Transmission Analyzer Covers Many Applications

E1/Datacom Transmission Data Analyzer

GAO Comm launched its E1/Datacom transmission analyzer, which integrates both E1 and datacom testing functions and is intended for comprehensive analysis, measurement and evaluation of the transmission quality of telecommunications systems. Its portable design, simple operation and multifunctional features make it an invaluable tool for field use.

Test and Measurement Basics of Microphones

Microphones are familiar sensors that transform sound pressure waves into electrical signals over a broad range of frequencies and amplitudes. They are an integral part of a variety of devices including tape recorders, hearing aids, telephones, and computers. They are also used in radio and television broadcasting and audio engineering. But another specific class, perhaps not as well known, is intended for the scientific measurement of certain types of sounds and noise levels, including ultrasound. Sounds monitored for test and measurement purposes are recorded, analyzed, and typically assigned somewhat different quantitative and qualitative values than voice and music in the entertainment industry. The most common types of microphones include carbon, magnetic, piezoelectric, and condenser.

Condenser microphones have a simple construction. As sound waves vibrate the diaphragm, the spacing, and consequently the capacitance, between it and the back plate varies. The varying capacitance, in turn, generates a fluctuating electrical signal that is amplified and otherwise processed, depending on the instrumentations’ needs.

Each type is best suited for a specific application. For example, carbon microphones were first used in telephones and for radio communications. They are relatively low-cost, rugged, two-terminal devices that contain a small cartridge of lightly packed carbon granules, usually biased with current or voltage. When acoustical pressure waves hit the microphone, the carbon granules compact and relax thus modulating the terminal resistance. The continuously changing resistance then generates an output signal of varying voltage or current in step with the sound pressure waves at the microphone’s input.

Magnetic microphones are dynamic transducers that contain a moving coil, and are based on the principal of magnetic induction. Here, a coil of wire attaches to a lightweight diaphragm, which is in the presence of a magnetic field. The coil moves and generates a voltage proportional to the applied acoustical pressure.

The third type, a piezoelectric microphone, uses either a natural quartz or manmade ceramic crystal. Although these microphones have relatively low sensitivity levels, they are durable and can measure high amplitude pressures. Conversely, their noise-floor level is generally high, which make them particularly suitable for shock and explosive-type pressure measurements.

Condenser microphones come in two types, externally polarized, and pre-polarized. They transform the sound pressure waves into capacitance variations, which are then converted to electrical signals. The unit’s cartridge comprises a small thin diaphragm spatially in parallel with, but not electrically contacting a stationary metal back plate connected to a voltage source. In the presence of oscillating pressure, the diaphragm moves and changes the gap between the diaphragm and the back plate. This produces an oscillating voltage output, which is proportional to the original pressure signal.

Free-field microphones are intended to pick up sound waves from a single source. They are calibrated to compensate for any sound-wave diffraction that might affect the sound pressure at high frequencies. The microphone’s presence in the field does not affect the measurements.

The voltage source for an externally powered condenser microphone is usually a 48 to 200 V power supply. By comparison, a newer pre-polarized microphone has an “electret” layer of charged particles deposited on its backplane to supply the polarization. The electret microphone can use inexpensive constant-current supplies instead of costly polarized power supplies. Also, more economical BNC coaxial cables or 10-32 connectors can be used instead of the LEMO-type, 7-pin connectors and cables. Coaxial cables can carry the signals over long distances without significant degradation. Modern pre-polarized microphones are becoming the preferred type for laboratory test, measurement, and field applications.

Selecting and specifying microphones
Most types of microphones can measure broadband sound pressure levels from a variety of sources, but high-precision condenser microphones characterize the sound better than most. When choosing the optimum microphone, a number of factors must be considered:  the application, the sound source, and the operating environment. Also, investigate the type of response (application) field, dynamic response, frequency response, polarization type, sensitivity required, and temperature range needed. A variety of condenser microphones are available for specific applications.

Precision condenser microphones work well in three common application fields: free-field, pressure-field, and random-incident (diffuse) field. Free-field microphones are intended for measuring sound pressure variations that radiate freely through a continuous medium, such as air, from a single source without any interference. The microphone is typically pointed directly at the sound source (0º incidence angle). Free-field microphones measure the sound pressure at the diaphragm; however, the sound pressure may be altered from the true value when the wavelength of a particular frequency approaches the dimensions of the microphone. Consequently, correction factors are usually added to the microphone’s calibration curves to compensate for any changes in pressure at its diaphragm due to its own presence in the pressure field. These microphones work best in anechoic chambers or large open areas where hard or reflective surfaces are absent.

