SCOPING IT OUT:using the oscilloscope

Jul 1, 1999 12:00 PM, Glen Ballou

We have all either used or abused oscilloscopes at one time or another. Many times, we did not see what we thought we should have seen, not necessarily because we did not know what it should have looked like, but because the scope did not have the right specs for the job we were doing. Ultimately, what do we need in an oscilloscope? That depends on what we are looking for. If we are looking at high input level and low frequencies, almost any scope will do. If, however, we are looking at low level RH signals or high-speed digital signals, we need a scope with high sensitivity, wide frequency response and a fast rise time. If we want to compare one signal to another, we need a dual beam scope.

Although it is nice to have a scope that can do everything, cost, size and complexity of operation probably dictate that we should buy a scope that only fits our needs or a little beyond. About 20 years ago, almost any scope was adequate for an A-Vservice technician. Sound and optical circuits were low frequency analog. Today, however, we see digital with fast rise times, VHF and UHF rf microphones and all kinds of TVs, many, like video walls, linked together through computers. Of course, we cannot forget that many sound systems are controlled by computers.

We also need to look at size. Everybody likes a large screen television in his home, but in a boat or camper where one must carry it around, we usually opt for a small portable unit. That also holds true to scopes. If we are using a scope only on the bench, then a 5 inch or 6 inch (127.5 mm or 152.4 mm) screen is very useful. If we use the scope on the road, a small, lightweight scope is probably best, such as a Fluke ScopeMeter. Remember that small scopes have small screens, which translates into lower apparent resolution and more eye strain, but this still may be much better than back strain. Another good idea when we are on the road is battery power. This is especially handy if we are in the bowels of the building, far away from the power outlet, servicing a system in the middle of a field, or in a country with power other than what we are used to. Another advantage of the ScopeMeter type of scope is that it is a digital multimeter.

What frequency response do we need? That again depends on the application for which we plan to use the scope. If we are only looking at audio signal, then a scope capable of reproducing from 20 Hz to 20 kHz would be adequate. If, however, we want to see distortion, noise and spikes, we will require at least a 200 kHz bandwidth. Today's scopes usually have at least a 5 MHz bandwidth, so frequency response is not usually an issue. If we are looking at RF, we might require a scope with a bandwidth of 200 MHz or 500 MHz. Frequency response costs money. A 20 MHz scope may cost $600, and a 500 MHz scope can cost more than $10,000.

The scope's time base is also important. A 20 MHz scope may have a time base from 0.1 us/div to 0.2 s/div. It is quite common to have five divisions on the screen. If we had a 200 MHz signal, which is equivalent to 5x10[superscript]-9 seconds per cycle (s/cycle), and we set the time base to 0.1 us/div (0.1x10[superscript]-6 s/div) we would have 100 cycles (20 cycles/div x 5 div) showing on the screen. A 20 Hz signal would have 20 cycles on the screen at 0.2 s/div. This is particularly important to know if we are planning to use the scope to determine frequency.

If we want to look at digital circuits (zeros and ones), we will need a scope that can measure DC. We should also have good rise time because a perfect digital circuit would go from zero to one instantaneously. If we have a narrow band scope, our signal may look like a sign wave when it is actually a square wave.

Remember that when measuring an AC signal riding on a DC signal, we are seeing both the AC and the DC signal at the same time. The DC signal looks like an offset with an AC signal riding on it. This can be a potential problem. For instance, if we have a 10 mVAC signal riding on a 10 VDC signal, the AC will hardly be visible. This is the reason scopes incorporate an AC/DC input switch. Look at the DC value at a vertical sensitivity that allows you to see it, then switch to the AC input and increase the scope input sensitivity to expand the AC signal.

Next, we need to look at vertical sensitivity. Vertical sensitivity is given in V/div and often goes from 5 mV/div to 5 V/div in 10 steps. The input also has a vernier for adjusting the signal to fit the screen; however, it is only calibrated in the calibrated position. If we want to look at such low-level circuits as micro output, we need a scope with 10 mV sensiti vity or better, one mV being much more acceptable. Unfortunately, high sensitivity costs money. If we only need to look at low level signals once in a while, we might be better off to insert a preamp between the device and the scope. Although it will not be calibrated, it will show us the signal.

Often we like to compare one signal to another. This is where a multi-input scope can come in handy. We can put one signal on the A channel and the second signal on the B channel, and so on. By superimposing the signals, we can see distortion, phase shift and gain or attenuation between the two signals.

Scopes have to be triggered to produce a steady trace on the screen. If they are not triggered, the waveform will move either to the left or to the right. There are a variety of ways to trigger a scope. If we are looking at a signal that is related to 60 Hz, we can trigger it off the power line. Of course, if the scope is operating on battery, we will have to trigger it internally. Normally, a scope is triggered by the signal we are looking at. If it is a sine wave, we can trigger it with either a positive or negative going signal. If, however, it is an asymmetrical signal, we will probably want to trigger it with the polarity, which is larger, often the negative going signal. Scopes often have a trigger that will sync to a video signal, whether it be NTSC, PAL, SECAM or HI resolution.

Triggers may be straight forward signal-sensitive triggers, or they may be delayed, one shot or input sensitive. The straight forward trigger starts the horizontal sweep at the left side of the screen immediately after the beam makes a sweep. This looks like a continuous display. All scopes have this mode.

More sophisticated scopes have delayed and/or one-shot modes. The delayed trigger starts the sweep a certain length of time before applying the signal, so the event shows up into the sweep rather than on the leading edge of the sweep. This is especially helpful when looking at an impulse wave or something that happens at the beginning of a cycle. If we tried to look at an impulse in regular mode, the beginning of the impulse would be cut off.

One-shot sweeps are useful when looking for an intermittent spike or signal. The scope sits at idle and will only trigger when a signal reaches the threshold we specify. The scope triggers and then shuts off the horizontal sweep until we reset the trigger. This way, we can now leave the scope unattended and still see the signal if it occurs.

Most scopes have a 5x mode that expands the horizontal sweep five times. This is useful for expanding the waveform currently under inspection. We can also expand or contract the length of the sweep, so a repetitive wave can be aligned between divisions on the screen. Of course, like the vertical vernier, the sweep is calibrated only in the calibrated mode.

Finally, if we have a dual beam scope, we can trigger one beam with the signal of the other beam. This can be useful when looking at time relationships between two events.

One final piece of food for thought. If you usually carry a portable PC with you on the job, you might want to think of using the the new PC-based oscilloscope. The Velleman PCS64i is a hardware device that interfaces with the PC through the parallel port. It is optically connected to the PC to prevent damaging the PC with high voltages and bad grounds. It operates like a multi-channel digital storage scope. It also includes an FFT to display the spectral content of the waveform and has some basic math functions. Bandwidth is DC to16MHz.