SPECIAL-PURPOSE PROBES [abc's of ELECTRONIC TEST PROBES (1966)]

Signal tracing and substitution offer two valuable methods for servicing electronic equipment. They are often quicker than voltage and resistance measurements or parts substitution, although not always more advantageous. The method to use depends on the complexity of the circuit and the degree of difficulty we experience. Signal tracing or injection are normally employed when the circuit, although not completely inoperative, is malfunctioning to such a degree that the difficulty cannot easily be found with the classic measuring methods . Even though voltages and resistances may be of the correct value, or within the limits specified in the applicable service literature, the circuit still may exhibit the wrong waveshape, or its frequency may not be correct. Some circuits may develop difficulties only when a signal is applied to them. A signal-injector probe duplicates this condition by substituting a signal .

SIGNAL-TRACER PROBES

A signal tracer is one of the simplest yet most effective instruments for rapid and accurate troubleshooting of electronic circuits. Its biggest advantage is that it permits the circuit to be checked under dynamic operating conditions. The technique of signal tracing is relatively easy and straightforward. A test signal, or one from a transmitting station, is applied to the circuit. Then the signal tracer is moved progressively from the input to the output of the equipment under test. At the same time, the test signal is checked to see if it is present, and if so, whether it has been amplified or reduced (or perhaps distorted) . An inoperative or maladjusted stage can thus be quickly localized and then other static measurements made, to further localize within that stage the defective component or components . The most popular tracer is the easy-to-operate untuned type, which has no controls except perhaps one for volume.

The probe required for a particular signal-tracing job depends on the type of signal, which in turn depends on the circuit under observation and whether the equipment under test can supply its own signal . (Before we can trace a signal, we must obviously have one to begin with!) The frequency and nature of the signal to be measured are also important. What kind of signal is it-audio frequency, low or high radio frequency, modulated, pulse, or what? A television station signal is usually traced with a semiconductor demodulator probe . The probe output is applied to the vertical-input circuit of an oscilloscope. If the television signal is weak, the display on the oscilloscope may be too small to be of any value . The thing to do here is substitute another signal for the one from the television station, or else use a probe which amplifies the signal before applying it to the oscilloscope.

A signal-tracer probe (as well as its output cable) must be shielded so it will not pick up extraneous signals or hash from stray fields and thus mask out our signal.

Our choice of a demodulator probe is also governed by the output desired. Do we want maximum output? Or will we be satisfied with a good match and the least loading? In a demodulator probe, sensitivity must be sacrificed for fidelity. For signal-tracing purposes, however, we are ordinarily more than willing to do the opposite-particularly in low-level stages like those in the r-f, mixer, and first i-f stages of a television receiver. For this reason, semiconductor probes intended for signal tracing usually compromise between providing the highest possible output while maintaining reasonable fidelity, so they will be suitable for observing video-amplifier output signals and the waveshapes in television sweep and sync circuits.

The classic approach to signal tracing involves simply moving the probe from stage to stage, starting at the front and moving toward the output of higher-gain stages, while noting the increase in signal at each successive stage. Initially, it is advisable to go from plate to plate, rather than from grid to plate or plate to grid, because the impedances are usually lower in the plate than in the grid circuits. Thus, the test probe does not affect the plate circuits as severely as it does the grid circuits . The ratio between the signal at the output of a stage to the one at the output of the preceding stage is the gain of the stage being measured.

A sweep signal can also be traced through the i-f-amplifier stages. However, the resultant pattern does not usually represent the true response of the stage or stages under test. The reason is that the loading effect of the probe alters the response characteristics of the circuit.

This loading effect can be reduced considerably by using a low-capacitance or cathode-follower probe ahead of the signal-tracer probe, or by making the plate load of the last tube nonresonant. The latter is done by shunting (swamping) the load with a resistor of a few hundred ohms.