The second type is called a pressure-field microphone. They measure sounds from a single source within a pressure field that has the same magnitude and phase at any location. In order to simulate a uniform pressure field, they are usually calibrated in enclosures or cavities, which are small compared to their wavelength. This minimizes any alterations in measurements due to the presence of the microphone in the sound field. They are also supplied with a pressure versus frequency-response curve. Such microphones measure the pressure exerted on walls, airplane wings, or inside structures such as tubes, housings, and cavities.

A pressure-field microphone is intended to measure sound pressure in a field that has the same magnitude and phase at any location within it. Unlike the free-field microphone, its presence in the field does affect the measurement, however this effect is typically compensated in its design.

The third type is called a random-incident or a diffuse-field microphone. They are omni-directional and measure sound pressure from multiple directions and sources, including reflections. They come with typical frequency response curves for different angles of incidence and compensate for the effect of their own presence in the field. An appropriate application for this type of microphone is measuring sound in a building with hard, reflective walls, such as a church.

Random incident microphones are not as common as the other types. Few manufacturers rate them as such; since the small (0.5-in. diameter) pressure field microphones operate similarly, when equal-pressure sound waves hit the microphone from all directions.

Dynamic response
The main criterion that describes sound is based upon the amplitude of sound-pressure fluctuations. The lowest amplitude that a healthy human ear can detect is 20 millionths of a Pascal (20µPa). Since the pressure numbers represented by Pascals are generally extremely small and not easily managed, another scale is more commonly used, called the decibel (dB). This scale is logarithmic and more closely matches the response of the human ear to pressure fluctuations. Some examples of typical sound pressure levels used as a reference include the following:

Table of Sound Sources and Their dB Ratings

Typically, the maximum decibel level is based on the physical characteristics of the condenser microphone. The specified maximum dB level refers to the point where the diaphragm approaches the back plate, or where total harmonic distortion (THD) reaches a specified amount, about 3% or less. The maximum level in dB that a microphone outputs in a certain application depends on the voltage supplied and its sensitivity. In order to calculate the maximum output for a microphone, use a specific preamplifier and its corresponding peak voltage, and then calculate the pressure in Pascals that the microphone can accept. The amount of pressure is determined from the following equation:

Where: P = Pascals, Pa
Voltage = the preamps output peak voltage, V.

After the maximum pressure level that the microphone can sense at its peak voltage is determined, it can then be converted to dB using the following logarithmic scale:

Where: P = Pressure in Pascals, Pa
Po = Reference Pascals, Pa (Constant = 0.00002 Pa)

The microphone’s cartridge thermal noise (CTN) rating indicates the lowest measurable sound pressure level that it can detect above the electrical noise inherent within the microphone. The inherent noise level of a microphone and preamplifier combination is highest at both the lower and upper limits of the microphone. Each microphone has a unique noise characteristic, and the diameter of the microphone strongly influences its frequency response and noise level.

Frequency response
After the microphone’s field response and dynamic range have been determined, find the usable frequency range from its specification sheet. Proper selection requires that the measured pressure levels fall between the microphone’s low-end noise level (CTN), and the maximum rated dB level of the microphone. In general, the smaller the microphone diameter, the greater the high-end dB level expected. The larger diameter microphones are recommended for lower frequency range amplitude (dB) measurements since the inherent noise or CTN specifications are typically lower.

Manufacturers typically specify a ± 2 dB tolerance on the frequency versus output amplitude. When comparing microphones, check the tolerance in the specific frequency range most needed. When an application is not critical, select a wider frequency range with a higher allowable output (dB) tolerance from the manufacturer’s specification sheet.

Polarization type
Condenser microphones are further classified in two categories; traditional externally polarized and pre-polarized microphones. Either type works well for most applications, but the pre-polarized units tend to produce more repeatable output in humid surroundings. Pre-polarized microphones are recommended where temperature changes can promote condensation and short circuits on the internal components of externally polarized microphones. Conversely, at temperatures between 120 and 150º C, externally polarized microphones are better suited, because their sensitivity level is more consistent in this range.

Microphone sensitivity is inversely proportional to temperature. Microphones operated or stored in high temperature environments may require calibration more frequently. However, a probe microphone is specifically intended for withstanding these kinds of harsh environments. It combines a microphone with a probe-type extension tube for mounting close to the sound source. The probe tip contains the microphone, and the signal-conditioning module may be remotely located, either in a less harsh environment or where access to the sound source is too small for a typical condenser microphone.

