Connect with us

Question: Learning How To Use Hand Held Oscilloscope

Discussion in 'Electronic Basics' started by Chris L, Nov 11, 2007.

Scroll to continue with content
  1. Chris L

    Chris L Guest

    Hello, I've been trying to learn how to use my new Protek 800 series
    hand held Digital Oscilloscope.

    So, I connected the single oscilloscope probe to the output of a used
    function generator that I bought on EBay.

    On oscilloscope auto, what I see is a waveform that I can only
    describe as a repeating sinc function. When I increase the frequency
    of the sine wave on the generator, I see more and more tiny waves
    appear in this sinc-like function. If I increase the amplitude on the
    generator I see these tiny waveforms in the sinc-like waves increase
    in amplitude.

    This seems strange to me because I set the function generator for a
    repeating sine wave, but what I see is this repeating sinc-like
    waveform. The sinc waveform becomes more and more detailed.

    Is the generator bad?

    How do I get a repeating sine wave without the sinc?

    I set the oscilloscope to auto and the resulting setting are:

    20V/div, 200mV/time div
    coupling is AC
    probe set to 1X
    Trigger set to edge and slope set to rising

    If I try fooling around with the various setting I cannot improve
    things.

    Setting trigger to pulse, pattern, or video did not help.

    Setting coupling to DC or ground did not help.

    If I set the time division very low (micro divisions) to zoom in on my
    tiny
    sine waves they still seem to want to float around.

    What do I do?

    Thank you,
    Christopher Lusardi
     
  2. Most function generators approximate a sinewave. They are great
    at generating triangle waves, and the square waves that are a byproduct,
    but the sinewave is created by "smoothing" the triangle wave, usually
    by a circuit of diodes and resistors.

    Function generators are general purpose devices, and their sinewave
    out put is good enough for what they are intended for.

    If you need a perfect sinewave, then you choose a generator that
    starts with a sinewave, or at the very least uses frequency filtering
    to get the sinewave. But, those are usually cumbersome to switch
    bands, and add a lot of overhead to the timing so rapidly sweeping
    the generator is not in the picture.

    If you are seeing the sinewave become smoother the higher the
    frequency, likely you are seeing the frequency response of the scope
    kick in. Once you start hitting its limits, its frequency response
    will affect the waveform. Use a high enough frequency, or a scope
    with a low enough response, and the scope loses much of its purpose,
    since you can't tell whether that sinewave is really a sinewave, or
    the frequency response of the scope is turning a signal into a sinewave.

    Michael
     
  3. What exactly do you mean by a "sinc function"? My first thought was
    that "sinc" was a typo for "sine", but it apparently isn't.

    Can you describe the display more clearly, or better, put a picture of
    it on a web site somewhere - then we'll have a better idea of what you
    are seeing.


    --
    Peter Bennett, VE7CEI
    peterbb4 (at) interchange.ubc.ca
    new newsgroup users info : http://vancouver-webpages.com/nnq
    GPS and NMEA info: http://vancouver-webpages.com/peter
    Vancouver Power Squadron: http://vancouver.powersquadron.ca
     
  4. Jamie

    Jamie Guest

    I think he has the unit set up incorrectly. It's generating 10% or duty
    cycle pulses for example.
     
  5. Peter,

    For info on sinc function, see
    http://mathworld.wolfram.com/SincFunction.html

    Chris L,
    I'm not familiar with your particular o'scope (and I haven't time to look up
    the user's manual), but check to see if it has an FFT function and that it
    is turned off.

    Good luck,
    Richard
     
  6. Bob Masta

    Bob Masta Guest

    You can use the FREE signal generator that comes in my Daqarta
    software package. That will give you known-good waveforms to test,
    and it also displays the waveforms (or spectra) with advanced
    triggering options so you can compare with the hand-held. The only
    downside is that since it uses your computer's sound card, you will be
    limited to audio-range signals (few Hz to 22 kHz, typically).

