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SETTING UP THE CRO

CRO's are complex instruments with many controls and require care to set up. It is quite easy to 'lose' the trace if the controls are wrongly adjusted!

There are some variations in the arrangement and labelling of the controls of most CRO's so the following instructions may need to be adapted for your instrument.
The following set-up applies to a Dual Trace CRO.
We will set up two traces and use the internal square-wave oscillator to produce two waveforms on the screen as shown in the second diagram below.
All CRO's have a "CAL" test-point. It is a "lug" with a hole so the hook of the probe (or both probes) can be connected. The test-point produces a 500mV square-wave signal @ 2kHz. This may vary with different manufacturers.
The photo shows an alligator lead connected to the "CAL" test-point with a single square-wave shown on the screen:


A lead connected to the "CAL" test-point

    This screen after setting
    up, with no input signal.
    The two probes
    connected to the
    "cal" test-point.
  1. Switch the CRO ON. (It takes a minute to warn up).
  2. Connect the input leads.
  3. Switch the mode to DUAL.
  4. Set the AC/GND/DC switch to AC for each channel.
  5. Set Trigger Level to AUTO or NORM.
  6. Set Trigger Source to INT (internal).
  7. Set the Volts/cm to 0.5v/cm for each channel
  8. Set the TIMEBASE (Time/div) to 0.2mS/cm
  9. Click the timebase VARIABLE to CAL.
  10. Adjust the left/right shift for two traces on the  screen without a gap at either side.
  11. Adjust the up/down shift for channel 1 to produce a trace across the middle of the screen. Adjust the up/down shift
    for channel 2 to produce a trace near the bottom of the screen, as shown in the diagram opposite.
  12. Adjust INTENSITY (brightness) and FOCUS to give bright, sharp traces.

We will now produce two square-waves on the display.
Connect both probes to the "CAL" test-point. This is at the lower-left-hand corner of the oscilloscope, as shown in the photo above.
Two square-waves will appear on the display.
The CRO is now ready to use.
Simply connect the tip of a probe (Ch A) to the circuit you wish to test and the earth clip to the 0v rail of the project.

The waveform on the display will show what is occurring at the point you are testing.
If the waveform has a repeating pattern, it will have a frequency that can be determined from the Time/div control (The Sweep control). To determine the frequency, you need to switch the Time/div control so that one cycle occupies exactly one 1cm (the distance between  lines on the screen).
The height of the waveform does not matter. Only the length of the cycle is needed. The length of the cycle is called the "period" and the following diagram explains this.


The wave-form must start on a grid-line to
make it easy to work out the frequency.
 

Adjust the waveform so that the 0v of the wave crosses or touches a grid line. This gives you an accurate starting point. Make sure the cycle rises to a maximum, falls to a minimum and returns to 0v at the next grid-line. This is exactly one complete cycle.
If one complete cycle fits between a square on the screen, you will be able to use the following SWEEP-TO-FREQUENCY CONVERTER. If a cycle does not fit between two grid-lines, see below.


SWEEP-TO-FREQUENCY CONVERTER
When 1 cycle = 1cm on the  screen,
use this converter to determine
the frequency of the wave.

The diagram above shows a sine wave but the following applies to any signal with a constant shape.

  • Amplitude is the maximum voltage reached by the signal.
    It is measured in volts, V.
  • Peak voltage is another name for amplitude.
  • Peak-peak voltage is twice the peak voltage (amplitude). When reading an oscilloscope trace it is usual to measure peak-peak voltage.
  • Time period is the time taken for the signal to complete one CYCLE.
    It is measured in seconds (s), but time periods tend to be short so milliseconds (mS) and microseconds (µS) are often used. 1mS = 0.001s and 1µS = 0.000001s.
  • Frequency is the number of cycles per second.
    It is measured in Hertz (Hz), with kiloHertz (kHz) (1kHz = 1,000Hz)  and MegaHertz (MHz) (1MHz = 1,000,000Hz).
    frequency  =           1             and     time period  =           1        
    time period frequency

If a cycle does not fit exactly inside a square on the screen, you will have to do some simple mathematics to determine the approximate frequency. Count the number of cycles in 5 squares, and if it is more than 5 cycles, the frequency will be slightly higher than shown in the Sweep-to-Frequency Converter above.


CONNECTING A 10X PROBE
 Most passive probes have an attenuation factor, such as 10x.
By convention, attenuation factors, such as for the 10X attenuator probe, have the X after the factor. In contrast, magnification factors like X10 have the x first.
The 10x (read as "ten times") attenuator probe minimizes circuit loading and is an excellent general-purpose passive probe.
Most CRO's have a 20v/cm setting on the Volts/cm control. The screen is normally 10cm (10 divisions) high. This allows a waveform of 200v to be viewed.
To observe larger waveforms, a 10X Probe can be used.
The photos below show 10X Probes:



However the best probe has a switch to change from 1:1  to 10:1

The switch in the photo above can be clearly seen. It allows finger-tip change between x1, x10  and reference ground.  "Ref" removes the signal from the input of the CRO and allows a "zero trace" to be observed. This is handy when checking to see which trace is being displayed and if the waveform has any "offset" (DC component)

The 10X position on the probe allows high voltages to be observed on the CRO. All Volts/div values on the CRO are multiplied by 10.
For example, 20v/div becomes 200v/div. The CRO can now show voltages up to 2,000v

The 10X feature has another advantage. The CRO has an input resistance of 1M and this can put a load on a high-impedance circuit. By using the probe in the 10X position, it puts less load on the circuit.

Connecting the Probe
Push and rotate the BNC plug to fit the probe to the input of the CRO:

The Ground Clip
Probes come with an alligator-clip for connecting to the circuit under test. It must be connected to a ground connection such the "chassis" or "frame" of a device, the negative lead of a battery or the "ground plane" of a circuit board.
 


 Page 6

26-02-04