first article in this series, we briefly used VA's oscilloscope
section to look at a test waveform. In this article, we look at
how to make the best use of the scope section.
Note that at the time of
writing, several functions were still not clear or behaving entirely to
expectation. To make these easier to find later, they have been
marked with (?).
Remembering that not all our
readers may be familiar with oscilloscopes and their use, let's do a
quick background. If that doesn't apply to you, feel free to skip
the next section.
Have you ever paused to
consider what it is that we electronics people do? That our job is
to manipulate invisible, powerful and sometimes deadly forces to make
them do our bidding? Sounds a bit like magic, doesn't it, and we
are going to need some magical tools to tame the beast without getting
bitten. The oscilloscope is one of the most powerful of these
tools, precisely because it renders that invisible force visible.
Without it, we're just hunting a
in the dark.
The early versions were
called Cathode Ray Oscilloscopes, abbreviated to CRO (and pronounced
either C-R-O or crow). You'll still catch oldtimers (ahem, like
me!) inappropriately use that term, even when there isn't a cathode ray
tube in sight. The cathode ray tube created, collimated and
focussed a fine beam of electrons, which were then accelerated towards
the screen, where a phosphorescent coating would glow to mark their
impact point. Plates at the top, bottom, left and right of the
screen allowed the dot to be moved around the screen. The
principle is the same as the TV tube, although TV tubes use magnetic
steering rather than electrostatic. Real analogue CROs are still
available, with prices starting at a few hundred dollars.
More recently have come
Digital Storage Oscilloscopes (DSO), using liquid crystal displays, and
offering a much wider range of extra facilities. These range in
price from a few hundred dollars to tens of thousands.
And, for those unable to
justify spending a few hundred dollars, we have oscilloscope software
for PCs, like VA. It has some limitations, which we will come to,
but it still does the basic job of an oscilloscope - it allows us to see
the invisible. I note that Alfredo uses the word Scope to describe
his oscilloscope section, so we will too.
Exploring VA's Scope
We'll pick up from where we
left our introductory article (check back there
if you forget how). We want:
You should see a pure sinewave on the
Scope (upper) screen, and hear the pure sound in your speakers. If you
don't, it might be that you left VA in some other setting last time. VA is
like hardware, the knobs stay where you left them last. If ever you can't
find your way home, press Settings, near top left. Now press the Default
Conf. button at the bottom right of the Settings window. It will warn you;
press OK. Now close the Settings window, also by pressing OK.
On my system, this also resizes VA
smaller, and I have to press the Windows Maximise button twice if I want to get
it back to full screen. It will also have turned the wave generator off, so
press Wave On again and you should see and hear the signal again.
In the old days, we had a knob that
enabled us to select the vertical sensitivity (or Y), calibrated in V/Div (volts
per division). On VA, the equivalent is the Zoom box, just to the right of
the Scope screen. It defaults to x1. You can enter a value in the
box, or use the up/down buttons beside it. Or, once you've clicked on the
box, your mouse scroll wheel.
The maximum zoom is 256. 10
produces a good sized image on my system.
Headroom, the amount of spare level you have left above the
current level, is always something to watch in digital systems. If you are
running a 16 bit system, you have 2^16 possible values (65536). Once you
have used them all up, that's it, you can't go any higher, and clipping will
occur. How can you then tell if clipping is occurring in the device under
test or in the soundcard? A peak-amplitude bar-meter at the right of the
Channel 1 controls monitors the channel's headroom, as does a dB figure just
It can be really handy to be able to invert (turn upside down) a
signal on screen. Just tick the Inv button. Watch the start of the
wave at left of screen. Instead of going up, it now goes down.
We also had a knob for the X axis,
calibrated in time, eg 2mSec/Div. VA starts with a time base of 1 pixel
per sample, which on my system is 2.1484mSec/div. I find I cannot enter my
own value in the box, but can select from values using the up/down buttons
beside it. A value of 0.3581mS/Div offers a well sized image on our 1KHz
Pressing the U button just above the
box returns us to the default 1 pixel per sample starting point. Try this,
then use the buttons to come back to 0.3581mS/Div.
Surfing the wave
The horizontal slider below the Scope
screen allows us to move along the wave, that is to say travel (just a little!)
in time. The range of time visible on screen is shown in the little window
at left. The mouse scroll wheel also controls the slider once you've
clicked on it, and the keyboard's left and right arrows offers a finer level of
control. Of not much interest in a totally repetitive wave like our signal
generator's, but very useful if examining a non-repetitive event. More of
this in later articles.
There is also a second little window
at the right of the horizontal slider, but its function is not yet clear to me,
other than it is related to time and not amplitude (?).
We need a way to be able to move the
trace up and down the screen, especially when we bring in the second channel.