    There is no need to purchase Daqarta for this. The signal generator
    and all the display functions (and most everything else except signal
    inputs) will continue to work after Daqarta's trial period expires.
    You are welcome to use it this way as long as you like.

    Best regards,


    Bob Masta

    DAQARTA v3.50
    Data AcQuisition And Real-Time Analysis
    www.daqarta.com
    Scope, Spectrum, Spectrogram, FREE Signal Generator
    Science with your sound card!
     
  7. Chris L

    Chris L Guest

    Hello, I went to Frys Electronics and bought a $350 function
    generator, hooked it up, and got the exact same plot on the
    oscilloscope.

    What the plot looks like is this: consider a repeating sine wave. It
    has 2 large side lobes which are smaller than the middle lobe. This
    repeats the same way a sine wave repeats. If I then increase the
    frequency on the function generator these repeating waves become more
    developed/detailed.

    It may be because the oscilloscope triggering is wrong!
    /\
    / \
    /\ / \ /\
    / \/ \/ \ ...


    If increase frequency I get :
    ./\.
    ./ \.
    ../\. / \../\.
    / \/ \/ \ ...
    /\
    Consider the dots to be / \. I.E.: Little sine waves

    What should I do?

    Chris Lusardi
     
  8. Bob Masta

    Bob Masta Guest

    What is the test frequency, and what is the sample rate of the scope?
    What you describe sounds like what you see when the sample rate is
    too slow for the test frequency. Adjacent samples on the display
    don't come from the same waveform cycle, but from subsequent cycles
    whenever the A/D gets back to fetch them. Since there is no sync
    between the sample rate and the input signal, they don't form a single
    complete waveform.

    You can try reducing the signal way down (say, a few 100 Hz) and see
    if that gives a clean trace, then slowly bring up the frequency.

    Digital scopes typically have a way to avoid this problem by sampling
    only every Nth sample on the first trigger, then on the next trigger
    they add a one-sample delay and again grab every Nth sample, etc.
    So if N = 4, the first trigger will get samples 0, 4, 8, 12 ... Then
    the second trigger will get 1, 5, 9, 13..., the third will get 2, 6,
    10, 14..., the fourth will get 3, 7, 11, 15... and the display will
    put them all in the correct order.

    I couldn't find any on-line info about the Protek 800, so I don't know
    what its intrinsic sample rate is, or what they label the controls
    that activate the above mechanism. But I bet it's covered in the
    manual. <g>

    Best regards,


    Bob Masta

    DAQARTA v3.50
    Data AcQuisition And Real-Time Analysis
    www.daqarta.com
    Scope, Spectrum, Spectrogram, FREE Signal Generator
    Science with your sound card!
     
  9. Chris L

    Chris L Guest

    Can the below information solve the problem?

    Christopher Lusardi

    The Operator's Manual says:
    Product Standards and Specifications

    Model 840
    Bandwidth/
    Sample Rate 400MHZ
    Real Sampling 100MS/s per Channel, 200 MS/s Single
    Channel with 2.5 GS/s Repetive Sampling

    2 Input channels
    input coupling: DC, AC, GND
    input impedance IM +/- 1%, 20pF +/- 2pF
    Probe attenuation x1 or x10
    Max Input 300V direct input, 600V through 10:1 probe

    Vertical
    vertical resolution 8 bit
    Volts/div range 5mV/div ~100V/div(1,2,5 step)
    offset position +/- 5 div from center
    vertical accuracy +/- 3%

    Horizontal
    Sec/Div Range 10ns/div ~ 50s/div
    time base accuracy +/- 0.01%
    Position Range pre-Trigger:20 div max. Post-Trigger: 1000div
    Delay Resolution 1/250 of screen diameter
    delay accuracy 0.04 divisins
    delta time measurement accuracy +/- 3%
    Modes Main, XY
    XY Yes
    Horizontal Pan and Zoom Yes

