I have recently been asked if I would provide some information regarding the use of an Oscilloscope, so I have put together this document as a start for those who wish learn a little of the use of the instrument in Amateur radio. If you have any sugestion for improving this document then please give me a call; my addresses will appear at the foot of this page. This information is not intended as a definitive work regarding the use of Oscilloscopes, merely a little information to get you started and show some of the possibilities.
The Oscilloscope (often called simply a "scope") is nothing more than an instrument for displaying electrical signals in the TIME domain. In other words, you can view waveforms on it. Some of the uses of the scope may not be obvious but if you already posess one, then you will most probably regard it as the most useful tool on your workbench.
A basic (and fictitious) Oscilloscope is shown below.
This is a Dual-Beam oscillloscope although most of the information presented here is also applicable to a single-beam Oscilloscope. The basic controls are:
The Scope will also have an input socket for each input channel, situated on the front of the instrument. There will most probably be more controls on your scope, and I will try to cover some of these later on.
On the trigger input select CHAN-A. Adjust the TRIGGER control slowly and at some setting of the control the display will become stationary. If the TRIGGER control has an AUTO position then select it and you do not need to adjust anything.
The waveform you see will not look exactly like that shown above, as there will be loads of distortion. This is due to many things mainly because you are picking up spurious signals radiated by household electrical equipment such as TVs, flourescent lamps etc. All these items distort signals in one form or another.
If you look at the waveform you will see that there are two horizontal red divisions on the graticule scale between two consecutive peaks. Since the timebase is set to 10mS/Div, it will take the spot 20mS to travel between the two divisions. The PERIOD TIME of the waveform is therefore 20mS, (or 0.02 seconds). The FREQUENCY of the waveform is 1 divided by 0.02 = 50 Hz.
Now look at the vertical scale. The centre-line is zero-volts and the waveform moves both 1.8 divisions above and below the centre line, and if the input level is set to 1volt/cm the level of the input is 1.8v + 1.8v = 3.6 volts PEAK-TO-PEAK. This equates to 3.6v times 0.35 = (about) 1.2 volts RMS, as you would read it on a voltmeter.
In this way, you can measure the FREQUENCY and VOLTAGE (AMPLITUDE) of just about any waveform.
Here, the oscilloscope has both its inputs connected to the DC output of a home-made 12v DC Power Supply Unit. Trace B is set to 5v/cm DC and only a single is displayed. The horizontal line will have moved up the tube face by just over 2cm, but it is straight and shows no deviations.
Trace A input is set to AC (DC block), and the input sensitivity is turned up to about 1mV/cm. The DC component would give a deflection of 1200cm but the DC has been blocked. The low-level ripple can now be clearly seen. In practice, with the circuit shown, little or no ripple should be seen, but when a load is placed on the power supply the ripple may even become quite objectionable, depending upon the component values chosen. It is also very interesting to see that the ripple frequency will be 100Hz or 10mS between the peaks. This is because of the action of a full-wave bridge rectifier.
The TIMEBASE selector switch will most probably have an 'XY' position even on the cheapest of oscilloscopes. This can be used to for a variety of new functions, such as:
The 'XY' timebase selector position disconects the internal generator that moves a spot across the screen. Input B will now move the spot across the screen, but input A will still make the spot move up and down.
If the two signals were EXACTLY identical, then the spot would move UP and to the RIGHT then DOWN and to the LEFT. This would be seen as a horizontal line diagonally across the screen. A loop would be seen if the two signals were exactly the same frequency, but different wave-form or phase. If the two signals were perfect sinewaves and differed by exactly 90 degree phase then you would see the following waveform:
If you have access to an AF sinewave signal generator, then connect an RC circuits to one input and a CR circuit to the other input of the sccope:
Notice how the circle slope changes as the frequency is altered.
If the phase were to constantly change (slightly different frequency) then you would see a SQUARE BLOCK formed by a moving image changing from a left -sloping line - a circle - a right sloping-line - a circle - then back to a left-sloping line again.
