Oscilloscope Basics for Synthesizers - Modular Synthesizers 102

This is part of the Elk Elektroniks synth class on the 21st November.

The class will be held in Wollongong, NSW, AUSTRALIA.

Check out the FB link for more details

https://www.elkelektronik.com.au/ 

https://www.facebook.com/elkelektronik/  

 

The first part of the class will cover the topic:

" What are Modular Synthesizers??"

We will be using scopes and the question of how to use them will probably also come up.

Thus this second part.

 

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Oscilloscopes look complicated. There are many thousands of models. 

The models used in this tutorial have 2 ports.  They accept probes with BNC connectors.

You will commonly see scopes with 1, 2 & 4 channels.

When buying a scope, there are a few things to consider.

 

The first is Bandwidth. This is the maximum frequency range the scope can measure.

The ideal is to choose a scope with a max bandwidth at least 5 times the maximum frequency you intend to measure. Most entry level scopes have a bandwidth of around 100MHz. This means that they are really only good at accurately measuring sine wives of around 20MHz.

Sample Rate:

This is really only applicable to digital scopes.

The greater the sample rate, the better. 

It's the number of times a second a signal is read. Get a scope at least 5 times your expected highest frequency.

Most entry level scopes have a sample rate of 1 to 2 giga samples/sec

 

                                                An Analog Scope - Using a cathode ray tube.
 

In the world of synths, being able to use a scope is really useful. 

Most electronic musicians have a multimeter. This allows us to measure & "see" a voltage at a specific time.

What a  scope enables  is the visualization of a "moving" or constantly changing voltage over a much longer

period of time. I like to use the analogy of a still photo vs a movie.

 

                                                         A modern Digital Scope
 

The Multimeter, takes a photo.

The Scope does the movie.

 

To properly take a photograph you need to set the shutter speed correctly & frame the image.

It's no different with a scope. You will even find a focus setting and filters on your scope.

Being able to visualise your sounds will make music production more predictable and you will be able to trouble shoot your DIY builds easier..

 

 All scopes whether they are analog or digital have 4 basic sections:

1. Vertical

2. Horizontal

3. Trigger

4. Screen (digital or analog CRO)

 

1. Vertical

 

This uses the volts/div control .

(remember Vertical = Volts)

It adjusts the amplitude (height) of the signal on the screen.

 

The vertical controls are specific to each channel. 

So if you have a two channel scope, you will have two individual  vertical controls.

Common vertical controls include:

+ Voltage Scale. (volts/division)

+ position / Auto

+ Coupling (DC/GDN/AC)

+ Invert (on/Off) 

+ Bandwidth Limit

 

 

 

Position

This control is useful for also pulling a signal into view (if you initially can't see anything on the display).

It moves the waveform up and down to exactly where you want it on the screen.

Some scopes have an "auto" button which helps to position your signal on the centre of the screen.

 

DC vs AC vs Ground Coupling

DC coupling shows all of an input signal.

AC coupling : you see the waveform centered around zero volts

Gnd  lets you see where zero volts is located on the screen.

Bandwidth limit

By limiting the bandwidth, you reduce the noise that sometimes appears on the displayed waveform, resulting in a cleaner signal display.

The BW limit setting can also effect (clip) the display of the actual waveform  ... be aware of this.

 

 

 

2. Horizontal

 

This is the time base.

 

 

This uses the sec/div control. 

It will position & scale the waveform correctly.

This control effects all channels

 

 

 

 

 

 

 

Common Horizontal controls are:

+ sec/div control.

+ Time Base

+ Delayed Time Base

+ XY Mode

+ Position

+ Z axis

 

sec/div control.

Here you control the time period that the signal is displayed on your screen.

 

The position moves the waveform left and right to exactly where you want it on the screen.

 

 

Many oscilloscopes also have what is called a delayed time base. Here, the sweep can start at a pre-determined time relative to the main time base.

 

 XY mode that lets you display an input signal, rather than the time base, on the horizontal axis. 

This allows you to do phase shift measurements.

 

 

3. Trigger

 

This is how you stabilize the waveform. 

Without choosing the correct trigger the scope will sweep horizontally across the screen,  and your display will repeat & appear to move.

Triggering will make the moving waveform appear static by repeatedly displaying the same part of the waveform.

 

The trigger setting effects all channels.

You need to also set a trigger level.

The most basic form of triggering is "Edge  Triggering". 

It could be on a rising or falling edge.

 

 

Common trigger modes include normal and auto:

(a) Normal mode: the scope only sweeps if the input signal reaches the set trigger point.

     (the screen will be blank (analog scope) or freeze  (digitalscope)

(b) Auto : the oscilloscope sweeps, even without a trigger. 

      A timer will trigger the sweep even if no signal is present, so the display will not disappear.

 

Most of the time, you just need to set the scope to trigger on the displayed channel.

Some Oscilloscopes can also use extra trigger sources that may not it be displayed.

So be careful not to trigger on channel 1 while displaying channel 2.

If you are trying to display a trace from a modular synth, the easiest way is to set the scope to external trigger mode (EXT).

Then connect a trigger or gate from your modular synth.

 

 4.  Screen

 

This is where your waveform will be drawn.

 

The screen is divided into a grid.

My analog scope has a 10 by 8 grid

The horizontal is your time base.

Vertical = voltage.

 

Each Vertical division can set to represent a specific voltage.

 

The horizontal can either be set by the user, or it may be an automatic setting (if you don't know much about the signal you are measuring, at the start).

 

My digital scope shows the actual horizontal time division

 (demo in class)

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Auto setting -- 

Great for quickly capturing a signal

(Digital scope obviously)

 

It's like the auto button on a camera. Great in most cases.

 But to capture a signal really well, the manual controls are the best.

The scope must be in the Green "Run" mode.

Like on a camera, you need to first zoom out to see the object you wish to photograph.

Then zoom in to get a detailed image.