    Trigger
    Sources Channel A and B
    Modes Normal, single, roll, auto trigger
    Advanced selections edge, pulsewidth, pattern video
    edge : trigger on a rising or falling edge of any source
    pulsewidth: trigger on a positive or negative pulsewidth of any source
    larger than, less than, equal to or not equal the duration
    width 200nS ~ 10S
    TV: NTSC, PAL, SECAM, Normal Video, Even field, Odd field, Line
    #select
    TV Sensitivity: 0.7 div trigger level
    auto scale : yes
    range: +/- 10 div from center of screen
    Level Accuracy +/- 0.4 divisions
    Level resolutions: 500 pixels
    sensitivity: DC 5MHz - 0.5 div 1.5 divisions
    noise rejections : yes
    Coupling Modes: DC: DC Coupled trigger
    AC: AC Coupled trigger cutoff frequency < 1HZ
    HF-Refject: > 50KHz
    LF-Reject: > 30KHz
    Noise Reject: 3 times the DC coupled limits

    Acquistion
    Real-Time: 200MS/s single channel
    Sample Rate : 100MS/s per channel
    Waveform Interpolation : Dot, Linear, Sine, Pulse
    Record Length: 125 Kbytes/channel
    Acquisition Mode: Sample, Peak detect, envelope, average
    peak detection: 10ns
    Average: 2-256
    Vertical resolution: 8 bit
    same rate accuracy: 100ppm
    Minimum sample rate: 100MS/s (250 ns to 50S/div)

    Measurement
    Automatic Measurement: Peak-to-peak, Maximum, amplitude, top,
    base, positive/negative overshoot, preshoot,
    RMS mean, one cycle mean, frequency
    period, + width, - width, + - duty cycle, rise
    time, fall time, delay, phase shift
    Waveform math: Ch A + Ch B, Ch A - Ch B, Ch B - Ch A
    Cursors : delta V voltage
    Delta V : voltage diff between cursors
    Delta T: Time difference between cursors
    FFT : Model 840
    Windows : Rectangular, Hamming, Hanning, Blackman-Harris
    Amplitude Display: Variable dB (1/2/5/10 dB)
    Maximum Frequency: 1.25 GHz
     
  10. <further specs snipped...>

    Not really - of more importance is the frequency and voltage of the
    signal you are measuring, and the sweep rate, vertical gain and
    trigger settings of the scope when you are doing these measurements.

    A silly question: do you have the scope ground connected to the signal
    generator? You can get very strange displays if you only connect the
    signal lead (tip of the test probe) without any ground connections.


    --
    Peter Bennett, VE7CEI
    peterbb4 (at) interchange.ubc.ca
    new newsgroup users info : http://vancouver-webpages.com/nnq
    GPS and NMEA info: http://vancouver-webpages.com/peter
    Vancouver Power Squadron: http://vancouver.powersquadron.ca
     
  11. Chris L

    Chris L Guest

    Your silly question has almost completly solved the problem!

    Can you give me more information?

    My function generator has 3 connections for oscillocope probes plus 2
    connections for a multi-meter. The oscilloscope connections are
    labelled,

    Input VCF
    Output TTL/CMOS
    Output 50 Ohms with a ground symbol

    The oscilloscope has 2 connections. One for channel A and one for
    channel B.

    Via buttons, if I set the oscillocope channels A and B both on. And,
    connect channel A to either Input VCF or Output TTL/CMOS with channel
    B connected to the Output 50 Ohms I get the correct sine, square, and
    trianular waves.

    Five questions:

    1. Is this the correct way to measure the signals?
    2. Why can I connect channel A to both Input VCV and Output TTL/CMOS?
    3. Does it matter what I connect channel A to?
    4. How do I keep my oscilloscope operating correctly?
    5. Is there anything else that I have to explicitly be reminded about?

