It may seem daunting to many CCTV installers, but using an oscilloscope will save you time and money
Of course the title assumes that you actually own or have access to an oscilloscope, but hopefully by the time you finish reading this you will want to rush out and purchase one, or clean the dust off the one that has been stored under the bench since it was bought a long time ago!

The CCTV industry has been slow to adopt the use of the oscilloscope as a means of verifying the quality of signals and for fault diagnosis in systems. And yet for these purposes there is no better test method than actually observing the signal. The reasons for the slow uptake are to some degree understandable. First of all, many CCTV engineers have come from an electrical or security alarm systems background where, in the main, a scope has no advantage over other, more simple, testing methods and, as such, these engineers naturally shy away from this somewhat daunting piece of equipment. Secondly, perhaps because many engineers do not use a scope, they do not appreciate just what it can do and how effective it can be in reducing time and costs when troubleshooting certain faults in CCTV systems. Third, for many years the scope has been a large, expensive item of equipment that is unsuited to site conditions.

No need to be an expert
The first factor is perhaps the most significant, yet the learning curve need not be very steep when coming to grips with the use of a scope for CCTV applications because the truth is, the CCTV engineer does not need to be an expert in the operation and application of a scope. This is because in CCTV, we are generally looking for just one waveshape; that is, one or two TV lines of a 1 Vpp CCIR video signal.

Lets begin with a brief recap on the make-up of a composite video signal. Fig 1 illustrates one line of the composite colour signal associated with a standard colour bar display. Here we see the classic 'staircase' which carries the monochrome information with the 'packets' of 4.43MHz colour signal sitting on the steps. The synchronising pulses which keep the VCRs, monitors and other equipment in step with the cameras are clearly evident, as is the colour burst signal which serves as a sync signal for the colour decoder in the monitor.

Default settings
From Fig 1 we see two important points. First, that the duration of one TV line (including the sync period) is always going to be 64ms. Second, the peak to peak value will always be 1 V when the signal is terminated at 75W (Ohms). Thus it stands to reason that in order to view this signal on a scope, the settings will always be the same. So lets look at the primary controls on a scope and note the settings required to display a video signal.

Referring to Fig 2, perhaps the two most important controls are the 'Volts/Div' and the 'Time/Div' adjustments. On a dual trace scope there is one Volts/Div control for each trace, however in CCTV you can get away with using the scope as a singe trace device, greatly simplifying the operation.

The peak to peak voltage is measured by counting the number of vertical divisions and then multiplying this figure with the volts/division control setting. The periodic time is found by multiplying the number of horizontal squares with the timebase control setting. Typical settings to produce a display of about one line of video signal are given in the example in Fig 3.

The function of the trigger control is difficult to explain without going into great detail, but suffice to say that it must be correctly adjusted if a stable trace is to be attained. In fact, if it is not adjusted correctly there is every possibility of having no display at all. Once again, for CCTV applications the adjustment of the trigger can be greatly simplified because in the majority of cases the automatic circuit is capable of producing a stable display where the input is a composite video signal. In other words, you should be able to set the trigger to the 'Auto' (sometimes called the 'Normal') position and leave it. Just make sure that the 'Trigger Input' selector is set to channel 1, and that you use only the channel 1 input.

Having identified the function and operation of the primary controls, here is a simple procedure that should produce a stable video signal display on the majority of scopes…

Switch on

  • Adjust INTENSITY (Brightness) and FOCUS controls to mid position
  • Set TRIGGER control to AUTO (NORMAL) position
  • Adjust X and Y POSITION controls for centre position
  • Set channel 1 Y GAIN/AMPLITUDE control to the 0.2 V position (or nearest value on your scope)
  • Set Timebase control to 10 ms/cm
  • Set TRIGGER INPUT switch to CH1 position
  • Set trigger switches to AC and + positions
  • Set INPUT COUPLING switches on channels 1 and 2 to 'AC' position
  • Connect the equipment under test to the channel 1 input
  • Adjust FOCUS and INTENSITY controls for best definition

Automated Scopes
Up to this point it has been assumed that you are operating a traditional style manual scope. However if you were considering purchasing a new scope, then it is well worth looking at some of the modern automated versions which in effect self-adjust the timebase and Y input gain to whatever signal is applied to the input. Furthermore, some models have a facility where the last input settings are memorised at switch-off and are re-applied the next time the scope is used. As you will appreciate, this is ideal for our industry where we are in general only looking for one particular waveform.