Special applications
Microphones are often designed for special applications. For example, corrosion-resistant microphones called hydrophones are used for testing, monitoring, and measuring sounds under water. Different models are available for different sensitivities, frequencies, dB levels, and operating depths.

Another particular type, sound level meters, are special instruments intended to read sound pressure levels quickly and conveniently. Portable units are usually small, handheld instruments, which include the microphone, preamplifier, power source, software, and display. They measure ambient noise levels on the street and artillery ranges; in factories, power generating plants, shopping malls, office buildings, and numerous other places.

Intensity probes are usually the best choice for measuring the magnitude and direction of a sound. The set up usually consists of two phase-matched microphones with a spacer between them. They measure the pressure level as well as the speed and direction of the propagating sound waves. The higher frequencies typically require a smaller spacer, while larger spacers are used for lower frequencies or where sounds might reverberate.

Array microphones are used for near-field, acoustic holography (NAH). These are applications where 3D field values are studied. A number of array microphones are placed in a predetermined pattern and combined with appropriate software to map the acoustic energy flow of a complex sound pressure field. Array microphones work especially well where a large number of microphones are used concurrently. Also, Transducer Electronic Data Sheet (TEDS) are recommended to be used with arrays, since they let the user quickly and easily identify a particular microphone within the array. TEDS chips and firmware are typically stored electronically in each microphone and list its model number, serial number, calibration date, along with the specifications of the microphones sensitivity, capacitance, impedance, and other information that can be downloaded to help ensure accurate test results.

Array microphones are usually free-field types, and are intended to be a low-cost application for multiple channel sound measurements. For example, they are used in acoustic holography and pressure mapping such as vehicle testing to record the sound level at different points around an automobile engine, body, or tire well.

Finally, outdoor microphones can withstand rigorous environmental exposure, such as in airports and on highways. Noise measurements here are crucial to providing information for improving human safety conditions. Environmental and outdoor microphones provide different levels of protection for the internal components, while maintaining their high-accuracy specifications.

PCB Piezotronics Inc

Automated Gearbox Testing builds in Consistency

Ann Arbor, MI – The U.S. military has demanding requirements for the hardware it needs. Take, for instance, a set of gearboxes built by Excel Gear Inc., Roscoe, Ill, (excelgear.com) for missile launchers on the U.S. Navy’s new DDG1000 series of ships. The gearboxes are drive elements for the servo systems that rotate and elevate the missile launcher. For good servo performance the boxes must meet the Navy’s requirements for stiffness, efficiency, and low backlash.

A prototype was tested using a time consuming manual method. Although satisfactory, the method required careful checking to prevent data entry errors. Requirements for the test system called for high accuracy, elimination of measurement errors, and elimination of data entry errors. The company’s experience automating the test procedures provides a useful design lesson.

After successfully completing the prototypes, Excel Gear president N.K. Chinnusamy, decided the production run needed improved assembly procedures and to automate the test methods. Preload on the bearings was identified as an important factor – too little preload allowed excessive backlash while too much decreases efficiency and generates heat. A measurement accurate enough to size an optimum preload spacer is difficult because before the spacer is in place, the bearing can tip from side to side.

The company manufactured a set of fixtures to prevent bearing tipping and improve the repeatability of the preloads. The fixtures also improved the efficiency of the first production boxes and reduced their backlash from what had been attained in the prototype boxes.

Types of tests

The units called for several tests. For example:

Temperature tests during run-in: The primary sources of heat in the gearbox are seal friction, bearing friction, and oil churning. The heat generated by oil churning distributes throughout the box and dissipates through the case. Seal and bearing friction are concentrated and, if excessive, will cause failure. Temperatures are checked near the bearings on the high-speed shaft, where measurements on the prototype boxes showed the highest temperatures. These areas were also near the seals. Although it isn’t possible to separate the heat generated by the bearings and seals, the seals seem to generate the most heat.

Temperatures were recorded for two hours with the box running at maximum speed. After cooling, the test was repeated with the box running in the opposite direction. In the test, temperatures rose rapidly at first and then at a decreasing rate. While the temperatures do not reach equilibrium, in operation the boxes will not run continuously for two hours, and they will reach top speed only intermittently.