The slider at right of screen does this. Note that there is a little arrow
in the green calibration band that moves with the trace to show us its centre.
Note also that the values on the gridlines also change automatically to reflect
the new position. Once you've clicked on it, your mouse scroll wheel will
also move it.
Resetting the Vertical Position to
centre could take a little fiddling, so our kind host has given us a little
button at the bottom of the Vpos slider to do just that. Try it.
Triggering is what makes the image stand still on the screen,
and not drift to left or right. At the moment, we are relying on VA's
set-up, which we can prove by this simple experiment:
Press the Wave button near top left of screen. A
screen opens. You'll see from boxes at left of screen that we have
separate control of the frequency of the test tone being fed to the left and
right channels, and that they are both set to 1000 Hz.
Press the Down button on the upper box once, and the
frequency going to the Left channel drops to 999Hz. You can see on the
Scope, the trace now drifts to the right.
Two presses on the Up button (1001Hz) and it now drifts to
We need to invoke triggering to prevent that:
Leave the Wave window open, but position it over the lower
part of the screen, out of the way of the Scope screen and its adjustments.
Tick the Ch A Trig button to assume control. Note that
a greyed-out section below it now becomes accessible. But the signal
We need to tell VA the exact point on the waveform we want
to trigger the trace at, and that's done by pulling down the Trig slider at
the right of screen. As we do, we see a dotted line across the screen
come down to identify the level at which the triggering will occur. As
soon as that dotted line intersects the waveform, the timebase locks, and
the image stops drifting.
Again, note that, once you've clicked on it, your mouse
scroll wheel will adjust it, and the keyboard's arrow keys will fine-tune
Watch the start of the trace as you manipulate the slider -
you'll see that you can control where the trace starts anywhere along its
rising slope. Now switch to Negative Slope, and you'll find you can
control where the trace starts anywhere down the trailing side.
The remaining Trigger section control is the Delta Th % box.
It controls the size of the triggering threshold, as a proportion of full
scale. As you can see, it defaults to 25%, but I see some triggering
instability (twitchiness) on the sinewave signal at that value. Try
bringing it down to 1%. Different types of signal will trigger better
with differing values. The moral of the story is, don't put up with
twitchy triggering - tweak it!
Note that now, pressing the INV button doesn't seem to
invert the signal. It actually does, but we can't see that as the
triggering control determines where the start of trace is. The signal
turns over sure enough, but then slides sideways to lock the triggering
again at the same value. But, if you had an asymmetrical signal like a
pulse, you'd see it invert. We'll play with that later.
Now we come to a quite remarkable feature Alfredo has programmed
into VA. As Alfredo notes (slightly edited):
VA has the unique capacity to perform a full real-time
Digital-Analogue conversion for the oscilloscope function, although it is
rarely well understood.
Assume the CD standard sampling frequency of 44100 Hz.
Other programs similar to VA simply plot the raw points on the screen, which
means you canít easily analyse signals with a frequency higher than
3000-5000 Hz, because there are limited points to plot. As an example, think
a sinusoidal signal of 20 KHz. You would have only two samples in each
complete sinusoidal cycle, and so only two points to plot!
The Nyquist theorem says that it is quite sufficient to RECONSTRUCT the
signal, that is, to re-compute ALL the points between those two points.
Standard software normally uses only those two points, simply connecting
them by means of a line. If you draw a 20 Khz sine wave with only two
points per cycle, without recomputing all the intermediate points, it will
appear like a triangular waveform!
Try the power of VA enabling the function "full D/A". Apply a
sinusoidal signal of 15-20 KHz (for example using the Waveform generator
included in VA). Use the "Time division" control for the selected
channel (mS/d) to display the signal at the desired detail level. You will
see a perfect waveform with all the points of the original signal (not only
Woah, a bold plan indeed, so let's see if it works:
Change the 1001Hz we left the wave generator set to by
editing it to 15000 Hz. Press Apply to accept the new value.
Speed up the timebase by pressing the lower button on the
Ch1 (L) mS/d box. Something like 0.1023 is good.
Look at the waveform - hardly a good sinusoid, eh?
Tick the Ch1 D/A button while watching the waveform.
Unfortunately this miraculous improvement cannot be made to
waveforms other than a sine wave, and any attempt just mangles them more.
But for viewing sinewaves, the full D/A is the way to go. The message D/A
On appears in the scope screen to remind you that it's enabled.
Now, reset the wave generator frequency to 1000Hz and the Scope
timebase to 0.3581mS/Div for later tests.
DC Removal (?)
I have to admit, I'm foxed by this feature at this time. I
hope it will become clear in which case I'll edit what I have to say below.
I can think of two possibilities:
One is that some soundcards might go down to DC (ie 0Hz), in
which case you might want to be able to dissociate the DC component.
