Chapter 2   Test Equipment.

By definition a VOM has no amplifying devices in it. It contains only resistors, a few diodes, perhaps one capacitor, some batteries and, of course, the meter movement.

As the name implies the VOM can measure voltage, current and resistance. Most VOMs will measure both AC and DC voltage while the current ranges are DC only. The internal batteries are used in measuring resistance. A VOM will not measure the reactance of an inductor or a capacitor.

Ranges and scales.

It is NOT correct to say "the meter is set to the 10 volt scale". It is correct to say "the meter is set to the 10 volt range".

It is correct to say "the scale reading is 5.3". Notice that there are no units given. The units and the power of ten by which the scale reading must be multiplied come from the setting of the range switch.

Reading the meter.

A typical VOM may have 8 voltage ranges, 6 current ranges and 5 resistance ranges. There are never separate scales for each range. It is necessary to multiply the scale reading by the correct power of ten to obtain the correct reading.

Example 2.1.

A VOM is set to the 2.5 volt range. There is no 0 to 2.5 scale but there is a 0 to 250 scale. The reading on this scale is 195. What is the voltage being indicated?

Solution:

The range switch is set to the 2.5 volt range (given), which means that the meter can measure voltages anywhere in the range of 0 to 2.5 volts. It is therefore impossible for the reading to be 195 volts. If the meter were indicating full scale (all the way to the right), it would be indicating 2.5 volts. Also, the pointer would be over the 250 mark on the scale. It is necessary to divide the scale reading by 100 to obtain the correct voltage reading. 195/100 = 1.95 volts, which is the correct reading.

Example 2.2.

A VOM is set to the 500 milliampere range and the scale indication is 27 on a 0 to 50 scale. What is the meter indicating?

Solution:

The 0 to 50 scale corresponds to the 0 to 500 milliampere range; therefore, the scale indication must be multiplied by 10 to obtain the correct reading. 27 times 10 = 270 milliamperes.

Notice that you do not multiply the range switch setting by the scale reading. You choose the scale which is related to the range switch setting by a power of 10 and multiply the scale reading by that power of 10 to obtain the reading.

Most experienced VOM users simply look at the scale, mentally put the decimal point in the right place and take the reading. As so often happens, the marvelous computer called the human mind makes all of these calculations without even thinking about them.

AC voltage.

Input resistance.

Example 2.3.

A Simpson 260 is set to the ten volt range and is indicating 5.1 volts. The sensitivity is 20,000 ohms/volt. What is the input resistance of the meter when set to this range?

Solution:

The voltage being indicated is extraneous information. The input resistance is the product of the range switch setting and the sensitivity. 10 volts times 20,000 ohms/volt = 200,000 ohms or 200 k ohms.

Example 2.4.

A Simpson 260 is set to the 10 volt AC range. The stated AC sensitivity is 5,000 ohms/volt. What is the input resistance?

Solution:

10 volts times 5,000 ohms/volt = 50,000 ohms or 50 k ohms.

It is a common mistake to multiply the scale reading by the sensitivity instead of the range switch setting. If that were so, the meter's resistance would be zero when its pointer was sitting on zero. The voltmeter would be a dead short and it would be impossible for a voltage to appear across it.

Voltage measurement and loading effect.

Example 2.5.

A Simpson 260 is set to the 10 volt AC range and is being used to measure voltages in a circuit. What is the maximum value the Thevenin resistance can have if the meter is to perturb the voltage by no more than 2 percent?

Solution:

Referring to example 2.4, the resistance of the meter is 50 k ohms. Solving equation 2-1 for RTH yields

R_Th = R_M x 0.02 = 50 k ohms x 0.02 = 1 k ohm

A Simpson VOM which is set to the 10 volt AC range can only be used to measure voltage in circuits with Thevenin resistances from 0 to 1 k ohm.

Example 2.6.

A Simpson 260 is set to the 2.5 volt DC range and is being used to measure voltage in a circuit which has a Thevenin resistance of 5 k ohms. The DC sensitivity is 20 K ohms/volt. What is the percent by which the meter will perturb the measurement?

Solution:

The resistance of the meter on the 2.5 volt DC range is 2.5 V x 20 k ohms/V = 50 k ohms. From equation 2-1

% error = 100% x (5 k ohms)/(50 k ohms) = 10%

Resistance measurement.

The scale for measuring resistance is not the same as the voltage and current scales. First of all, the zero is at the wrong end of the scale. There is a very good reason for that. The scale is also very nonlinear. This scale is always at the top of the meter and is clearly labeled "OHMS".

The circuitry within a VOM applies a voltage from a battery to the resistor under test and measures the current. Ohm's law tells us that I=V/R. This is a nonlinear function, which is the reason why the OHMS scale on the VOM is nonlinear.

