I'm Travis, and I'm an engineer on the academic team

here at National Instruments.

And today I want to show you a new approach

to teaching frequency response calculations of filtering

circuits.

As you know, active filters are very common building blocks

of electronic circuits.

When a student is introduced to the concept for the very first

time, a single low pass op-amp filter, such as the one shown

here, is a good example to explain the circuit theory

and how to mathematically calculate the cutoff frequency.

However, when circuit complexity increases,

such as in the case of this third order low pass filter,

the theory becomes much more challenging.

In this new approach, we can start

in the same place is traditional coursework by using

an exercise in a popular textbook

to theoretically calculate the frequency

response of a cascaded filter.

But I won't stop there.

I'll then force that theory with SPICE simulation and lab

exercise.

So now I'm going to jump into a SPICE simulation circuit, where

I can build the same circuit with SPICE models of industry

standard parts commonly used in professional designs.

Here I'm using multisim as my SPICE simulator.

I'm going to go ahead and open my three stage low pass filter.

And on the circuit schematic, I also

have multiple virtual instruments

that allow me to quickly evaluate the circuit behavior

and experiment with an infinite number of scenarios.

Iterating different component values, circuit topologies,

and different types of op-amp SPICE models.

Over here I have a function generator generating

a sinusoidal input, and I also have an oscilloscope

and bode plotter instruments connected

to the input and the output of this circuit.

Let's take a look at that.

Now if I go ahead and run the simulation,

and open my bode plotter, and we see

that we get that output bode plot of this three

stage low pass filter.

But we're not just going to stop there.

Let me go ahead and stop this simulation.

So now we're going to use some hardware

to reinforce that theory.

In this case, I'm going to use the NI Elvis

II, which seamlessly integrates with this multisim environment.

On the Elvis we have a built circuit

that shows the three stages here with their three cutoff

frequencies.

Now our Elvis board allows us to blow out

a signal with our function generator,

and read back that signal with an oscilloscope

the same way we did in the multisim environment.

Let's take a look.

So here, I'll just go ahead and choose the actual Elvis device

instead of a simulated device.

And I'll make this stimulus channel-- or the channel

I'm going to look at for the signal I'm providing-- the AI0.

Then I'm going to make the response channel AI1.

And this is going to look at one of the stages of our low pass

filter.

So let's go ahead and look at that.

I'm going to go ahead and allow real data to overlay

on top of the simulated data.

And as you can see, from a single stage

we have a low pass filter characteristic.

But it's roll off is nowhere near our simulated data.

But that's because we're looking at one stage.

Now let's look at the response channel AI2

to look at what happens from two stages.

I'm going to go ahead and run that again.

And now we're seeing that our roll off

is getting a little bit closer to what our simulated data had.

But once again, this is two stages.

So I'm going to go ahead and add in that third stage

by looking at AI channel three and running our simulation

one more time.

And as you can see, the hardware output

matches very closely to the simulated data.

Now we can ask questions like, why doesn't it match exactly?

But that's for the textbook.

So we just finished looking at a three stage low pass filter

in not only the textbook, but in simulation and hardware.

For students, this experience of going from theory

to simulation, and then to actual interaction

with the circuits, leads to a much deeper comprehension

of the theory.

To further explore this approach to teaching analog circuits,

or active filtering specifically,

download the free courseware linked on this page.