Welcome back to Linear Circuits. This Dr.
Ferri.
This lesson is a lab demo. It's on guitar string filtering.
So, filtering again is our last topic
here and very important application of reactive circuits.
So, we're going to be looking at guitar string, and we want to do filtering on it.
So, we're going to use a tone control filter.
I got this from this website right here.
But you, if you play electric guitars, you may be
familiar with tone control, where you would take the output
of your guitar pick up, and then you put it through a circuit.
And, then you look at what goes to your amplifier.
And in tone control, you can adjust certain frequency ranges
and make maybe mid range frequencies higher or, or lower.
So, in this particular circuit, we're
looking at these parameters that we choose.
In this configuration, it's going to be a
low pass filter where we have two potentiometers.
One adjusts the, the tone of it or
adjust the frequency shaping and the other for volume.
So, in a general sense, going back to
what we were looking at with frequency response and
filtering, if this is my circuit, the one
that I just showed you for the tone control.
And this is the input from the guitar pick up.
And this is the output that would be sent to an amplifier.
If I had an input signal that looks like this.
And this was the shape of a low
pass filter, the output would look something like this.
Now, the formulas for it, well, if my input
is a sine wave as I'm showing here, then
the output amplitude, the output corresponding output sine wave
is related to the input amplitude by this magnitude.
So, in other words I can talk about is the magnitude of the input
times the magnitude of the transfer function
gives me the magnitude of the output.
In ability plot I
find the Bode plot by taking 20 times the log of this to give me this right here.
So, in a logarithm scale when I multiply things and then take a log,
it's the same as adding once I've taken the log to get this signal right here.
So, the log scale, I'm adding terms
whereas, in the linear scale I'm multiplying terms.
So, let's do, let's look at the lab demo for the guitar string filtering.
Now, let's take a look at this circuit.
We've built the tone control circuit on this bread board.
These are the two potentiometers.
This is the capacitor right there and the two resistors are right there.
The output of the guitar pickup goes into the circuit
and then the output of the circuit is over here.
So,
remember the way that a guitar pick up works is that it generates electricity.
When I move this and it vibrates, its vibrating metal, vibrating inside of a
magnetic field, it generates it's own electricity
so it generates a signal that way.
And then the output of the circuit is over here and
we're going to measure the output of this circuit with this mideck.
So, let's go ahead and look at the frequency response.
I start out, we start this looking at this display.
What we see before I've done any signals is that there's some noise in here.
Noise at about 3000 hertz and close to 2000 hertz.
That's just and it noise electrical magnetic interference.
Lets ignore that and just concentrate on.
The noise, the signal that we induce when we pluck the string.
[MUSIC]
Okay, we plucked the string, we see all the harmonics in
there, first harmonic, second harmonic, and so on, and so on.
And this is the row signal that is not filtered.
Now, let me change this to look at the other channel.
This is the output of our circuit.
Let me, me start this.
[SOUND]
And what we see is the filter, this is a low pass filter.
It got rid of a lot of these. It really attenuated this high frequency.
Oh, quite a bit in this signal.
So, that's the effectiveness as a low pass filter.
It might help for us to actually see this
filter and see the Bode plot of the filter.
I'm going to go ahead and generate this.
This is just doing a sign sweep. Oh, I need to
take out the guitar string and plug in the
function generator. because that generate it's a sine wave.
Now, let me run this.
So it's automatically generating the Bode plot.
You can see the magnitude part, it's going down like a low
pass filter with a bandwidth that's in the range of about 1,000 hertz.
Now, let's take a look at these signals a little bit more carefully.
This is the free, the frequency response of the low pass filter.
And I've just highlighted the magnitude part of it and if
we look at this at the low frequency it's about minus ten.
So, going over here minus three db from
that is somewhere in this region so the bandwidth
is close to a 1,000 hertz
for this circuit.
Now, the input and output spectra is shown here.
First of all, this is the input spectra.
And again, we've got the different peaks of the harmonics there.
And then this is the output.
So, this is the input to my tone control circuit and then
this is the corresponding output for that same time that I plucked it.
And what you'll find here is that there's a ratio between the initial peak at
the low frequency and than the higher frequencies.
The bandwidth is right around here.
So, prior to this bandwidth, its passing it through without much attenuation.
And then it starts attenuating and the further
out to the right the more it attenuates.
And so if I look at the, the ratio between these
first peeks its a little bit lower here, this being the output.
Again, this is the bandwidth here.
So, the ratio is down a little bit because it does lower it as
we go higher in frequency, but the ratio down here this is attenuated much more.
This peak right here is attenuated much more than this one and we can
see it by looking at the ratio of this peak to the initial peak.
So, this peak if I, if I look at these, these like here,
it goes down much more this does over here.
And that's that additional amount is due to that filtering at the high frequencies.
So, in summary, what we seen is we've
examine the input output relationships on a linear scale.
I multiply the two things.
I multiply the amplitude of the input times the
transfer function and in the bode plot I add them.
So the, so the bode plot of the
filter decreases the value at high frequencies for a
low pass filter.
And we found that the first order filter has
a roll off of minus 20 decibels per decade.
Passive filters are made of R, L and C components.
That's what we built here, was a, one with a RC components.
And it required no power supply. Now, it's very common to build something
called active filters, where we use resisters and capacitors and outbacks .
And
those will require a separate power supply, but
they are much versatile than a passive filter.
And you can get a lot more characteristics and higher order filters.
So, commonly when people have to build filters for
specific measurements, they often times do it out active filters.
Now, beyond the scope of this class but
it's something that you might want to investigate.
Active filters, outback filters, if ever you have to actually build one.
But the fundamental concepts are the same.
So, this is the last lesson in this module prior to the wrap up, so make sure that
you review the wrap up lesson and also go
to the forum to ask questions and answer questions.
And make sure you do the homework and the quiz for this question.
Thank you.