Termination
This is something that we have to consider when using a scope to measure a CCIR standard wave-form because the signal is only ever at 1 Vpp when it is applied to a 75W load. CCTV equip-ment is all designed to provide this load, either through a manual switch or, more commonly, via automated switching in the BNC input connectors. However a scope is designed to have a very high input impedance – typically greater than 1 MW - and thus if you simply connect the output of, say, a camera into a scope, the signal level would be greater than 1 V (typically 2-3 V) due to the lack of correct termination.

Therefore, whenever you use a scope in a CCTV system you must always ensure that the signal has a means of termination. Methods of doing this are shown in Fig 4.

Referring to Fig 4, making a measurement at any of the outputs labelled A you would use method A where the scope is made to break into the system via a BNC T connector fitted to its input. In this case termination is still provided by the following item of equipment. Note that the scope itself will have no effect on the signal owing to its very high input impedance.

Making measurements at the outputs labelled B may require method Bi or Bii, depending on whether or not the monitor employs manual of automated termination switching. In both methods the signal can usually be taken from the loop output connector on the monitor. If the monitor uses a manual termination switch, then you must leave this in the 75W position. However, if the monitor employs automatic switching, applying the scope lead to the loop output will remove the 75W load. Therefore you must add your own, and the simplest method is to use a BNC T connector as shown in method Bii. Note, if you don't happen to have a spare terminator with you, simply use any spare input socket in the surrounding equipment.

Other Waveshapes
In Fig 1 we looked at the 'ideal' waveshape produced by a test signal generator producing a standard colour bar display. It is important to note that it is only possible to accurately measure a 1 Vpp signal when a display containing peak white is present, which is not always the case when looking at a camera output.

There are occasions where it is necessary to monitor the 'live' video signal emerging from a camera, perhaps to check its level, or perhaps to check its quality (more about this in a moment). In this case what you will be viewing is a waveshape made up of regular line sync pulses with a random pattern in between which represents the instantaneous video information.

With such a signal the peak to peak level will depend on the image being produced by the camera and therefore you may think it not possible to verify the camera output level. However this is not the case, because in a CCIR standard video signal the line sync pulses will always be at 0.3 V into 75W. Thus you can use the sync pulse to verify that the camera is delivering a correct output level.

Another useful technique is to force the camera to produce a white level by pointing it to a white or bright source. External cameras can simply be pointed to the sky (not the sun!) when even a cloudy day will often produce a white level. Internal cameras can be trained onto a piece of white paper as long as the illumination is high enough.

There are occasions when you need to view the signal at field rate (50 Hz) rather than line rate. In this case you will be viewing 312 lines very close together, separated by the vertical sync and flyback blanking period, which is actually a sequence of line sync pulses.

The field period is 20ms and therefore to produce a suitable display you should adjust your scope timebase speed to around 5ms/Div. A typical scope will then display about two TV fields (i.e., one frame).

Test Probes
When you purchase a scope you are normally provided with a pair of test probes. Because of the nature of the tests we are making in CCTV these are rarely required as we are usually able to connect directly via a BNC connector. However there are occasions where you need to be certain that the impedance of the scope is not affecting the circuit under test and in this case it may be necessary to employ a X10 probe.

A X10 probe actually reduces the signal voltage by a factor of 10 – which is why it is called a X10 probe because you have to multiply the voltage reading obtained from the scope trace by 10 to obtain the true reading.

Using a X10 probe increases the input impedance of the scope to around 10MW, ensuring that it will not have any effect on the system or signals being measured.

A high impedance probe is also necessary when looking at the mains voltage waveform because there are few scopes that are capable of displaying signals in the order of 650 Vpp (remember that the 230 V level is the RMS of one half cycle; the peak to peak mains potential is 230 x 1.414 x 2 = 650 V). However it is a highly dangerous practice to use a standard X10 probe for this purpose, and special isolated test kits are available which is a handy 'extra' to pack with your scope.

