science student and although I am a little ashamed
to be doing this question, I really need to do it as I have a
practical exame this week.

One of the tasks we have to do is to make a frequency analysis of a
circuit in order to get a bode plot. I know how to do it making manual
increments of frequency and registering the amplitude. However, I have
to do it automatically and without using multisim. Somewhere in my
notes I have written that it has something to do with the SWP mode of
the function generator but I still don't know how to do this kind of
plot automatically using only a function generator and an
oscilloscope.

Your function generator needs to have a sync
output of some sort, which you use as the trigger
input for the scope. Even if there is not a
separate pulse output for this, there is likely a
VCO control voltage. It will be a slow sawtooth
that rises in voltage as the function generator
rises in frequency. If you trigger on the
*falling* edge of this, at the end of a sweep, the
scope trace will start at the next sweep start.

You will of course need to set the scope sweep
very slow, similar to the function generator sweep
time.

Alternatively, you could put the scope in X-Y mode
and use the VCO signal as the scope sweep itself.

If you view the circuit response with the scope,
you won't get a single-line trace like the typical
Bode plot. Instead, you will get an envelope
whose upper edge is the same as the Bode plot.
There will be a symmetrical negative edge on the
envelope, and between them will be more-or-less
solid light.

If you want a conventional single-line plot, you
will need to rectify and filter the circuit output
before feeding it to the (DC coupled) scope input.
It won't be a conventional log-log Bode plot...
for that you'd need to add log converters.

Though you may need to use the class-supplied
scope and function generator for this assignment,
note that for audio frequencies you can do
everything with a computer and sound card. It can
generate the sweep and plot the response,
including showing it in log-log format.

Best of all, it can use faster (and often better)
methods than a simple sweep. See the
Frequency Response Measurement topic for my
Daqarta software at:

<http://www.daqarta.com/dw_0akk.htm>

Best regards,



Bob Masta

DAQARTA v5.10
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, Sound Level Meter
Frequency Counter, FREE Signal Generator
Pitch Track, Pitch-to-MIDI
DaqMusic - FREE MUSIC, Forever!
(Some assembly required)
Science (and fun!) with your sound card!