Some signal-tracer probes are so sensitive that, when held close to a tube pin or on top of an insulated wire carrying a signal, they pick up enough signal by capacitive coupling to give a suitable indication. This type of signal pickup gives considerably less circuit loading than some others.

The main purpose in signal tracing is not to show the exact waveform or faithfully reproduce the signals at a particular point of a circuit. Rather, it is to show the presence or absence, or the strength or weakness, of a signal at that point.

Signal tracing from stage to stage by means of an oscilloscope affords a rapid and convenient method of locating a defective circuit.

Many circuits (such as sweep circuits) generate their own waveforms ; therefore, no external signal is needed. On the other hand, audio and video amplifiers and sync circuits do require an external signal, from either a broadcasting station or an audio generator or oscillator.

Aside from showing the approximate gain in each stage, a signal tracer will also meet the challenge of a dead receiver in which all voltages seem normal. Starting at the antenna, we work from plate to grid to plate, etc., until we reach the point where we completely lose the signal. The trouble is between this point and the one where the signal was last encountered.

A signal-tracer probe will help us locate an intermittent. As we trace the signal, we tap the chassis or tube gently (preferably with the eraser end of a pencil) until we find the intermittent. A signal tracer will also tell us whether or not there is a signal . As we go toward the output of an audio amplifier, we will notice a substantial increase in the signal. However, going from the primary to the secondary of the output transformer, we will observe just the opposite. Because we are moving from a high-impedance, high-voltage primary to a low-impedance, high-power secondary, we will obviously get a reduction in voltage. After all, a signal tracer indicates only voltage-not audio power.

Fig. 6-1. Signal tracing an r-f converter.

A demodulator probe and an oscilloscope can be used to trace a modulated r-f signal-from the antenna, all the way through the detector stages-at frequencies of up to several hundred megacycles.

Tune the receiver to some frequency within its range and feed a fixed signal (from a signal generator) into the antenna terminals. (The signal from a transmitting station may be used, but one from a signal generator is preferred because it is steady and can be kept under control at all times.)

Fig. 6-2. An a-f/r-f signal tracer probe giving both visual and aural indication.

Fig. 6-3. An a-f/r-f signal tracer for use with headphones or a-f amplifier. The r-f signals must be modulated.

Fig. 6-4. Schematic of RCA WG-302A signal-tracing probe.

An attempt to evaluate gain may sometimes lead to confusion because the capacitive loading effect of the probe may detune the circuit.

For example, in Fig. 6-1 the tracer probe (connected to an oscilloscope) is shown checking either side of the coupling capacitor, between the r-f amplifier and converter stages. (This capacitor usually has a value of between 5 and 10 pf.) At the plate of the r-f amplifier we may get a deflection of, say, ten units on our oscilloscope. However, at the grid of the converter stage, we may get a deflection of only six or seven units. This is rather confusing, to say the least.

There is nothing wrong with the circuit--it is just the additional capacitance of the signal-tracer probe that is giving us trouble. The probe input capacitance is somewhere around 10 pf. When applied at the grid of the converter tube, this additional capacitance forms a voltage divider with the coupling capacitor. The full voltage at the plate of the converter is now divided between the input capacitance of the detector probe. We read on the scope only the voltage across the detector probe. Therefore, this voltage will be lower than the one that would normally reach the grid of the converter tube if there were no additional shunt capacitance from the signal-tracer probe.

Fig. 6-2 shows a semiconductor-diode signal tracer which requires no external power supply and provides audio output as well as a meter indication. This instrument can be used for troubleshooting the r-f, detector, oscillator, i-f, and audio stages of the receiver, as well as in audio amplifiers. A 1N54 high-efficiency germanium crystal diode improves the performance . The 0.01-mfd r-f bypass capacitor at the input protects the diode, headphones, and meter from any DC voltage in the circuit under test, but passes audio and r-f signals. The 200,000 ohm gain control in the meter circuit allows the meter movement to be adjusted for a suitable deflection. The stronger the signal, the more the resistance inserted into the circuit, and vice versa. Either the meter or the earphones can be plugged into the jack, depending on whether a visible or aural indication is desired. Note that the signal must be a modulated r-f or an audio one before an aural indication can be obtained. However, an indication will be obtained whether the r-f signal is modulated or not, because the meter responds to the DC level of the rectified signal.