The range switch is not marked the same for resistance ranges, as it is for voltage and current ranges. A typical set of resistance ranges are as found on the Simpson model 260 VOM. These ranges are RX1, RX100 and RX10,000. These are read "Resistance times one", "Resistance times one hundred", and "Resistance times ten thousand".

This indicates that the scale reading is to be multiplied by the range switch setting.

Example 2.7.

A VOM is set to the RX1 range and the ohms scale is reading 12.5. What is the resistance being measured?

Solution:

To obtain the resistance, multiply the range switch setting by the scale reading. 12.5 times 1 = 12.5 ohms.

Example 2.8.

A VOM is set to the RX10,000 range and the scale is reading 1.6. What is the resistance being measured?

Solution:

1.6 multiplied by 10,000 = 16,000 ohms or 16 k ohms.

Adjusting the ohmmeter.

1. Set the range switch to the range you intend to use.

2. Clip the two test leads together.

3. Adjust the ZERO ADJUST knob on the VOM to bring the pointer over the zero mark on the ohms scale.

4. Unclip the leads from each other and clip them to the resistor to be tested.

5. Read the scale and multiply by the range switch setting.

6. If the pointer is far to the right or left of the scale, the reading will not be very accurate.

7. Every time you change ranges you must repeat steps 2 through 5.

The first form of electronic voltmeter was the vacuum tube voltmeter VTVM. For several decades the VTVM was the most common instrument to be found on the test bench.

When the age of the semiconductor came upon us, the vacuum tubes in the VTVM were replaced by field effect transistors. The name FETVM was too clumsy to catch on. TVM (Transistor Voltmeter) never caught on either. The name EVM (Electronic Voltmeter) was never tried. Heath company tried SSVM for Solid State Voltmeter. I don't really know if that one caught on with other manufacturers. In this text we will use EVM as a generic term for an analog meter containing vacuum tubes or transistors.

The input resistance of an EVM (electronic voltmeter) is typically 10 megohms. More costly units have an input resistance of 100 or even 1000 megohms. unlike the VOM the input resistance is the same for AC and DC. Also, the resistance is constant on all ranges. An EVM loads the circuit under test in exactly the same way as any other meter. The only difference is that the circuit resistance can be higher before the error becomes significant.

One reads the EVM in exactly the same manner as the VOM. The terms "range" and "scale" have the same meanings and should be used the same. Remember, the range is the setting of the range switch and the scale is the set of markings on the meter face.

The resistance ranges may operate differently than on a VOM. If you are using an EVM you should consult the instruction manual for that meter to determine how to adjust and read the meter on the resistance ranges.

The AC Voltmeter.

This is the only area where analog EVMs are still extensively used. A typical frequency range for an analog AC Voltmeter (AC only) is from 5 hertz to 4 megahertz. These meters are AC only, If you try to measure DC you will get no reading or one which makes no sense.

The input impedance of such a voltmeter is typically 10 megohms in parallel with 100 picofarads.

An AC Voltmeter is used in trouble shooting wide band amplifiers such as video amplifiers where frequencies exist to which VOMs, other EVMs and even DMMs (digital multimeters) cannot respond. The AC Voltmeter will also find uses in low-level audio equipment where signal levels are too low for other meters to indicate.

Digital Multimeters are smaller, lighter, cheaper and more accurate than their analog counterparts. It is no wonder why you almost never see an analog EVM anymore. The only area where DMMs are behind is in measuring AC over a wide range of frequencies. A typical frequency range for a DMM is 40 to 5,000 hertz although more expensive models will usually read accurately out to 20,000 hertz. A typical analog AC Voltmeter (AC only) may have a frequency range of 5 hertz to 4 megahertz. But when it comes to DC voltage, current and resistance, the DMM is unsurpassed.

Little thought is required to use a DMM. The measurement units V (volts), mV (millivolts), k ohms (kilohms), M ohms (megohms), etc. are usually indicated in the display window. The decimal point is always put in the right place. If the display window contains 1.364 k ohms or 103.8 uA, you have the measurement. What you see is what you get. Some DMMs even select the correct range automatically, removing almost all operator intervention in their operation.

The input resistance of most low cost DMMs is 10 megohms. Higher cost instruments may have input resistances of 100 or even 1000 megohms. DMMs load the circuit under test as do all voltmeters. Equation 2-1 applies.

Example 2.9.

A DMM which has an input resistance of 10 megohms is being used to measure voltage in a circuit where the Thevenin resistance is 100 kilohms. What is the percent error introduced by the meter loading the circuit?

Solution:

From equation 2-1

% error = (100 k ohms)/(10 M ohms) x 100% = 1%

Although 1% error is small by analog standards, it is large by digital standards. If you are making a measurement under the conditions of example 2.9, the meter will give you 3 or 4 significant digits, but you must remember that the reading is 1% low.

Correcting Measurements.