    Many thank yous,
    Christopher Lusardi
     
  12. That is not a "connection for an oscilloscope", it is an input signal
    which can be used to modulate the output signal. You don't need to
    worry about this.
    That is a 5V square wave output. You won't get a sine or triangle wave
    out of this.
    This is the "normal" sine/square/triangle output.
    Yes. With the exception of connecting to the VCF input which is
    incorrect.
    You can connect either the TTL output or the "50 ohm" output to either
    channel on your oscilloscope.
    Connecting the VCF input will do nothing.
    You connect Channel A to whatever signal you want to measure with
    Channel A.
    Ensure you know how to use it correctly. An oscilloscope is a complex
    measurement tool that can operate in many different ways, you need to
    read some tutorials that explain how they work and how to use them.
    Type "oscilloscope tutorial" into Google.

    Dave.
     
  13. What you have here is a "digital storage" oscilloscope, not exactly
    the best tool for a beginner.
    You should be starting out with a basic "analog" oscilloscope.
    Digital storage oscilloscopes have many ways in which they can play
    tricks on you if you don't know how to use them correctly.

    Dave.
     
  14. (I assume you mean function generator connections)
    That would be an input to the function generator - from the "VCF" I
    might guess that applying a varying voltage to that connector would
    vary the output frequency of the generator.
    Probably a signal intended to be used to trigger the sweep on an
    oscilliscope.
    That would be the actual signal output.
    You also need to look at the scope trigger settings. If you connect
    the 'scope's "A" input tor the signal generator main output, you
    should set the 'scope trigger input to "A", and other trigger settings
    to "AC" and "Auto", as well as setting the time base to something
    suitable for the signal you are measuring.

    Sometimes it is useful to trigger the scope directly from the signal
    generator, rather than from the signal you are measuring - for that
    use, you could set the trigger source to "B", and connect the "B"
    probe to the TTL/CMOS output of the signal generator. Many scopes
    have a separate trigger input for this sort of application.

    It depends a lot on what you are trying to measure, and why.
    Carefully study the manuals for both your signal generator and
    oscilliscope, and try playing around (sorry - experimenting with) the
    various controls on both the signal generator and scope to see what
    happens. You are unlikely to damage anything (unless you try
    measuring 120VAC). As others have said, search the web for
    oscilliscope tutorials. (You may find some useful tutorials on
    Tektronix and other scope manufacturers' web sites.)
    --
    Peter Bennett, VE7CEI
    peterbb4 (at) interchange.ubc.ca
    new newsgroup users info : http://vancouver-webpages.com/nnq
    GPS and NMEA info: http://vancouver-webpages.com/peter
    Vancouver Power Squadron: http://vancouver.powersquadron.ca
     
  15. Chris L

    Chris L Guest

    Well my current investigation has hit a roadblock where I have to try
    and alternate route.

    I was successful at using channel A for input and channel B for ground
    a few days ago with sinewave, triangle wave, and square wave. I
    connect channel B to the function generator with a banana clip, but I
    just touched the channel probe A to the function generator output.

    I cannot, now, reproduce what I did earlier in the week! With the
    manual on my lap, I tried and tried. The only thing I get is a square
    wave which I can change the frequency but not the amplitude.

    I was thinking the reason it initially worked was I worked a long time
    with one probe and channel A. And, the scope requires the probe to be
    tested (calibrated) before use. Maybe after the scope was shut off I
    calibrate the probe(s) incorrectly!

    Chris L.
     
  16. Bob Masta

    Bob Masta Guest


    Hmm, this all sounds really strange. What sort of connectors do you
    have on your scope inputs? Typically, scopes use BNC connectors,
    where the outer shell is ground and the inner pin/socket is the
    signal. If your signal is going into channel A input, you would
    connect its ground via the channel A connector. The scope probe
    should have a point or a clip for the signal, and probably a little
    collar for the ground lead. The ground lead will usually have an
    alligator clip on the end, and a "U"-type clip to attach to the probe
    collar.

    Channel B is for the purpose of seeing a second signal trace on the
    screen at the same time as A. It is not for grounding A.

    Best regards,


    Bob Masta

    DAQARTA v3.50
    Data AcQuisition And Real-Time Analysis
    www.daqarta.com
    Scope, Spectrum, Spectrogram, FREE Signal Generator
    Science with your sound card!
     
