Digital Oscilloscope Tutorial by K7QO
Version 0.5
August 2, 2008
compiled by
Chuck Adams, K7QO
chuck.adams.k7qo@gmail.com
1 Preface
This tutorial is written for the purpose of introducing you to the new
technology of digital storage oscilloscopes (DSOs). They are now at
the price point where you and I can buy one or more without getting
ourselves into a deep financial hole in the process.
I am hoping to take a methodical and somewhat scientific approach
to reviewing one oscilloscope that I have so far found to be the best
for the money. I’ll start with what I know and develop all the steps
that I think will review the strengths and weaknesses of the product.
Time will tell how well I do. Give me time to do the work, but I
will post results as they develop and then tell you when I’m through.
There will be mistakes in spelling and some sentence structures, but
I promise to clean and polish as I go. I read my work over and over
to double and triple check for errors of all kinds. So be patient.
2
2 Introduction
A human being, for purposes of this tutorial, basically has five main
senses for response to external and internal events. These are
• sight
• hearing
• touch
• smell
• taste
Let’s not get into the additional thermal, balance, and time passage
responses.
Our response to external stimuli is not that fast and there are some
things that we have difficulty sensing. For electrical and electronic
devices we need help in determining exactly what is happening when power
is applied to a circuit or combination of circuits. For assistance
in the electrical world, we rely upon all types of meters and instruments.
I will assume that you are familiar with some of the basic ones and
how to use them without doing too much or any damage to things you are
experimenting on.
What we are interested in here is the purpose and the use of an oscilloscope.
An oscilloscope is used to give us a visual display of electrical signals
that may be varying or oscillating at very high frequencies or even
at audio frequencies. We just don’t have the built in sensory mechanisms
for detecting electrical signals except at dangerous voltage levels.
3
3 Basic Sections of an Oscilloscope
An analog oscilloscope typically has four main sections:
• input amplifier
• time base
• power supply
• display
4
3.1 input amplifier
There is an input amplifier for each channel of an oscilloscope. This
usually is the one component of the device that determines the final
cost. The higher the response frequency or bandwidth of the input channel
the better off you are. Spare no expense, if you can, and get the best
response that you can afford. There is no limit to how much you can
spend on a scope.
Dual-channel scopes are typically the norm so that two input signals
may be displayed simultaneously on the display screen.
3.2 Signal Source for Initial Tests
For initial testing of the DSO I am using a NorCal FCC-1 and FCC-2 DDS
generator to generate an almost pure sine wave signal up to 20MHz. We’ll
do some more complex signals later on as we progress. I’ll also be
using a Tektronix 191 Constant Amplitude Signal Generator that will
go from 350KHz to 100MHz with variable output levels. I’ll attempt
to get a picture of it here ASAP.
As you can see in the photo I have an aluminum plate with a right-angle
bend to hold two BNC connectors. One is the 50-ohm output and the other
I am using a 10-turn 10K pot to allow me to vary the voltage out through
the other BNC connector.
5
NorCal FCC-1 and FCC-2 combo.
6
4 DSO-2250 Digital Oscilloscope
The DSO that I will discuss and experiment with the most is from TaoMore
at
http://www.taomore.com/
and click on test equipment. They have three models DSO-2090 (100MS/s),
DSO-2150 (150MS/s), and DSO-2250 (250MS/s). Their costs are 131.25,
168.00, and 257.25 US dollars repectively. I’d go with the best you
can afford. There is about 28.00 US dollar shipping charge and it comes
via express mail. It took me 4 days after the order to get the package.
We just gotta hope they don’t raise the pricing after this tutorial
gets discussed and orders pour into China.
DSO-2250 from TaoMore.
7
After easily installing the software on my WinXP system and starting
up the system, I must say that I was blown away with the interface.
TaoMore did all the things right that I wanted previously with the other
DSO discussed in the previous section.
The windows were easy to save with the push of a button. You can
save the display in JPG or BMP format and I chose JPG. The controls
for the scope and the spectrum analyzer are all on the display with
pull-down menus or buttons for selections.