Referring to figure 2.1 the Thevenin resistance of the circuit and the resistance of the meter make up a voltage divider. If we rearrange the voltage divider equation we have

where R_M is the resistance of the meter, R_Th is the Thevenin resistance of the circuit, V_M is the voltage being read on the meter and V_Th is the voltage across the terminals when the voltmeter is not present.

Example 2.10.

A DMM with an input resistance of 10 M ohms is being used to measure the voltage in a circuit where the Thevenin resistance is 220 k ohms. The meter reads 12.52 volts. What was the voltage before the meter was connected?

Solution:

Using equation 2.2 we have

V_Th = 12.52 v ((10 M ohms + 220 k ohms)/10 M ohms) = 12.80 volts.

Strong Electromagnetic Fields.

Modern day DMMs are housed in plastic cases. Plastic does not shield against electromagnetic fields. Many instruments have a shield consisting of a layer of metal foil which is coated with plastic to prevent shorting out the circuit board. This meager shield is totally inadequate to the job of preventing electromagnetic fields from entering the circuitry of the instrument.

Electromagnetic fields can so totally jam the circuits in a digital meter as to prevent it from working. Even if the meter appears to work, its readings will not be reliable. The circuits in an electronic analog meter (EVM) can be adversely affected by strong electromagnetic fields. Only the VOM has no internal devices which can be affected by these fields.

Meter Characteristics.

The ranges listed in the table refer to full-scale ranges, not minimum measurement capability. For example, the VOM is listed as having DC current ranges from 50 microamps to 10 amps. That does not mean that 50 uA is the smallest current that can be measured; it means that the lowest range is 0 to 50 uA. The smallest current which can be measured with any reasonable accuracy is 5 uA.

Table 2.1.

Summary of Test Meter Characteristics.

EMI = Electromagnetic Interference.
* Data for AC-only electronic voltmeter.

When an EVM is set to the DC voltage range a 1 Megohm resistor is connected in series with the input at the probe tip. This is done either by using a special probe only for DC voltage or by a switch on the probe tip. The purpose for this resistor is to filter out AC including RF that might be present on the measured voltage or picked up by the leads. Many EVMs use a shielded lead after the resistor.

A few examples will give you some practice in choosing the correct meter for a given task.

Example 2.11.

You need to measure voltage, current and resistance in a laboratory and the accuracy needs to be as good as possible. What kind of meter would you use?

Solution:

EMI (electromagnetic interference) is not likely to be a problem in a laboratory setting. The need for accuracy indicates the use of a DMM (Digital Multimeter).

Example 2.12.

You need to measure AC voltages over a range of 2 to 10 volts and a frequency range of 20 to 20,000 hertz. The Thevenin resistance of the sources can be as high as 5 k ohms. There are no nearby sources of EMI. Which meter would you use?

Solution:

The frequency range of 20 to 20,000 hertz eliminates the DMM from consideration. A VOM set to the 10 volt AC range will have an input resistance of 5 k ohms/V x 10 v = 50 k ohms. The circuit resistance of 5 k ohms gives an error of (5 k ohms/50 k ohms) x 100% = 10%. The VOM is eliminated. That leaves us with some kind of EVM. An AC-only model would be preferable but an electronic multimeter would serve the purpose.

Example 2.13.

You need to measure AC voltage in the neighborhood of 2500 volts. Which meter would you use?

Solution:

The only meter with ranges above 1000 volts is the VOM.

Example 2.14.

You need to make measurements on an operating microwave oven. Which meter would you use?

Solution:

A microwave oven with its service covers removed is likely to be a strong source of EMI. The old reliable VOM is the instrument of choice here.

Example 2.15.

You need to measure voltages in the range of 0.5 to 3 volts at frequencies from 100 kHz to 1 MHz. Which meter would you use?

Solution:

While the voltage range might be accommodated by other meters, the frequency range dictates the use of an AC- only EVM.

There are many cases in which an adjustment must be made for maximum or minimum current or voltage. While it is possible to use a digital readout for such an adjustment, an analog meter makes it much easier.

Suppose you are adjusting a control for a minimum current. When using an analog meter you do not actually read the scale of the meter. You watch the pointer moving to the left as you turn the control. When the pointer starts moving to the right, you reverse direction on the control and bring the pointer back to its left-most position. It is a matter of eye-hand coordination.

On the other, hand if you are using a digital readout to make the same adjustment, you do have to read the number on the display. As you make the adjustment you continuously read the number and do a comparison to the previous one. It's no longer a matter of eye-hand coordination; now the mind must remember a number and do calculations of sorts: "Is this number larger than or smaller than the other one?" This remembering and calculating takes more time and requires more mental effort than does eye-hand coordination.