Practical Applications
Perhaps the most common application of a scope in CCTV is in the adjustment of line correction and twisted pair matching equipment. In both cases it is usually necessary to adjust internal controls to match the equipment to the system, and if this adjustment is not performed correctly then the signal emerging from the equipment will not be true CCIR standard.

The adjustments in line correction equipment are normally related to frequency response and their action is not unlike that of a graphic equaliser in a hi-fi system; i.e. each control affects a different frequency band within the signal spectrum. Incorrect setting can result in excessive high frequency (hf) components which appear in the scope display as overshoots, or excessive low frequency (lf) cut, which appear as a rounding of vertical edges on the scope.

On the picture, excessive hf gives a sharp dark edge to the picture which can be further emphasised following recording and playback on a VCR. Low frequency cut appears as a general lack of contrast, which too often is covered up by turning up the contrast level on the monitor.

The use of a pulse and bar generator is normally recommended for the setting up of this equipment, however a quick check to ascertain general condition of the signal emerging from the equipment is to look at the line sync pulse. This should be a perfect square wave. If it shows signs of overshoots, there is too much hf lift; if the pulse has rounded corners then there is too much lf cut. These effects are illustrated in Fig 5, (drawings I made whilst investigating problems in an actual system).

Fault location
Another use for a scope is in fault location. In reality, if all you are concerned with is signal tracing then it is far simpler to employ a test monitor or a signal level meter. However where you are attempting to isolate the source of RFI, EMI or other interference, a scope can prove invaluable as it will display the noise over the video signal waveshape. (It is often better to look at fields rather than lines for this type of investigation).

Noise signals can enter either through the mains supply or via direct injection into the cables (co-axial in particular). A scope will quickly prove which of these is the source, however remember the precautions we discussed when checking mains potentials with a scope.

I have made use of a scope to deal with telemetry problems, in one case proving that the cause of intermittent drop-out of some PT units was due to the cabling problems; the video signal appeared reasonable on the screen, however when tested it proved to be down to around 0.5 Vpp and the subsequent reduction in telemetry (which in this case was transmitted during the field blanking period) resulted in intermittent loss of control.

Go for 'basic'
If you are thinking of purchasing a scope then you may very quickly become overawed when you see the range (and prices!). Here are a few practical guidelines.

First of all, you only require a 'basic' model for CCTV field work, however if you hope to attain stable displays easily you should look for a model with a bandwidth of at least 20 MHz …but 30 MHz is ideal.

We mentioned automated scopes earlier, and if you are looking to buy new then it is worthwhile considering these. However, you will generally find yourself having to pay a little more than you would for a comparable manual version. Remember, even with an automated scope you only require the basic functions, and should you purchase an 'all singing and dancing' model, then you might well find yourself saddled with something that is beyond the scope (excuse the pun!) of the novice operator. Furthermore, you will be paying for many features that you simply will not need for CCTV work.

You will require something that is portable, and, ideally, you will want a suitable carry case (usually an 'extra') to protect against damage in transit and the climatic conditions experienced on site.

Portability is one of the problems associated with the use of scopes on site; finding a suitable power source in the middle of a field is not always easy! There are a number of hand held units on the market, but my advice to you is that you insist on being given the opportunity to try out a model on site before you buy. Many of these can be very difficult to become familiar with unless you are already proficient in both the use of a scope and the operation of menu driven equipment.

Also, many of the 'budget' models do not provide a clear real time display and, from my experience, are incapable of displaying rapid signals such as the presence of hf noise.

Conclusion
I am well aware that the majority of CCTV work does not require the use of an oscilloscope. However, when we come to the point where we are commissioning or fault finding in systems, the use of a scope can save a lot of money in the long run on repeat calls and the replacement of equip-ment that has incorrectly been diagnosed as faulty.

I hope that this article proves to be of a practical help to readers. Perhaps these few pages are ones you may wish to tear out and keep with your scope for future reference.

… And finally, I would like to thank Barry Ross of Hameg Ltd for his help in the preparation of this article.

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