Another simple signal-tracing probe is shown in Fig. 6-3. It also is an r-f/a-f type of probe, except that the DC component of the r-f signal is blocked by the 0.1-mfd capacitor. The audio component can then be applied to headphones or to an amplifier or scope. The probe should be shielded to prevent pickup of extraneous signals. When used with headphones, this setup is known as a radio stethoscope.

An interesting r-f/i-f (video-frequency) signal-tracer probe is the RCA Type WG-302A. Its schematic is shown in Fig. 6-4. This is a slip-on probe used with the WG-300B direct-low-capacitance probe.

Fig. 6-5. Diagram of a high-frequency signal-tracer probe voltage-doubler type.

The circuit of the WG-300B was shown in Fig. 1-5. The signal-tracer probe in Fig. 6-4 contains a semiconductor diode and an r-f filter housed in a plastic case. When using it with the WG-300B, set the switch on the latter to the Direct position. The time constant of the rectifier circuit is such that when the slip-on probe is used in high-frequency circuits, the low-frequency modulation is separated from the amplitude-modulated r-f carrier and fed to the oscilloscope input through the direct probe . The waveform is centered vertically on the zero axis of the screen when an AC RC-coupled oscilloscope is used.

On a direct-coupled oscilloscope, the waveform is displayed vertically, the distance being proportional to the DC voltage resulting from rectification of the r-f carrier.

When this signal-tracing probe is used with an oscilloscope, and a sweep generator is employed to sweep the picture or sound-i-f amplifier of a television receiver, it is possible to observe the response curves of tuners and of picture and sound i-f and video amplifiers, plus the overall response curves in all high-frequency sections of the television receiver, without upsetting the performance of the high-frequency stages.

The low (3-pf) input capacitance permits the probe to be used in such critical circuits without seriously detuning the amplifiers. Because its capacitance is lower than that of the kinescope grid circuit, the probe can also be connected to the video-amplifier output without affecting the circuit.

Fig. 6-6. Transistor amplifier for signal tracing probe.

The probe extends the range of an oscilloscope to 50 mhz-enough to cover the i-f and video-frequency sections of television receivers . A three-inch ground lead, connected between the probe and the low side of the circuit under test, will extend the usable range to 250 mhz for signal tracing in tuners. To add the ground lead, remove the nylon screw in the body of the case ; then install the ground lead here, except use a metal screw and insulate the screw head.

Fig. 6-7. Single transistor demodulator signal tracer probe.

The WG-302A r-f/i-f/v-f signal-tracing probe is an indicating device rather than a voltage-measuring instrument. For voltage measurements, the probe and oscilloscope should be calibrated against a known voltage.

The high-frequency voltage-doubler probe in Fig. 6-5 is designed for signal tracing in television, video and i-f circuits . The semiconductor diodes are so connected that positive-going sync pulses are provided to the input circuit of the oscilloscope . The low internal capacitance of the series diode (D2) , being effectively in series with the cable and scope capacitances, isolates the latter from the input circuit. Hence, the input capacitance is kept low, without the need for a series isolating resistor.

The input signal capacitor of only 10 pf further reduces the input capacitance to an absolute minimum. For this reason, the probe has excellent 60- and 120- hz rejection, and can therefore be used to trace r-f interference in B+ and filament circuits . A signal here would constitute an undesirable voltage, which could be cross-coupled between circuits and thereby cause the receiver to operate improperly.

The amplitude of the output signal is about twice that of a single diode probe.