"But wait a minute" I hear some of you saying. "What about bar graph displays?" Bar graph displays usually have ten elements which gives only 10% resolution. In tuning the output circuit of a radio transmitter the capacitor is adjusted for minimum amplifier current. This setting gives maximum power output and maximum efficiency of the amplifier. If a bar graph were used for this purpose the amplifier current would have to change by 10% of full-scale before any change could be detected by the operator. If a transmitter's output stage is operated 10% "off the dip" the output could be down by as much as 30% and the output amplifier could even be damaged.

This is but one example; there are many others in the field of electronics. It can be argued that there is no reason why a bar graph must be limited to ten elements. There is a reason, money. To match the resolution of an analog meter a bar graph would have to have at least 50 elements and 100 would be preferred. At the present state of the art, a 50 or 100 element bar graph readout is so costly as to be unfeasible. And don't forget that matter of EMI. Analog meter readouts will be with us for many years to come.

In service work there are many service adjustments which require making an adjustment for zero, minimum or maximum voltage or current. That is one of the strongest arguments for keeping a VOM on the service bench.

The technicians of those days had an oscilloscope calibrator of a sort. It was a source of square-wave voltage with "calibrated" dials on it for voltage and frequency. The idea was to switch from the wave being observed to the calibrator, match the voltage and frequency to that of the original wave and read the dials on the calibrator.

Enter the Tektronix company. Their first models had the calibrator built in and a convenient switch to select the input wave or the calibrator wave. Then one of their engineers asked the question, "Why not calibrate the scope the same way a voltmeter is calibrated, with ranges?" The rest, as they say, is history.

Today we take the calibrated oscilloscope for granted. I wonder how many students know that the scope began life as described above.

The instrument is known by several different names: "oscilloscope", "o'scope", "'scope", "scope" and one author referred to it as the "CRO" (cathode ray oscilloscope). Whatever else you may call it, call it the most useful instrument on the service bench.

What an Oscilloscope Can Do.

The modern-day oscilloscope is very accurately calibrated in volts per division on the vertical axis and in seconds per division on the horizontal axis. The display is a visual display of instantaneous voltage versus time. Such displays can give information not available using any other instrument.

For example, ripple in a power supply can render circuits inoperative but may not show up on any test meter. The problem will be readily apparent on an oscilloscope.

The following is not intended as a thorough tutorial on "how to use an oscilloscope". That can only be done if the tutorial is written for a particular make and model of oscilloscope. It is assumed that the student is familiar with oscilloscope controls and operating procedure. This section should bring together all of the fragments of knowledge you have acquired in the past few years.

Reading An Oscilloscope.

To obtain the voltage between two points on a wave, it is necessary to perform the following steps: a) Measure the vertical distance between the two points (in divisions and fractions of a division). b) Multiply the distance by the setting of the volts/division range switch. c) If a times ten probe is being used, multiply the voltage by ten.

Example 2.16.

For a wave on the screen of a scope the distance between the positive peak and the negative peak is 5.6 divisions. The setting of the vertical range switch is 100 mv/div. A times one probe is being used. What is the peak to peak voltage of the wave?

Solution:

The peak to peak distance has been given as 5.6 divisions. 5.6 divisions times 100 mv/division = 560 millivolts or 0.56 volts. The use of a times one probe means that the voltage must be multiplied by one.

To obtain the time between two points on a wave, it is necessary to perform the following steps: a) Measure the horizontal distance between the two points (in divisions and fractions of a division). b) Multiply the distance by the setting of the time/division sweep range switch.

Example 2.17.

For a wave on the screen of a scope the distance between two successive positive going zero crossings is 8.3 divisions. The setting of the sweep range switch is 0.5 ms/div. A times ten probe is being used. What is the period of the wave?

Solution:

The fact that a times ten probe is being used has no effect on the time reading. The wavelength has been given as 8.3 divisions. 8.3 divisions times 0.5 ms/division = 4.15 milliseconds.

The Times Ten Probe.

The resistance portion means that if we want plus or minus 2% accuracy the highest Thevenin resistance a circuit could have is 20 k ohms. That would considerably restrict the use of a scope but that's not all.

Capacitance added in parallel with a circuit presents a load at higher frequencies and slows down the rise time of square-waves and pulses. In most cases, calculation of this effect is too complex to be worth the effort. The best approach is to make the capacitance as small as physically possible.

The input capacitance of the scope by itself would not be so bad but there is more.

If you have ever touched the input terminal of a scope, or connected an unshielded wire to the input of a scope, you know that there are signals in the air which can be picked up by the wire (or your body) and displayed on the scope. If you are trying to look at a 130 kHz triangular wave, you don't want 60 Hz mixed with it. That is what you can have if you don't use shielded cables to connect circuits under test to the input of an oscilloscope.

Shielded cable consists of two conductors separated by an insulator, the definition of a capacitor. Typical cable capacitance is 30 picofarads per foot of cable. Typical cable length is 4 feet. The input capacitance now becomes 4 ft. x (30 pf/ft.) + 30 pf = 150 pf.

