Digital Readout Bridge:
The tremendous increase in the use of digital circuitry has had a marked effect on electronic test instruments. The early use of digital circuits in bridges was to provide a Digital Readout Bridge. The actual measuring circuitry of the bridge remained the same, but operator error in observing the reading was eliminated. The block diagram for a Wheatstone bridge with Digital Readout Bridge is shown in Fig. 11.16. Note that a logic circuit is used to provide a signal to R3, sense the null, and provide a Digital Readout Bridge representing the value of Rx.
Microprocessor Controlled Bridge:
Digital computers have been used in conjunction with test systems, bridges, and process controllers for several years. In these applications, computers were used to give instructions and perform operations on the data measured. When microprocessors were first developed they were used in much the same way as digital computers. However, real improvements in performance occurred when the microprocessor was truly integrated into the instrument. With this accomplished, microprocessors cannot only give instructions about measurement, but also they can change the way the measurements are taken. This innovation has given rise to a whole new class of instruments, called Intelligent Instruments.
The complexity and cost of making analog measurements can be reduced using a microprocessor. This reduction of analog circuitry is important, even if additional digital circuitry must be added, because precision analog components are expensive. Also, adjusting, testing and troubleshooting analog circuits is time consuming and often expensive. Digital circuits can often replace analog circuits because various functions can be done either way.
The following are some of the ways in which microprocessors are reducing the cost and complexity of analog measurements.
- Replacing sequential control logic with stored control programs.
- Eliminating some auxiliary equipment by handling interfacing, programming and other system functions.
- Providing greater flexibility in the selection of measurement circuits, thereby making it possible to measure one parameter and calculate another parameter of interest.
- Reducing accuracy requirements by storing and applying correction
Instruments in which microprocessors are an integral part can take the results of a measurement that is easiest to make in a given circuit, then calculate and display the value of some other desired parameter, which may be much more difficult to measure directly.
For example, conventional counters can measure the period of a low frequency waveform. This is then converted to frequency either manually, or using extensive circuitry. On the other hand, such calculations are done very easily by a microprocessor. Measurements of resistance and conductance, which are reciprocals of each other offer another example. Some hybrid digital/ analog bridges are designed to measure conductance by measuring current. This measurement is then converted to a resistance value by rather elaborate circuitry. With a microprocessor based instrument, a resistance value is easily obtained from the conductance measurement.
Many other similar examples could be presented. However, the important thing to remember is that the microprocessor is an integral part of the measuring instrument. This results in an intelligent instrument that allows us to choose the easiest method of measurement and requires only one measurement circuit to obtain various results. Specifically, one quantity can be measured in terms of another, or several others with completely different dimensions, and the desired results calculated with the microprocessor.
(One such microprocessor-based instrument is the General Radio model 1658RLC digibridge.)
Such intelligent instruments represent a new era in impedance measuring instruments. The following are some features of these instruments.
- Automatically measures R, inductance L, capacitance C, dissipation factor D and storage factors for inductors
- 1% basic accuracy
- Series or parallel measurement mode
- No calibration required
- Ten bins for component sorting/binning (equivalent, binary number)
- Three test speeds
- Three types of display-programmed bin limits, measured values or bin number.
Most of these features are available because of the use of a microprocessor, e.g. the component sorting/binning feature is achieved by programming the microprocessor.
When using the instrument in this mode, bins are assigned a tolerance range. When a component is measured, a digital readout (bin number) indicating the proper bin for that component is displayed on the keyboard control panel.
Impedances at AF or RF are commonly determined by means of an ac Wheatstone bridge. The diagram of an ac bridge is given in Fig. 11.17. This bridge is similar to a dc bridge, except that the bridge arms are impedances. The bridge is excited by an ac source rather than dc and the galvanometer is replaced by a detector, such as a pair of headphones, for detecting ac. When the bridge is balanced,
where Z1, Z2, Z3 and Z4 are the impedances of the arms, and are vector complex quantities that possess phase angles. It is thus necessary to adjust both the magnitude and phase angles of the impedance arms to achieve balance, i.e. the bridge must be balanced for both the reactance and the resistive component.