Hardware oscilloscopes do this by an input switch marked DC-Gnd-AC or
something similar. It would be great to be able to go down to DC, but
I'm not aware of soundcards offering that feature. Correct me if I'm
My second thought is that perhaps some soundcards
artificially input some DC offset of their own, and this feature fixes that.
But again, I'm open to correction!
I note that the Nuova Elettronica magazine front end for VA
is AC coupled, so it doesn't appear to relate to that.
Values is a feature much more associated with the later DSO
(Digital Storage Oscilloscopes) than the earlier CROs (Cathode Ray
Oscilloscope), although I do fondly remember a portable Techtronics oscilloscope
that had some multimeter functions built in. It was a brilliant one-stop
Do be aware that, because we haven't yet calibrated the input
level (a later article), all our amplitude (vertical) values are just given in %fs
(Percent of Full Scale). After calibration, they'll be much more
Ticking the Values box opens a column at right of screen with
all sorts of juicy information:
The Frequency of the viewed signal, in Hz, tells us
how many times per second the signal cycles. VA's wave generator is
currently set to 1kHz, so we see 1000Hz.
Note that this is the frequency of the viewed signal, not the generated
signal, although in our simple case, these are the same. The frequency
readout will also work with an external oscillator.
I understand that, in the case of a complex waveform, it will read the
frequency of the highest amplitude harmonic, which is usually, but
not necessarily, the fundamental (the lowest frequency partial). Some
musical instruments make sounds where the highest amplitude partial is the
second or higher harmonic, and so may give misleading results. Most
electronic test signals will respond normally. Perhaps when we get to
the page on the Filter section, we'll see if we can fool the frequency meter
by filtering the fundamental out of a square wave!
Note the unusually high resolution of the frequency reading (0.01Hz).
We'll look at that in more depth when we get to the Frequency Meter page.
Mean Value. This should always be zero in an AC
coupled system, which takes us back to the conundrum we faced under DC
Removal. We'll come back to that when we crack the conundrum.
T RMS stands for True RMS, meaning VA has computed
the Root Mean
Squared amplitude of the waveform, a fabulously useful feature for AC
Crest Factor of a waveform is the amount by which the peak amplitude
exceeds the average value.
Value is the amplitude of the highest (ie negative or positive) peaks
with respect to the mean.
Peak-to-Peak Value is the amplitude when measured from the lowest to the
highest point of the wave. In a symmetrical wave, it will be double
the Peak value.
is the ratio of the RMS value to the average value (mathematical mean of the
absolute values of all points on the waveform).
You'll notice that a lot of the values provided above tend to
jump around a lot, making taking a reading difficult. The Infinite
Average tickbox dramatically smooths that out, while not making the system
too sluggish to change.
Measuring frequency manually
We saw above in the Values section how VA already measures a lot
of stuff for us, and to a far greater degree of accuracy than we could estimate
off screen. But sometimes we want to measure something manually. Try
Click your mouse at one zero-crossing of the waveform, then drag
to the next zero-crossing of the same phase (ie, miss one zero-crossing).
We have now identified a full cycle, and are rewarded with L=1000Hz (or
Measuring time manually
We often want to measure the passage of time, eg the width of a
pulse, or period between pulses. At this stage, I can't see any way to do
this other than count graticule divisions and multiply by the timebase speed.
We optimise our accuracy by expanding the wave to one cycle almost filling the
screen. I used 0.1023ms/Div, and nudged the keyboard arrow keys to centre
the cycle. Then I used the vertical slider to make the wave pass through a
left most intersection on the graticule. I counted squares over to
the other point where the wave crossed the same horizontal, and estimated it as
9.7 divisions. Multiply by 0.1023mSec/div and I get 0.992mSec, a good
approximation to 1mSec, which is the period of a 1000Hz wave.
I'd like a mouse-drag method of measuring time, similar to how
we measured frequency above. E.g. Shift-mouse-drag? Alfredo?
Measuring amplitude manually
Alfredo has given us a mouse-drag method for measuring
amplitude. Just click on the bottom of the waveform, drag to the top and
read off the value. (or the othere way around.) Or you could use the
calibrations in the green right hand edge of the Scope screen.
A reminder that, once calibrated, we'll see amplitude values in
much more useful units, like Volts.
Now there's only one more feature I want to show you in this
article, and that's how to change the graticule. But to do that we need to
open the Settings section (top left of screen), and chose the Scope tab.
Here you'll find all the same controls as we've been using, some a little
differently operated. Feel free to mess with them!
But near bottom right, you'll find the Scope Grid buttons that
control the number of grid lines on the Scope screen. They default to 10 x
8, but you can make them anything you want between 2 and 20.
We'll probably go on to 2 trace operation in the next article,
but, at this stage, I think both of us deserve a rest!