The spectrum analyzer has the dB scaling and the x- and y-axis are
labeled for easily reading. You can slide the display up and down and
left and right for display of desired sections.
The scope display can be slid right and left to look at various sections
of the waveforms.
This is a keeper. And the $257 price tag blows away a number of
other choices that you may find by searching the Internet. Faster sampling
at 250MS/s with a single channel input and 150MS/s for the dual-channel
mode. This will do for me and my experiments in RF design and measurements
at radio amateur HF frequencies.
DO NOT every use this scope to measure line voltages or tube voltages
or anything where more than 35V may be present. You will destroy the
input circuits and you’ll be very very sorry that you destroyed such
a fine instrument. Keep it away from AC line volatages, switching power
supplies, etc. Use only at QRP levels for transmitters. I’ll give
you some examples later on.
8
DSO-2250 INPUT SPECS
Sample Rate (Max) 250MS/s using one channel
125MS/s using two channels
Channels 2
Bandwidth 100MHz Analog
Vertical Resolution 8 Bits/Channel
Gain X1 Probe: 10mV/Div, 20mV/Div, 50mV/Div, 100mV/Div,
200mV/Div, 500mV/Div, 1V/Div, 2V/Div, 5V/div
Gain X10 Probe: 100mV/Div -> 50V/div
Range 8 Divisions
Offset Level +/- 4 Divisions
Coupling DC, AC, GND
Offset Increments 0.02 Div
Impedance 1M ohm
DC Accuracy +/- 3%
Input Protection 35V Peak (DC + Peak AC < 10KHz,
without external attenuation)
Display Mode Y-T, X-Y
DSO-2250 TIMEBASE SPECS
Timebase Range (/Div) 4nS -> 1Hr
4nS, 10nS, 20nS, 40nS, 100nS, 200nS, 400nS,
1µS, 2µS, 4µS, 10µS, 20µS, 40µS,
100µS, 200µS, 400µS, 1mS, 2mS, 4mS,
10mS, 20mS, 40mS, 100mS, 200mS, 400mS, 1S, 2S,
4S, 10S, 20S, 40S, 10min, 20min, 40min, 1Hr
Acquistion Mode Real-Time Sampling: 4nS/Div -> 400mS/Div
1S/Div -> 1Hr/Div
Range 10 Divisions
Buffer Size 10K -> 512K Samples
9
DSO-2250 Trigger Specs
Mode Auto, Normal and Single
Type Edge Trigger: Rising or Falling Edge
Autoset Yes
Range 10 Divisions
Trigger Level +/- 4 Divisions
Setability 0.02 Division Increments
10
Above is the box in which the DSO-2250 is packaged in. You’ll note
there is a 500Ms/S part number, the DSO-2500, which is not listed on
their web page yet. It will be very interesting to see what the price
point is for it.
The box was surrounded by styrofoam packing and well protected for
the trip from China to AZ. You will get a tracking number and you will
google for Worldwide Express Mail Service and get a URL in China. You’ll
see it leave China and arrive at the Customs entry point and then get
to the USPS Express Mail route and tracked to your destination. You
just gotta love the Internet.
11
As you can see, the unit is bubble wrapped for additional protection
for the long trip to its new home.
12
Here we have the DSO still in the bubble packaging.
You have a plastic pouch for holding the two scope probes that come
with the oscilloscope.
The USB cable is on the right side.
13
Here we see both PP-150 probes on the BNC ends with labels. Note
that at 10X, where the input voltage to the scope is divided by 10 and
thus reduced to the DSO, you get a higher resistive impedance seen at
the measurement point and 18.5pF capacitance between the point and ground.
This should reduce the effect of taking the measurements in a lot of
active circuits.
The X1 value of the probe shows a much larger capacitance at 115pF,
thus greatly effecting a large number of measurements in active circuits.
I’ll demo this later in this paper.