  17. Chris L

    Chris L Guest

    I went home and tried it because it was on my mind.

    If I hook only one probe up to the function generator (by touching the
    point of the probe for channel A to the f.g. output, and clip the
    banana clip from the metal base of the probe to the function generator
    ground) I get square waves.

    If I switch the f.g. function from square waves to sinwaves or to
    triangle waves the oscilloscope still displays square waves.

    I am able to increase or decrease the frequency, but tweeking the f.g.
    amplitude knob does nothing.

    (I also, later, tried substituting a channel B probe [after
    calibrating the channel A probe] for the banana clip to the f.g.
    ground but without success.)

    Happy Thanksgiving,
    Chris Lusardi

    PS: I do own copies of the following books but they have not helped,
    yet: The XYZ's of Oscilloscopes by Tektronix , How to Use
    Oscilloscopes and Other Test Equipment by R. A. Penfold, and 101 Ways
    to use your... Oscilloscope by Robert Middleton.
     
  18. Ecnerwal

    Ecnerwal Guest

    Sounds like you are on the "sync" output of the function generator, not
    the actual normal output - behaves exactly as described - constant
    amplitude (generally TTL logic levels) and variable frequency, but
    always square waves. Used to synchronize a scope or some other thing
    (nice sharp trigger signal) to the normal output (different waveforms
    and amplitudes).
     
  19. Sounds like you are using the TTL output of the generator. That output
    will *always* give you a square wave at the *same* voltage, onyl the
    frequency will change. The "50 ohm" output is the one that produces
    sine/square/triangle with varying amplitude.
    You only need *one* connection between your function generator and
    oscilloscope.
    That connection should be a BNC cable, with a BNC connector on both
    ends. If your function generator is an older style one then it may
    have banana plugs, in which case you simply connect your CRO probe to
    the two banana terminals, ground clip on the black plug, tip of the
    probe on the red plug.

    Then it's simply a matter of operating both your oscilloscope and
    function generator to get the correct waveform. Forget channel 2, it
    is obviously confusing the situation.

    For the purposes of getting a waveform on the screen your probe does
    not need to be calibrated.

    Post a photo so everyone is clear what is happening here.
    And get yourself an analog oscilloscope.

    Dave.
     
  20. Chris L

    Chris L Guest

    I think I've got it, and I helped a little!

    If I use one probe with an attached wire/clip at the base of the probe
    I am able to to see sine waves, triangular waves, and square waves. In
    addition, I am able to adjust the amplitude.

    The function generator has an output labelled output 50 Ohms with a
    ground symbol. This output is a connection for a BNC probe.

    When I touch the pointer of the probe into the hole of this function
    generator output with the clip attached to the raised portion of the
    connection for the BNC probe, it works.

    Question: What is so great about an analog oscillocope? Can't I just
    live with what I have.

    FYI, I can, now, attempt to use the function generator and digital
    oscilloscope with my expensive Lab-Volt system (I got it on EBay at a
    great discount). This system comes with a computer, program lessons,
    and 16 circuit boards. The circuit boards look exactly like regular
    circuits with transistors, resitors, voltage source, function
    generator connections, etc, etc, etc.

    There are many holes in the circuit boards where I can connect the
    oscilloscope, and function generator probes. The system has a few
    small 2 post connectors to connect different portions of the circuit.
    The Lab-Volt system comes with its own power source +/- 15 Volts.

    For example, the generator symbol on one of the boards has holes where
    you can hook-up the function generator outputs. Question: What is the
    best way to do this? I'm going to try to connect the function
    generator probe with a clip wire on the metal base of the probe to
    opposite holes of the function generator symbol on the boards. The Lab-
    Volt system has a few metal posts that I can stick into the holes for
    the probe clips.

    The power source symbol on the same board has an arrow through it, and
    two holes on either side of it. The positive side is the one closest
    the arrow head. Question: What is the best way to hook-up the
    oscilloscope to the board? I'm going to experiment with this.


    Thanks and I hope I did not bore everyone,
    Christopher Lusardi
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-