Sometimes a signal-tracer probe does not deliver sufficient signal for a readable deflection on a vtvm or oscilloscope . If not, a single-transistor amplifier stage inside the probe will add sufficient gain to permit r-f measurements directly at the television tuner. Figs. 6-6 and 6-7 show these circuits. There may be a slight deflection in the meter circuit even with no signal applied, due to leakage within the transistor.

Select the transistor that gives the least initial meter deflection. The one in this circuit is a pnp. If an npn transistor is used, the battery voltage and probe cable terminals must be reversed.

Fig. 6-7 shows the demodulator-tracer-amplifier all in one . The unit, together with its battery, should be encased in a plastic tube and connected to the oscilloscope through a shielded cable . The output may also be connected to a pair of headphones. The biggest advantage of the battery-operated signal tracer is that no transformer and no isolation from the line are required. Both amplifiers have a gain of 10.

Fig. 6-8. An a-f signal-tracer probe.

Fig. 6-9. An a-f signal tracer probe with volume control.

Just about the simplest signal tracer imaginable is an r-f signal tracer consisting of a pair of high-impedance earphones connected to a blocking capacitor encased in a probe. The capacitor prevents the DC points from short-circuiting through the earphones. We simply listen to the audio signal at various test points in the amplifier, and then judge its quality and volume by ear. Such a probe (Fig. 6-8) consists simply of a 0.1 -mfd blocking capacitor mounted near the tip of a probe. Probe housings are available, making it a simple matter to insert the capacitor into a probe and solder it to the tip. If the probe is plastic, it should be shielded to isolate the circuit under test from hum pickup and body capacitance. This is done by lining the inside of the probe with metal foil. The shield must be connected to the braid of the flexible coaxial cable coming from the headphones.

This elementary signal tracer is used frequently for making rapid analyses in audio circuits. However, it also has some disadvantages.

It tends to load the circuit because of the relatively low impedance (2000 ohms) of the earphones and the negligible reactance of the capacitor at audio frequencies. Furthermore, the human ear is insensitive to any small changes in volume, or even to relatively large amounts of distortion. Nor is there any provision for adjusting the volume. So, as we approach the high-level output stages of an amplifier, the signal grows louder and louder in the earphones, until it becomes quite uncomfortable to the listener. Crystal earphones, the impedance of which is close to 100,000 ohms in the audio range, will not load the circuits under test as much as the high-resistance magnetic earphones.

The somewhat advanced version in Fig. 6-9 has a volume control and earphone jack, usually mounted in a small metal box. Crystal earphones are used to minimize circuit loading. If desired, the output signal can be fed to an oscilloscope or AC vtvm, or to an amplifier that has its output connected to a speaker or meter.

For the sake of operating simplicity, most signal tracers have no tuned input circuits. Instead, the output from a simple demodulator probe is fed into one or more audio-amplifier stages. Although adequate for most work, such a tracer sometimes is not sensitive or selective enough. Amplifiers can be used after the probe, but because of broadband response of the probe, plus the fact that there is no frequency selectivity, the amount of gain is rather limited. A tuned signal tracer overcomes some of these disadvantages. It is many times more sensitive (and somewhat more complex) than an untuned tracer.

With it we can narrow our response down to the frequency or band of frequencies we are interested in, and thus exclude all other signals.

Such a tracer must be resonated at the frequencies being measured.

The tuned circuit need not be in the probe. (For this reason the probe, which sometimes contains a vacuum tube, can be made rather compact.) The output from the probe is fed into the instrument, which contains several tuned circuits. Here, it is demodulated and then fed into a conventional audio amplifier.

Fig. 6-10. Capacitance type signal tracer probe and equivalent circuit.

A tunable signal tracer is appropriately called a channel analyzer.