150 picofarads has a reactance of 20 kilohms at a frequency of 53.1 kilohertz. That says that 150 pf is much too much to have in parallel with a test instrument.

The frequency response of the circuit will be flat if

The effective input resistance is R1 in series with R2 which is 10 M ohms. That's also a lot better.

To sum it all up, by using a times ten probe the input resistance goes up by a factor of ten and the input capacitance goes down by a factor of ten. The voltage applied to the input of the scope is 1/10 of that applied to the probe tip.

Example 2.18.

The following information is obtained using an oscilloscope. Peak to peak voltage is 125 millivolts and period is 22 milliseconds. A times ten probe is being used. What are the actual voltage and period.

Solution:

The voltage is ten times what is measured, 125 mv x 10 = 1.25 volts. The times ten probe has no effect on the period measurement.

Adjusting the Times Ten Probe.

All oscilloscopes have a square-wave output which has been put there for exactly this purpose. If the probe has a switch to select X1 (times one) or X10 (times ten), be sure it is in the X10 position. Connect the probe tip to the square-wave output and adjust the controls on the scope to obtain a stable display of the square-wave.

AC, DC and Ground.

When this switch is set to the AC position, a capacitor is connected in series with the input to the scope. This capacitor will block DC from the input but will pass AC. The size of the capacitor is usually chosen so that the low frequency limit will be about 2 hertz. The upper frequency limit is set by the amplifiers in the scope and is uneffected by the setting of the switch.

In the DC position the scope responds to both AC and DC. The capacitor mentioned above is shorted out. The response of the scope is from DC (zero frequency) to the upper frequency limit of the scope. This makes it possible to use the scope to measure DC voltage the same as you would with a voltmeter.

When the switch is set to the GND (ground) position the input to the vertical amplifier is grounded. The input resistance (as "seen" by the circuit under test) is not affected so that the circuit under test will not be damaged. In many measurements, especially those involving DC, it is essential to know where the trace would be if the input voltage to the scope were zero. Instead of disconnecting the probe, all that is necessary is to flip the input switch to the GND position, note the position of the trace (or use the positioning control to put it where you want it) and then flip the input switch back to DC or AC.

Example 2.19.

Figure 2.4 represents the display of a DC + AC voltage on an oscilloscope. When the input switch is set to ground, the trace is positioned one division up from the bottom of the screen, or at Y = -3 divisions. In figure 2.4a the scope operator has used the horizontal positioning control to set the most positive peak on the line of hash marks. In figure 2.4b the horizontal positioning control has been adjusted to put the least positive peak on the line of hash marks. This is what you would do if you were in a laboratory instead of reading a book. The settings of the scope controls are as follows: input switch to DC, Vertical range to 0.2 v/div and sweep range to 5 ms/div. A times ten probe is being used. What is: (a) the peak to peak voltage; (b) the voltage of the most positive peak; (c) the voltage of the least positive peak; and (d) the period of the wave?

Solution:

The only difference between figures 2.4a and 2.4b is that the horizontal positioning control has been changed. Figure 2.4a reveals that the most positive peak is 3.4 divisions above the center line and figure 2.4b reveals that the least positive peak is 0.6 divisions below the center line.

(a) The peak to peak distance of 3.4 div - (-0.6 div) = 4 divisions. 4 div x 0.2 v/div x 10 = 8 volts peak to peak.

(b) The voltage of the most positive peak is measured from the zero voltage line. As given, the zero volt line is one division up from the bottom of the screen or 3 divisions below the center line. The distance between zero volts and the most positive peak is 3.4 div - (-3 div) = 6.4 divisions. 6.4 div x 0.2 v/div x 10 = 12.8 volts.

(c) The distance between zero volts and the least positive peak is -0.6 div -(-3 div) = 2.4 divisions. 2.4 div x 0.2 v/div x 10 = 4.8 volts.

(d) We must use center line crossings in order to have the hash marks to read. Selecting two successive positive going center line crossings we have the wavelength as 4 divisions. The period is 4 div x 5 ms/div = 20 ms. The times ten probe has no effect on the time measurement.

Getting Best Accuracy.

For voltage and period measurements keep the vertical deflection large. The large divisions are subdivided by the hash marks into 5 subdivisions. It is possible to mentally insert 4 sub-subdivisions between the hash marks. That means that the best resolution is 1/20 of a large division. That is the ideal case. In actual practice it is more likely that the best resolution is 1/10 of a large division.

If you only have a vertical deflection of one large division, the typical accuracy is about (0.1 div)/(1 div) x 100% = 10%. If the vertical deflection is about 6 large divisions the typical accuracy is about (0.1 div)/(6 div) x 100% = 1.7%. 2% accuracy is about the best to be expected.