14
15
Here is the probe tip along with a plastic cap contained in the zip
lock bag along with spare coloring rings for the probes and the adjustment
screwdriver.
This tip will keep you from damaging circuits by avoiding shorts
to ground with the silver ring behind the probe time contact. Don’t
lose these. They are wonderful for keeping you out of trouble.
16
Here is an example of using the probe tip with the protective cap
to take a measurement on one of the pins of an integrated circuit (IC).
It is almost impossible to short two adjacent pins using this ingenious
device. Those of us that have, over the years, made numerous measurements
certainly wish we had these things decades ago.
Don’t abuse them. They are your friend.
17
Here is a photo showing the adjustment capacitor in the probe and
the adjustment screwdriver that is in the zip lock bag. DO NOT adjust
the cap. It was set at the factory and should not require any attention
from you for a long time to come.
18
Here is the back of the DSO showing where the USB cable plugs in
and also that there is a calibration signal available from the unit.
19
Here is the front of the DSO unit showing the BNC connectors for
the channel 1 and 2 inputs along with the external trigger input via
another BNC connector.
20
5 Beginning Input to the DSO2250 DSO
The following pages will take simple steps through basic one frequency
sources as input to the scope and show displays for different input
levels and different frequencies from 1MHz to 100MHz.
The next chapter I’ll do more complex waveforms to show more complex
frequency spectrum diplays.
21
Here we see the scope display as it first appears when you start
it up. The control panel just to the right of this where you select
display options and input channels.
I have the input to CH 1 grounded via the control panel button. That
does not mean the probe is grounded. For the DSO it means that the
signal processing software is set to zero input and you can set where
the zero line is on the display. The green pointer just to the left
of the horizonal line with the number 1 allows you to move the line
up and down with the mouse. Also I have disabled CH 2, so the sample
rate is at 250MS/s (250 MegaSamples/second) shown on the bottom line,
second quantity. 40nS per horizontal division.
The "T" on the right hand side sets the trigger level and it is at
-2mV as shown on the bottom right of the display.
22
Here is what the display will look like at almost 1280x1024 pixels
on a high resolution screen capture.
It looks a lot better on a real screen than here after it is captured.
I use my PDF reader at 200 per cent so that the page is almost edge
to edge on the 19 inch 1280x1024 pixel display. Play with your Acrobat
reader or whatever to get the page size comfortably large as possible.
23
Here we have nothing connected to CH 1 input and the input set for
DC. This is the A/D noise that is always present. The A/D have a bit
error rate and can rarely show zero input for all samples. It is something
that you have to get used to. Different than an analog scope.
24
Here we have nothing connected to CH 1 input and the input set for
AC. This is the A/D noise that is always present. The A/D have a bit
error rate
25
Here we have the FFT window. You obtain it from the Acquire pull-down
menu at the top menu items in the scope display window and then select
FFT.
This shows the dB display for the noise in the previous scope display.
Using Rectangle window FFT calculation for the noise as shown here.
26
Hanning window for the FFT of the noise. There is not a 1MHz signal
present. The calculations seem to show such a signal, but it is an
artifact of this calculation sequence.
27
Hamming window for the FFT of the noise.
28
Blackman window for the FFT of the noise.
29
Using the NorCal FCC-2 DDS I have a 1MHz 60mV amplitude RF signal
into a 50ohm non-reactive load with a tee to use a 1m coax segment to
get the signal to the DSO.
Note the noise due to A/D conversions as wiggles in the waveform.
30
The FFT with a linear vertical display to show the results for the
1MHz input. The horizontal scale only goes to 62.5MHz. You’ll see
the amplitude shown as 57.653mV. This is caused by 1MHz not being at
the frequency point of the 256 discrete points in the 62.5MHz range.
If you have a variable frequency source, as you vary the input frequency
you will see the amplitude vary due to the calculations being done at
the 256 points and not at the input frequency of the source.
31
Same 1MHz signal with the FFT done in a Hanning window. Linear vertical
scaling.