It can be distinguished from the simpler tracers discussed so far by the fact that it is tuned to the frequency (intermediate frequency or radio frequency) at which the measurements are made. Any possibility of a spurious indication from signals, other than those we are interested in, is thus eliminated. Because this signal is often taken across tuned circuits, the capacitive loading effect of the probe must be small enough not to detune the circuit under test. Such a probe, together with its equivalent circuit, is shown in Fig. 6-10. The input or coupling capacitor is a 1- or 2-pf miniature ceramic mounted as close as possible to the tip and inside a shielded test probe. This arrangement is much like the one discussed in the section on capacitance-divider type high-voltage probes, except the input capacitor is not a high-voltage type because the probe is not designed for high-voltage circuits. However, the shunt capacitance from the input circuit of the signal tracer, as well as the capacitance of the shielded cable used with the probe, must once again be taken into account.

Fig. 6-11. Tuned signal-tracer.

Fig. 6-11 shows a simple but quite useful tuned signal tracer. Plug-in coils of different values can be constructed to make this tracer tunable from 400 kilocycles to 30 megacycles in four ranges. The input stage consists of a single tuned circuit and a 1N34 germanium diode detector. This is essentially a crystal receiver. A 2-pf capacitor, encased in a shielded probe and connected through a shielded cable, connects the probe to the tuned circuit under test. This one is like the shielded test probe discussed before. Proper impedance match is obtained by connecting the probe to a tap on the coil. The detector output is fed to a high-gain audio amplifier terminated by an output meter or other indicator--or if required, by a speaker. If ultimate sensitivity is not required, a pair of earphones can be substituted for the 5000-ohm resistor and high-gain audio amplifier. Because of the fact that the signal tracer is tuned, its sensitivity will be greater than if it were not tuned to the circuit.

Tuning is accomplished by two parallel-connected 365-pf variable capacitors. The tuning range includes all the i-f, r-f, and oscillator frequencies ordinarily encountered in broadcast and short-wave receivers. The coil can be a simply-wound plug-in type to simplify range changing. If desired, a rotary selector switch can be added for greater convenience in changing from band to band. A dial and knob should be connected to the tuning capacitor, and a calibrated dial constructed. The dial can be graduated in kilocycles and megacycles by feeding a modulated signal at integral frequencies from a signal generator to the tracer. In addition to acting as a tunable signal tracer, the instrument can also be used for checking r-f oscillators and signal generators, transmitters, and carrier-controlled equipment.

We know that tuned signal-tracing probes select only the signal desired and exclude all undesired ones. Thus, before we can trace a buzz pulse through a video- or video-i-f amplifier, we must exclude the video signal .

(A) For 4.5- mhz probe. (B) Adjustable to receiver intermediate frequency.

Fig. 6-12. Tuned heads for signal-tracer probes.

Any of the untuned signal-tracer probes discussed can be equipped with a tuned head to make the probe suitable for tracing buzz pulses in intercarrier television receivers. Fig. 6-12 gives specifications for two probe heads . One is tuned to 4.5 megacycles; the other can be tuned to the i-f amplifier frequency of television receivers. The inductance in Fig. 6- 12A is one of the video-peaking coils in the television receiver.

The coil (L1), together with a small trimmer capacitor, should be made to resonate at 4.5 megacycles so it will respond to the 4.5- mhz audio i-f signal , but not to the video signal . The probe coil is held near the peaking coils in the video amplifier. Energy is thus picked up by inductive coupling. The demodulated output signal from the probe can be observed on an oscilloscope or listened to through headphones or an audio amplifier.

The circuit in Fig. 6-12B shows a coil and capacitor resonated to the i-f amplifier frequency. They are also coupled inductively to the i-f coil of the receiver, and the output observed in the same way. Both coils pick up the desired signal , but eliminate all video information (which would completely mask the buzz waveform). Sometimes it is impractical to use inductive coupling because the coils are shielded or the receiver is so crowded the probe cannot be brought close enough to pick up sufficient signal . Instead, capacitive coupling (through a 1- or 2-pf capacitor) can be used between the probe and the "hot" end of the coil being tested.