For period measurements have several cycles on the screen. Also, measure from zero crossing to zero crossing, not peak to peak. The horizontal position of the rounded peak of a sine wave cannot be determined with any accuracy.

If the vertical deflection is large and there are several cycles on the screen, the zero-crossings will be almost vertical. Suppose you tried to measure the period from one positive peak to the next. You would be trying to determine the exact point of tangency of the curved peak of the wave to the scale, not an easy thing to do. When an almost vertical line crosses a horizontal scale it is easier to determine the exact point where the crossing takes place.

If you measure the time for four or five cycles, the accuracy of the measurement will be improved. If you measure the period of just one cycle, the distance will be rather small and the same argument will apply as for the vertical deflection. Measuring over several cycles increases the distance of the measurement and increases the accuracy.

Trigger Modes.

The key difference is that an oscillator keeps on running at approximately the same frequency even in the absence of an input signal. A triggered sweep begins when a trigger pulse is received, completes one cycle and then waits for the next trigger pulse before beginning the next cycle. If the trigger pulses stop, the sweep stops.

The "auto" mode is to keep a visible trace on the screen in the absence of an input signal. If you put the input switch in the GND position, you want the trace to be on the screen so you can see where it is. A special circuit senses that there are no trigger pulses and generates artificial trigger pulses. These pulses are removed when real pulses are present. Like any other automatic circuit it can sometimes get confused and cause the pattern on the screen to be unstable. When this happens, you should switch out of the auto mode. If you want to use the GND position on the input switch, you will have to put the trigger back into the auto mode. Someday it will occur to some engineer (who can do something about it) to cause the trigger circuits to go into the auto mode whenever the input switch is placed in the GND position.

The Slope switch selects whether the sweep will trigger on the positive going or negative going slope of the input wave.

The level control sets the voltage level at which the sweep will begin.

Trigger coupling modes come in a wide variety of shapes and sizes. A few of them are DC, AC slow, AC fast, AC lf, AC lf rej, (reject), TV, TVh, TVv, TVl and TVf. DC is a full spectrum coupling. It is used for slowly changing, infrequently occurring or DC restored video signals. AC slow and AC lf are for low frequency sine or sine like waves. AC fast and AC lf rej are for square-waves, pulses or high frequency sine waves. TV is a general television trigger mode. In some scopes the horizontal or vertical sync pulses are selected depending on the setting of the sweep range switch. TV h (horizontal) and TV l (line) select the horizontal sync pulses from the TV signal to be fed to the trigger circuits. TV v (vertical) and TV f (frame of field) select the vertical sync pulses.

For complete information you should consult the instruction manual for the particular scope you are using.

Measuring Current and Resistance Using a Scope.

Measuring current is easy and maybe some of you have figured that one out already. Simply place a resistor, say one ohm, in parallel with the input of the scope. The ranges on the vertical range switch become current ranges. For example, using a 1 ohm resistor the 100 mV/div range becomes 100 mA/div. The resistor should have sufficient wattage rating to carry the current being measured. Also remember that if the voltage drop becomes too great, you will perturb the circuit under test and the current measurement will be in error.

Measuring resistance requires a bit more ingenuity. All scopes have a square-wave signal output for compensating the probe. This output is used in the circuit of figure 2.5. Since one side of the circuit is connected to the scope's own ground it is not possible to measure the voltage across each resistor individually. Measure the voltage across the reference resistor RR and then measure the voltage across the series combination of RR and RX. This last voltage is of course the output voltage of the square-wave output. For best accuracy the voltage across the reference resistor should be about half of the square-wave output. The unknown resistance is given by

where RX is the unknown resistance, RR is a known reference resistor, VS is the voltage of the square-wave source and VR is the measured voltage across the reference resistor.

Operational differences.

The trigger point has been moved from the left edge of the screen to the center. You don't need to adjust the control on the edge of a hair trigger to be sure you are seeing the entire rise of a square or pulse wave. Just turn up the sweep speed and the rise will sit right there in the center of the screen as it stretches out for easy measurement.

Range Settings are on the screen rather than on the panel. In fact if you start turning any knob on a DSO you will never come to a stop. None of the controls are detented wafer switches or potentiometers (pots). They are all shaft encoders. The range settings are displayed on the screen because that is the only way to show them. This was done in a few models in the analog era and it worked pretty well.

Cursors for more accurate measurement are a common feature of DSOs. This was also tried in the analog era but they weren't very successful. All they did was to make the calibration procedure longer and more complicated. Cursors can be lined up with traces with better accuracy than a trace by itself can be read. And of course the cursors have digital readouts.

Direct digital readout of commonly performed scope measurements are provided by firmware built into the scope. Some of these are; peak to peak, RMS, average, rise and fall time, pulse width, frequency, period, and even fast Fourier transform. Scopes that have just the minimum of these features essentially give you a scope, DMM, and frequency counter all in one box. Full featured scopes are more like having a scope and computer in the same box. The screen can get rather busy with all of these digital readouts on it which is why advanced features such as cursors and direct measurement can be switched off if they are not needed at the moment.