32
Same 1MHz signal with the FFT done in a Hamming window. Linear vertical
scaling.
33
Same 1MHz signal with the FFT done in a Blackman window. Linear
vertical scaling.
34
Here is the 1MHz signal using the dB vertical scaling and Rectangle
window FFT.
35
1MHz signal with Hanning FFT window and dB vertical scale.
36
1MHz signal with Hamming FFT window and dB vertical scale.
37
1MHz signal with Blackman FFT window and dB vertical scale.
38
Here is a 5MHz signal using the dB vertical scaling and Rectangle
window FFT.
39
If you set the vertical scale incorrectly, you will clip part of
the input waveform. As you see in the following FFT calculations you
get harmonic distortion and incorrect results.
40
Here is the rectangle FFT of the clipped 5MHz signal. Note the significant
harmonics in the resulting calculations.
41
5HMz input signal with no overload condition. FFT with rectangle
window.
42
5HMz input signal with no overload condition. FFT with Hamming window.
43
5HMz input signal with no overload condition. FFT with Hamming window
and double frequency bins. Note the extended frequency scale to 500MHz.
44
Here is the maximum frequency that you can get from the FCC-2, 21MHz.
45
FFT of the maximum frequency that you can get from the FCC-2, 21MHz.
46
FFT of the maximum frequency that you can get from the FCC-2, 21MHz,
with double frequency scaling to expand the horizontal frequency scale.
47
Now, using the Tektronix model 191 constant amplitude frequency generator
here is 500KHz.
48
500KHz from the Tektronix signal generator.
49
1MHz for the Tektronix.
50
1MHz signal.
51
1MHz signal.
52
10MHz signal.
53
20MHz from the signal generator.
54
40MHz signal.
55
100MHz signal.
Here is where the FFT gives you some artifacts and aliasing. Since
we are sampling at 250MS/s, then the Nyquist frequency is 125MHz. You’ll
see the 100MHz signal reflected about 125MHz to give us a 150MHz point.
100MHz is 25MHz from the 125MHz, so we also get a 25MHz signal in the
FFT. And 50MHz from the difference between the 100 and 150MHz points.
This does get complicated.
56
100MHz signal.
57
100MHz signal.
58
6 Analysis of a Working SWL-40+ Transceiver
I thought it would be of interest to first demonstrate the use of the
DSO-2150 scope on a working Small Wonder Labs SWL-40+ transceiver. See
http://www.smallwonderlabs/ for online manuals and photos of the rig.
I love it when a manufacturer has the construction manuals and schematics
online for prospective customers to download. It also helps those that
buy a used rig that doesn’t come with the manual. There are several
QRP kit manufacturers that still refuse to put their manuals online
and charge a high price for them. It’s their business but it just doesn’t
make sense to me personally. My opinion and I’ll stick with it.
On the following page is a schematic of the SWL-40+ and Dave gives
me permission to use the thing. I have a 11x17" version on my web page
that you can download and take to Staples or Office Max and get printed
for less that 50 cents.
On the following page is a block diagram of the functions that we’ll
use to go through the rig stage by stage.
59
60
Block Diagram of SWL-40+
And wouldn’t you know it, I had a line from the VFO to the RCVR MIXER
and it got removed. I don’t have the initial file, so you’ll have to
remember it’s there. I’ll try to come back and recreate the drawing.
61
DSO-2150 Scope Probes
The DSO-2150 comes with two PP-80 scope probes. There is a plastic
pouch to hold them along with the manual and a zip-lock baggie that
holds an adjustment screwdriver and spare color rings for the probes.
Keep the probes and everything else shown here in the pouch when not
in use. You do not want to abuse these nor do you want to lose anything
shown here. You can not pick up spare parts at Wal-Mart.
These probles will cost you on the order of 30 to 40 dollars at Fry’s
Electronics and I’m sure that Radio Shack doesn’t carry them. I did
a search at http://shop2.frys.com/product/2369089#detailed and found
each probe to cost on the order of $32.99, so don’t damage them.
62
