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~ OSCILLOSCOPES 14.55 FIGURE 14.58 approximately 50 pf. This much capacitance will seriously affect the operation of many circuits and attenuate fast transients; hence this probe configuration is seldom used. For example, if Zs is a 200-Q resistor and the totalload capacitance is 60 pf, then the response is bandwidth limited to 13 MHz. The "ten-to-one (lo X or 10:1) probe" reduces the capacitive 10ad on the user's circuit at the price of a IO-fold reduction in signal amplitude (Fig. 14.59). A parallel RC circuit, Rl and Cl' built into the probe tip combines with the probe cable capacitance and the oscilloscope input resistance and capacitance to form a voltage divider. A further idealization of the divider circuit is shown in Fig. 14.60. Making Rl = 9Rz sets the dc attenuation factor at lo, and choosing Cl = FIGURE 14.59 Connecting lo the user's circuil with a 10:1 passive probe. Vs c, i FIGURE 14.60 The idealized compensated divider equivalent circuit for the 10:1 passive probe..
14.8.2 Res,,-- -- I -
~ OSCILLOSCOPES 14.57 FIGURE 14.62 Resistive-divider passive prore. Vs
14.58 CURRENT AND VOLTAGE MEASUREMENT INSTRUMENTS 14.8.4 Diffl c: o) <J c: co "'C Q) a..~... ::J a. c: 14.8.5 CUri Frequency, Hz FIGURE 14.64 Input impedance at the probe tip for high-impedance and resisti vedivider 10:1 probes. between the probe cable and the oscillo scope input, but then the dc and some low-frequency ac signal information is lost. Figure 14.64 is a graph that compares the loading characteristics of 10:1 high-impedance and 10:1 resistive-divider passive probes. The high-impedance probe has an input capacitance of 7 pf and has a lower input impedance than the resistive-divider probe at frequencies above approximately 45 MHz. 14.8.3 Active Probes Both types of 10:1 passive.probes discussed above seriously 10ad the user's circuit when highfrequency measurements afe made. An active probe offers a lower input capacitance and a high input resistance combined in the same unito It has a physically small amplifier built into the probe body very near the probe tip so that the probe input capacitance can be kept small, usual1y less than 2 pf. The amplifier output drives a 50-0 transmission line which is terminated at the oscilloscope input with a 50-0 resistor. Active probe models with different divider ratios afe available, ranging from I: 1 to 10: 1. The amplifier input is connected directly to the probe tip on al: 1 model, while units having the largest attenuation incorporate a compensated divider built into the probe tip ahead of the amplifier. The divider uses the same R1C1 = R2C2 principle used in 10: 1 high-impedance passive probes, so a model with larger attenuation will bave a correspondingly smaller input capacitance. Some active probe models provide a removable 10:1 compensated divider. Compared with passive probes, active probes afe generally larger, heavier, more expensive, and less rugged. Because the active probe uses an amplifier, the signal dynamic range is limited, and peaks above a certain amplitude, usually a few volts, will be clipped.
OSCILLOSCOPES 14.59 14.8.4 Differential Probes A "differenti al" probe is an active probe which has two inputs, ODe positive and ODe negative, and a separate ground lead, and it drives a single terminated 50-Q cable to transmit its output to ODe oscilloscope channel. The output signal is proportional to the difference between the voltages appearing at the two inputs. Both inputs can bave active signals simultaneously, but they must be within a few volts from ground to stay within the dynamic signal range of the probe. The average of the two input voltages is called the "common mode signal." A differential probe is designed to reject (not respond to) the common mode signal, but inevitably there will be a small error response. Common mode rejection capability is easily measured by connecting both inputs to the same signal simultaneously and observing the probe response. The rejection is best at dc and low frequencies and deteriorates with higher-frequency signals. Some active probe models provide a removable lo: l balanced two-input compensated divider. Tbe differenti al probe has disadvantages similar to those listed for the single-input active probe. 14.8.5 Current Probes A transducer that generates a voltage output proportional to a current in the user's circuit is called a "current probe." A resistor and a 1:1 voltage probe do that (Fig. 14.65) and they aie a usefu1 method in certain situations. However, inserting a resistor into the user's circuit has some disadvantages. Generating a large enough vo1tage drop to register adequate1y on the oscilloscope could adverse1y affect the circuit operation, and so might connecting the probe ground lead to the circuit at the point current is to be monitored. Using a differenti al probe would permit measuring current into or out of nodes which cannot be grounded. -------------- FIGURE 14.6~ Adding a resister in series with a branch in Ihe user's circuìt uit to lo measure mea""e currenl "lirr..n' Current probes afe availabie that use a transforrner to, in effect, insert a small resistor into the user's circuit (Fig. 14.66). Located in the probe tip, a transforrner having a secondary winding of ns turns drives a 50-Q probe cabie which is terrninated at the oscilloscope input with a 50-Q resistor. The current to be measured in the user's circuit is conducted through a singieturo primary winding. Since it depends on a transforrner, this method does not measure dc currents; Le., it is ac-coupied. Using the equations for an ideai transformer, it is straightforward to show that (1) i = i In" (2) the sensitivity of the probe is R. In V/A, and (3) the apparent resistance in the prismary winding is Rjln/. When the transform~;' c~rrent probe is used to sense current in a wire in the user's circuit, a resistance R. In 2 appears to be added to that wire so that the Ioading effect of the current measurement can b~ d~termined. UnfortunateIy, probe manufacturers do not always state the number of turns on the transformer secondary winding explicitly, but the terrnination resistance and sensitivity afe given, and the number of secondary turns can be easily ca1cuiated.
14.60 CURRENT AND VOLTAGE MEASUREMENT INSTRUMENTS Current probes afe also available that lise "Hall effect" or a "Hall generator" to directly sense magnetic flux intensity in the transformer core to generate a voltage signal that is amplified and fed to an oscilloscope input. This method allows measurement of dc currents but is inherently limited to measuring relatively low frequency signals. Hybrid current probes combine a transformer and a Hall generator into an integrated unit and combine their outputs to provide the best features of both types. Measurement bandwidths from dc to approximately 50 MHz afe available. In one type of transformer current probe, a short length of wire is fed through a hole in the probe body, passing through the transformer core, and the wire is then inserted into the branch of the user's circuit to be measured. Other models arrange the transformer core material into two movable segments so that the transformer can be placed around an existing wire without first disconnecting it. The transformer turns ratio, and probe sensitivity, can be changed by looping two or more turns of the current-carrying wire through the transformer. Addition or subtraction of currents in different branches of the user's circuit can be performed by linking the curients through the current probe simultaneously, but there will be some cross-coupling between the branches measured. 14.9 HOW TO BUY AN OSCILLOSCOPE -' ',' "',.. In deciding whether to buy an oscilloscope or which model to buy, the first step is to careful1y define the measurement requirements it is to satisfy. Ask and answer questions lite these: What quantities need to be measured and to what accuracy? Where will it be used (laboratory, field service, etc.)? How long will the measurement requirement exist? Will the measurement requirement become more difficult? What is the technical skillievei of the operator(s)? What is the equipment purchase budget? What afe the probing requirements? As these questions afe addressed, other significant considerations or questions may be identified. Then a list of specifications that the oscillo scope must meet can be generated, perhaps with an additionallist of desirable features. The next step is to identify and evaluate the oscillo scope models that could potentially satisfy the measurement requirements. Contact the various oscilloscope manufacturers' sales repre-