The input resistance and units of measurement can be changed in some high end models. The input resistance, impedance to some, can be switched between 1 meg ohm and 50 ohms. Any communications engineer or technician will immediately understand the usefulness of that feature alone. The units of measurement can also be switched between voltage and current. I'll bet that power measurement isn't far off if not already here. I have yet to see one with the power feature.

Four function math involving the two traces is also a feature of a DSO. The most that could be managed in the bad old days was add and subtract. Digital technology has added multiply and divide. Below you see what happens when you multiply two triangular waves that have a 90 degree phase difference.

Below that you see that channels 1 and 2 are set to the 500 mV/div range with DC coupling and 1 Meg ohm input impedance. The channel 1 offset is 500 mV while the channel 2 offset is -500 mV. Channel 2 has the band width limit turned on while channel 1 does not. When on, the bandwidth of the channel is 20 MHz. This is a 200 MHz scope.

The yellow triangle at the right of the grid is the trigger level setting. The blue triangle at the top center of the screen is the trigger delay. The selected trace triggers at the intersection of the two lines drawn from the symbols. Both the trigger level and the delay are adjustable. The delay takes the place of the horizontal position control from analog scopes. In fact on this particular scope the control is called "position". On other DSOs it may be called delay or trigger delay.

The sweep speed is displayed just to the left of the word delay.

On the left edge of the grid you see three icons with characters inside them. The yellow one has a 1, white has an M and purple has a 2. These indicate the zero points of channel 1, the results of the math operation and channel 2 respectively. The math has its own positioning and range switch. On this scope the range switch is not labeled because it doubles as a control for the digital option which I do not have installed. There are only two ways of knowing this. Trial and error or reading the manual. Other scopes may be labeled a little better.

Along the bottom of the screen you see the soft key menu for the math function. There are hard keys to the right of the screen that select different menus. When a soft key is pressed one of two things happens. If there are only two alternatives the one that is currently selected changes to the other one. If there are 3 or more alternatives a pop up menu appears showing them. You can see that the operation is set to * (multiply), source A to channel 1, source B to channel 2, and invert to off. The empty space indicates that there are two more soft keys which are not used on this menu.

If you press the hard key for channel 1 or 2 the menu for that channel comes up. Some of the items are familiar and some are not. The first one is for selecting DC or AC coupling and ground. The next is bandwidth limit on/off. Next is adjust course/fine. This takes the place of that Vernier control that was concentric with the range switch on analog scopes. The fine setting of this item causes the range switch to change in tiny increments. So you can set a square wave amplitude to the dotted lines and easily identify the 10 and 90 % points on the rise and fall. Next is the probe selection key which doesn't just have X1 and X10 but values ranging from X0.1 to X10,000 set in a 1, 2, 5, sequence. Next is the 1MΩ/50Ω selector. The last key brings up the second page. First on the second page is V/A. Next is Deskew and has a number in nano seconds that can be changed by rotating a multifunction knob. I don't really understand this one. And finally is invert on/off. Oh yes, there is another key that brings back the first page.

When I was exposed to my first DSO I was able to puzzle it out with only a little help from the manual. If you find yourself employed by a research lab that is part of a large university or large company it is a virtual certainty that you will find a DSO on your workbench. The only places you are likely to find an analog scope is in the labs of a small college or high school in a not poor but not rich school district.

Low end DSOs really don't work very well. Given the choice I would rather have an analog scope from the end of the analog era than one of these poor excuses for a DSO. You would probably see an analog scope in a TV repair shop if they still existed, but they don't. The manufacturers have made TVs unrepairable and no one tries. Buy it, use it, and when a 2 cent resistor fails throw it away and buy a new one. That benefits the manufacturer and the big chain stores that sell the sets but does harm to the consumer and the environment.

If you prefer you can substitute a 6AQ5 for the 6V6. No component changes are necessary to change the tube. Remember that the pin numbers are different.

Oscillation.

As you can see this is nothing more than an audio amplifier with a whole lot of gain. It is useful for tracing signals through the audio section of a radio. A shielded probe should be used and a scope probe is a very good choice. Get one that has a times 1/times 10 switch on it. Of course you will have to mount a BNC connector on the tracer to make use of a scope probe.

Radio Frequencies and Intermediate Frequencies.

This probe has the advantage over the one supplied with the Heathkit T-4. It filters out low frequencies and responds only to frequencies higher than 50 kHz. In a departure from the usual quality Heath engineering the T-4 was supplied with a simple probe that didn't work very well. The manual admits this by saying that the probe should not be connected to the plate of a tube because all the operator will hear is 60 Hz hum from the power supply. Using this probe that will not be a problem.