Fig. 6-13. Signal-tracing probe using grid leak detection.

Fig. 6-13 shows a very sensitive r-f signal-tracing probe. This grid-leak detector probe not only detects a very weak signal but-unlike semiconductor or other vacuum-tube types-amplifies it, too. The resistor and capacitor in the grid circuit form the grid leak and thus provide demodulation. Such a probe is useful for signal tracing in the front ends of radio and television receivers, where the r-f signal voltage is usually in the microvolt range . The probe is so sensitive it can pick up a signal at the antenna of a receiver. In fact, it will give an indication when held in the hand or applied to a short wire! The grid-leak detector operates as a combination diode-detector triode-amplifier.

In effect, the grid and cathode of the tube act as the plate and cathode of the diode detector. The grid, plate, and cathode then operate as a high-gain triode amplifier.

The values are typical: the input capacitor is usually between 10 and 100 pf, and the grid-leak resistor, anywhere from 5 to 25 megohms.

The capacitor in the plate circuit bypasses any r-f signal that may have passed through the tube . If too large, this capacitor will also bypass the audio signals we are interested in. It should be no more than 100 pf or so. The plate resistor is the load, across which is developed the output signal representing the demodulation envelope of any amplitude-modulated signal. This output is then applied to an audio amplifier. Both demodulation and amplification are supplied. The probe is so sensitive it is easily overloaded. Therefore, it is suitable for relatively low signals only. High fidelity also is not one of its advantages. Filament and plate voltages are required; they are usually supplied by the amplifier.

Because of this sensitivity, the grid-lead detector is ideal for tracking down the source of hum, which is sometimes rather difficult to do.

We can locate hum by tracing the signal at each grid and plate, moving toward the output stage until we reach the point where the hum increases markedly. Here is the villain! It may be emanating from a defective tube, an open bypass capacitor, or any other hum-producing element.

The probe can also be used for checking screen and cathode bypass capacitors. If we find a signal at a screen or bypassed cathode, we know immediately that the bypass capacitor is either open or too low in value.

An unusually small and effective signal tracer is the fountain-pen size Stethotracer shown in Figure 6-14. The instrument is transistorized and completely self-contained. It operates from a single AAA size, 1.5 volt cell and is completely insulated from the AC power line, thereby eliminating such hum problems as might exist with line operated tracers. The Stethotracer has an audio gain of approximately 1000 (60 db) at 1 khz and is supplied with attenuator probes for operation in higher-level circuits. An interchangeable RF crystal detector head for use in the r-f and i-f regions extends the useful frequency

Fig. 6-14. Stethotracer with earphones and three interchangeable attenuator probes.

HUM-FIELD TRACING PROBE

The probe in Fig. 6- 15 is useful for exploring hum fields and checking hum currents in audio, radio, and television equipment . It is connected directly, or through an AC voltage calibrator, to the vertical-

Fig. 6-15. Hum-field tracing probe.

amplifier input terminals of an oscilloscope . The oscilloscope gain must be reduced so no or negligible residual hum( due to stray pickup in the room) will be displayed on the screen. When the probe is placed in a hum field, the increase in vertical amplitude will then be proportional to the field strength.

The hum probe can be used for determining the best orientation of transformers, chokes, and leads carrying alternating current. It can also be used to check hum currents in chassis, by pressing its nose to the chassis and observing the amplitude of the hum pattern on the oscilloscope screen range to 200 Mhz. The instrument will provide a maximum output voltage of 0.5 volts peak-to-peak and can be used with an oscilloscope or a vtvm as a preamplifier to extend their sensitivity.

SIGNAL-INJECTOR PROBES

Unlike the signal tracer, the signal-injector probe works from the output to the input. For example, in a superheterodyne receiver we inject a signal at the output stage ; then we work toward the power amplifier, the first amplifier, the second detector, the i-f amplifier, the mixer, and the r-f amplifier, until we reach the antenna.