If the two sine-waves were to differ in frequency by EXACTLY 2:1 then you will see something like this:
Notice that in the vertical plane (Y axis = CHAN 2) there is only one peak but in the horizontal plane (X axis = CHAN 1) there are two peaks. CHAN 1 is therefore twice the frequency of CHAN 2. With other frequency combinations the waveform may become even more complex, for example 3:2 8:3 2:5 etc.
Here we can see that the left hand peak is not quite as high as the right hand peak which means that the IF response is not perfect, although it is not bad enough to be noticed by the ear.
Most single beam oscilloscopes have a TIMEBASE output which can be used to make a voltage controlled oscillator sweep through the frequencies of interest.
I will be posting a simple Wobulator circuit soon. It has been tried and tested over a number of years.
Here we see that there are several radio signals displayed simultaneously. This would be typical of a display from 0 to 9MHz with a scale of 1MHz per division. The large spike at the left of the screen would be the 0MHz marker. This display shows a couple of small signals at about 1.5 and 1.9 MHz, 6.0MHz, 7.1MHz and 7.9MHz. Larger signals are shown at about 2.5MHz, 3.2MHz, 4.3MHz, and 5.5MHz. The baseline is cluttered with noise, often refered to as 'GRASS'.
Harmonics and other spurious emitions may be observed from an amateur transmitter. Many HF radio receivers have a narrow-band spectrum analyser built into the receiver. These are operated from the receiver IF amplifier, before the filter. In this case they are called a PANORAMIC DISPLAY, PANORAMIC ADAPTOR or something equally obscure. QRP/DX signals would be at very low signal levels 'in the grass' so a Pan.Adaptor is not particularly useful.
Above is the block diagram of a simple spectrum analyser that will cover the complete HF bands from 0 to 30 MHz. I have done some experiments with this in the past and I may post circuits sometime in the future.
With the above setup you would see a trapezoidal waveform something like this:
CD minus AB divided by CD is equal to the modulation depth. In this example the modulation depth is (6-2)/6 or 4/6 = 66%. The upper and lower slpoing lines are straight showing that the modulation characteristic of the transmitter is linear.
CW may be monitored by LOOSELY coupling one input channel of the oscilloscope to the transmitter antenna (just place it near the antenna coax or connect it to the transmitter chassis) and set the scope timebase to (say) 100mS/Div. Send a load of DITs and observe the waveform.
Nice rounded edges are ideal, but square 'envelopes' are the sign of a badly adjusted CW transmitter. Leading spikes would render the transmitter illegal in most countries (except Saudi Arabia!). If you wish to do continuous measurements then it could be a good idea to make a multivibrator oscillator to switch a transistor in the transmitter key input. This will give a regular and continuous CW envelope for design/development of CW transmitters.
If a two-tone audio signal is connected to an SSB transmitter microphone input and the oscilloscope is LOOSELY coupled to the transmitter antenna, the following waveform will be displayed when using the scope's internal timebase:
A flattening of the peaks indicated that some stage has been overdriven, and a lack of dip between envelopes shows an excessive carrier level. This display can show quite a lot of things about the waveform, but this is beyond the scope of this page.
Here we see that the 1000000 ohm resistor is placed in series with a capacitor (the coaxial cable) which is a simple 6dB/octave low-pass filter. A square waveform should look like A below, but with the above probe it will look more like C below. If the 1000000 ohm resistor is shunted with a small capacitor, as in B above, it will correct this situation, but the value of the capacitor is quite critical. If the value is too small then the waveform will look like C below. If too large a value it will look more like B below.
If the correction capacitor is corectly selected then the waveform will look waveform A. Many oscilloscopes have a 1KHz square wave reference output for calibration purposes. The waveform is usually 1v peak-to-peak so the oscilloscope display should be one cm high using the 1v/cm input level setting. If your oscilloscope does not have a calibration output then you can easily build one using a CD4060 IC as an oscillator/divider from a 8192KHz (8.192MHz) crystal.
This circuit has the added advantage that there are also other outputs at much higher frequencies, up to 1.024 MHz, for even more accurate HF correction of your homebrew scope probe.
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