The probe should be shielded to avoid the effects of hand capacitance but the metal shield should be covered with some kind of insulating material. It is not safe to work on a radio that has lethal voltages in it while holding a grounded probe in your hand.

Another use for the probe is to measure RF voltages. The DC output of the probe is very close to the peak to peak value of the input wave. If it is a good sine wave the RMS value of the wave is Vdc / (2 Square root of 2).

Also, do not be tempted to leave out the power transformer! Some of the radios you will be working on will be of the line connected chassis type and having two pieces of hot chassis equipment on your workbench is a catastrophe waiting to happen.

Signal Sources.

There are three general categories of signal sources (more often called signal generators). They are 1) audio frequency, 2) radio frequency, and 3) pulse generators.

Audio frequency generators come in a wide variety of makes and models. The minimum acceptable frequency range is 20 to 20,000 hertz but most technicians prefer a range of 5 hertz to 1 megahertz. Generators are available which provide low distortion sine waves (.05% or less). The most familiar, however, is the function generator which provides sine, square and triangular waves. The distortion of the sine wave is usually in the range of 0.3% to 1%. A typical frequency range is from 1 hertz to 2 megahertz. The Thevenin output resistance of these generators is usually 50 ohms. Some function generators also provide outputs which are compatible with digital logic circuits. For details on operating procedure consult the instruction manual for the particular make and model you are using.

Radio frequency generators are further subdivided by the part of the radio spectrum they cover. For example a Hewlett-Packard model 606A is called an HF (high frequency) signal generator. It covers the range 50 kHz to 65 MHz. Granted, the HF spectrum is from 3 to 30 MHz but they didn't want to call it a "most of LF, MF, HF and part of VHF signal generator". As you can see from this example there is considerable overlap of AF (audio frequency) and RF (radio frequency) generators.

The one feature which distinguishes an RF generator from an AF generator is calibrated output voltage and power level. All RF generators worthy of the name use a combination of an output level meter and a well-calibrated attenuator to provide an output level which is known. The level is measured across a 50 ohm load and usually ranges from 0.1 microvolts to about 1 volt. Upper limits vary from one model to the next and can range from 0.2 volts to 3 volts.

Some high-priced AF generators may also have calibrated output levels, the minimum level usually about 1 millivolt. Only an RF generator will have a minimum output of 0.1 microvolts.

Pulse generators are used to stimulate logic circuits. They have outputs which are compatible with various integrated logic families such as TTL (transistor transistor logic), CMOS (complementary metal oxide semiconductor), ECL (emitter coupled logic) and others.

The pulse width and frequency are independently adjustable. There are frequently provisions for external triggering or triggering from the operation of a pushbutton switch.

Logic Probe.

If the logic probe is testing a pulse train of say 20 PPS (pulses per second) or higher, the lights will flash off and on too fast for the eye to perceive as flashes and they will look as if they are on continuously. A good logic probe will contain circuits which force the lights to flash slowly enough to be perceived as flashes.

Thus you have an unambiguous indication of logic low, logic high, open circuit or pulse train.

A logic probe must be compatible with the logic family with which it is being used.

Continuity Tester.

Many logic probes incorporate a continuity test mode.

Continuity testers are good for testing fuses, switches, line cords and even diodes. Although they are employed primarily by technicians who work on power or telephone circuits they can be a handy tool for the electronics technician as well.

Test Lamp.

In its most common form a test lamp is a neon lamp with a current limiting resistor in series with it. It may be housed in a small plastic cylinder and be equipped with a pocket clip. It will have two test leads emerging from it. The test lamp is used to check for the presence of AC line voltage.

You can also make your own test lamp. A six volt lamp makes an excellent quick tester for the presence of five volt power in a computer circuit.

Clamp On AC Ammeter.

These meters have a spring loaded clamp which can be opened by squeezing a handle on the meter. The clamp is opened and allowed to close around, not on, the wire in which the current is to be measured.

The clamp must be placed around one wire. If you put the clamp around a lamp cord (two wires) the meter will read zero. The reason for this is each wire in the lamp cord is carrying current in the opposite direction to the other one. Because this is a series circuit, the current in each wire has the same magnitude. The two equal and opposite currents cancel each other out and there is no reading on the meter.

These meters are often called "amprobes" but Amprobe is a registered trademark.

Non Contact Voltage indicators.

If you have encountered a familiar piece of equipment such as an oscilloscope, you may feel as if you know how to use a scope and don't need the manual. But if it is one of those new digital models (with no knobs, just push buttons) you will likely be sufficiently stumped to need some help.

If the equipment is unfamiliar you should by all means "read the book". There are only a few combinations of control settings which will work and an infinite number which will not work. The odds of hitting a working combination by random turning of knobs are vanishingly small.

Don't be ashamed to "read the book". There's no shame in that. No one was born all knowing.