Fig. 6-16. Schematic of a signal injector probe using a 12AX7 tube.

A signal-injector probe furnishes its own signal; thus, it does not have to depend on an external one. Some sort of output indicator is needed, such as a speaker, amplifier, or radio. As long as the circuit is operating, we will hear the output as we move the probe toward the front. But the moment we pass the dead stage, the output will be lost completely (or will drop if the stage is defective but not dead) . Most signal-injector probes are considered to be noise generators.

They are usually vacuum-tube or transistor multivibrators, or blocking oscillators, operating at a fundamental frequency of around two to ten kilocycles . The output signal is a rather rough square wave which is rich in harmonics. Thus, it can be applied to both audio and r-f circuits of up to several megacycles. Adjustments or a change of probes is not needed. The very broad-band signal is, therefore, suitable for all types of circuits.

Fig. 6-16 shows the schematic of an easily constructed injector probe. A miniature 12AX7 dual-triode is used in the multivibrator circuit, which operates at approximately 10,000 hz. The available signal is coupled to the probe tip through a 0.0022-mfd capacitor. A four-conductor cable (it does not have to be shielded) should be used to connect the power source to the probe.

Fig. 6-17 shows a signal-injector probe using two transistors, also in a multivibrator circuit. This probe and the one just discussed can be put together very easily. The vacuum-tube probe requires an outside voltage source and must therefore be connected to a filament and plate supply. The transistor probe can be made completely self-contained.

Fig. 6-18. The "Mosquito" transistorized signal-injector probe sliding the pocket clip forward. The signal can be heard in the speaker or observed on an oscilloscope.

Fig. 6-17. Diagram of a transistorized signal-injector probe.

For this reason, it has the tremendous advantage of being portable-it can be applied anywhere and at any time, because no interconnecting wires are needed.

An interesting signal-injector probe-called the "Mosquito" because the signal sounds like a mosquito in flight-is shown in Fig. 6-18 . Fully transistorized and powered by a single penlight cell, it houses a transistor oscillator operating at about two kilocycles. Its waveform is square with a sharp spike-very rich in harmonics extending to the i-f and r-f ranges. This probe is turned on by simply

Signal-injector probes can be inductively coupled to all magnetic sensitive circuits and pickups. No direct connection is needed.

CLAMP-ON AND CLIP-ON PROBES

Usually when we want to measure current, we must break into the circuit and insert the meter connection. Not only is this inconvenient, but in some circuits, particularly those containing transistors, the resistance of the moving-coil instrument is often so high (compared with the resistance in the circuit) that it can become intolerable. An ideal current-measuring instrument would have zero series impedance and would therefore present no reactive loading to the circuit under test.

Courtesy Hewlett-Packard Co.

Fig. 6-19. Clip-on probe used for direct-current measurements.

The clip-on DC milliammeter probe in Fig. 6-19, together with the instrument for which it is designed, makes the measurement of direct current in low- and high-impedance circuits very simple and convenient. The DC current range covered extends from about 0.3 milliampere to 1 ampere. The fact that the probe introduces no DC loading is a particularly valuable property when currents are measured in low-impedance transistor circuits, because this can be done without disturbing any operating conditions. The jaws of the probe open for clipping around the conductor, and only a half-inch conductor is necessary for a correct current reading.

The probe senses the strength of the magnetic field produced by the current under observation. This sensing requires no energy from the field ; therefore, the probe introduces no resistance into the circuit being measured-a most desirable situation. Since it is a direct-current probe, the direction of the current can be determined because the probe is marked with an arrow which shows the current direction for an upscale reading. The probe itself contains a magnetic amplifier which provides an AC output signal proportional to the magnetizing force produced by the direct current being measured. This AC output signal is somewhere around 0.01 volt peak at a frequency of 40 kilocycles. It is amplified in the meter and then applied to a phase-sensitive detector, the output of which feeds the indicating meter.

Fig. 6-20. Clip-on probe showing method of increasing pickup sensitivity.

Fig. 6-20 shows the probe with a conductor looped through it several times. This is done to increase the effective magnetizing force of the current and thus to make the instrument more sensitive. The readings obtained under these conditions must be divided by the number of loops in the conductor.

Courtesy Stoddart Aircraft Radio Co., Inc.

Fig. 6-21 . Clip-on probe and amplifier for use with an oscilloscope.

Fig. 6-21 shows a similar-looking clip-on probe designed for use with an oscilloscope, together with an appropriate amplifier. It will display current over an amplitude ranging from 1 milliampere to 15 amperes peak-to-peak and covering a frequency range from below 50 hz to 8 megacycles . This probe consists of a wide-range current transformer with a split core which is again clamped over the wire carrying the current which we want to observe. The basic schematic of this probe is shown in Fig. 6-22. We see that the current-carrying conductor is, in effect, a single-turn primary for the transformer. The probe output is fed to the amplifier, the output of which is connected to the oscilloscope . The amplifier converts a sample of the current which is induced in the probe to a proportionate voltage with a current-to-voltage conversion factor of one millivolt per milliampere . This results in a convenient one-to-one relationship so that a one-volt output from the amplifier indicates that one ampere of current is flowing in the circuit under test. The frequency response of the amplifier is ±3 db from 25 hz to over 20 mhz. As a result of applying the probe, the circuit under test will see an additional impedance of less than 50 milliohm in series with an inductance of about 0.05 micro-henry . This is approximately the inductance of one and one-half inches of hookup wire. Only one-half inch of wire and sufficient room to clamp on the probe are required for a measurement.

Fig. 6-22. Circuit representation of probe and amplifier in typical setup.

Fig. 6-23. Clamp-on r-f current probe.

The ferrite core used in the probe has magnetic properties which make it suitable over such a wide frequency range. Magnetic (as well as electrostatic) shielding is incorporated to minimize response of the probe to fields other than those of the current being measured.

Direct current up to a half-ampere or higher will not have any noticeable effect on any measurement. The sensitivity of this probe can also be increased, as we did with the DC probe, by increasing the number of turns which act as the primary. The increase in sensitivity will be directly proportional to the number of turns.

This probe can be modified somewhat to act as a magnetic search probe and indicate the direction and magnitude of AC magnetic fields.

This is done by placing the probe around a single shorted-turn coil. The magnetic fields will induce, in the coil, eddy currents which will in turn be indicated by the probe and displayed on an oscilloscope. We can thus observe the direction and strength of the AC field.

The clamp-on current-measuring probe affords the maximum in flexibility and ease of operation. Fig. 6-23 shows a clamp-on r-f current probe. It is used for measuring radio interference in order that the intensity of the r-f current in an electrical conductor or group of conductors can be accurately determined. Here, again, the conductor under test acts as a one-turn primary winding. The unique mechanical design of the probe permits it to be used around any insulated cable up to 1% inches in diameter. This probe is suitable for use over a frequency range extending from 14 khz to 100 mhz and is capable of measurements in circuits where the r-f current may be as high as 1 ampere. Essentially, this probe is a radio-frequency toroidal transformer designed to deliver voltage through a 50-ohm coaxial cable to any receiver having an input impedance of 50 ohms and covering the frequency range at which we are making our measurements.

Fig. 6-24. Snap-on high-current probe (tong ammeter) and adapter for extending current range.

A snap-on high-current measuring probe (also called a tong ammeter) for appliance and electrical testing is shown in Fig. 6-24. It, too, operates on the transformer principle, where the wire carrying the current acts as one turn on the primary of a current transformer.

An accessory, available for use with this probe, extends the current range by a factor of ten. This adapter, called a Deca-Tran, is shown together with the probe. Here, again, the sensitivity of the probe can be increased by wrapping two or more turns around the clamp.