Gear-train stiffness: This characteristic, measured with the output shaft locked, was the ratio of the input-shaft motion to the torque applied, in Nm/rad. Torque was applied using a hydraulic actuator with two opposed cylinders driving two racks against a pinion. The racks are held in the pinion by a bushing. This results in friction force opposite to the direction of motion. Because the friction in the hydraulic actuator would cause measurement inaccuracies, the torque is measured between the actuator and the input shaft using a Dataflex 42/1000 torque transducer. This sensor has a capacity of 1,000 Nm in either direction. Torque is determined by measuring the twist in the transducer shaft using rotary encoders in a differential circuit. An encoder rotor is mounted at each end of the shaft. Because the encoder read heads are mounted to the stationary part of the transducer, there are no slip rings. The A-quad-B output from the transducer is converted to a voltage by an encoder electronic box. The voltage output range is 0 to 10V with a no load value of 5V, and the calibration constant is 0.200 Nm/mV.

Rotary motion was measured using a 2,000 line rotary encoder (resolution of 0.018° ). Because the input shaft extends through the box, the encoder is mounted on the opposite end of the shaft from the hydraulic actuator. When the box is in operation, a brake is mounted on this end of the shaft.

The A-quad-B output from the encoder is converted to a voltage by the programmable encoder-control box. The range and number of volts per degree can be set depending on the amount of rotation to be measured. The output has a range of 0 to10V. For this test, the output was 8.100° per V and the no-rotation voltage was 5V.

Gear train backlash: Some backlash is necessary to provide running clearance for the gears. Too little backlash results in overheating and premature failure, and too much degrades servo performance. Backlash is determined from the data collected for gear train stiffness.

Breakaway torque: For these boxes, it was low and measured manually using a snap-torque wrench. Although automated tests are usually preferred, a few are so simple that the programming required is not justified. This test, for instance, was the only one not automated.

Gearbox power losses over the full range of speeds: Input torque was measured with the gearbox running at a set of speeds both clockwise and counterclockwise. The torque is a nearly linear function of speed with a small component of stiction. This is preferred in a servo system because it contributes to servo loop damping. Because the torque is nearly a linear function of speed, the power-loss curve is nearly parabolic.

Test equipment

Accompanying images show the test equipment and The test hardware table lists a few of its details. The software used, DASYlab, is a graphical programming language. It is programmed by placing block diagrams representing data collection operations on a screen and connecting them with “wires” to control data flow. In the system used here, processed data is written to disk in a tab-separated format suitable for further analysis using Microsoft Excel.

Programming the data collection: The DASYlab block diagram provides an example programming screen. Each block represents an operation on the data such as collecting, scaling, saving to disk, and displaying. This programming method is faster than writing code. For example, the voltage output from the torque transducer, encoder, and thermocouples was connected to the electronic interface box. This box has a built in reference junction for the thermocouples. The device also has digital and analog outputs but these were not used. The interface box scans its inputs and converts from analog to digital values. These are passed to the computer through a USB connection at about 1Hz. This is relatively slow for data acquisition, but more than adequate for these quasistatic tests.

In the DASYlab program, the first box is an input box that “talks” to the hardware and places the input values on its outputs in digital form. Typically this is a special module that works only with particular hardware. Most other boxes do not depend on the type of hardware in the system. The other boxes used are numerical displays, graphical displays, and output boxes to record the data on disk. These boxes have corresponding components on a display screen. This screen can be a virtual instrument, that is, it can look like instruments such as voltmeters, oscilloscopes, and chart recorders. For this test the program converts the inputs to engineering units and displays the values several ways. Digital displays show the current numerical values of inputs.

Another display, an XY-plot, shows rotation on the Y-axis and torque on the X-axis. This feedback gives a preview of results. It can save much time because if something is wrong, such as a broken wire or failed thermocouple, it quickly becomes apparent. The test can be stopped, the problem corrected, and the test resumed. A problem that goes undetected until the data is analyzed wastes the entire test period.

A disadvantage of DASYlab is that this program had to be written with the computer attached to the interface hardware. It would be a great advantage to write the program sitting in front of a desktop computer rather than working in the test area using a laptop.

Details of the analysis

Backlash and Compliance: One advantage of automated data collection is the larger amount of accurate information than can be manually collected in a reasonable period. The additional data gives a better picture of the equipment characteristics than would otherwise be possible. In Backlash and compliance, the red line simulates points collected when the torque was varied from zero to maximum, to minimum, and back to zero three times. (The data shown are not actual values but they are an accurate representation of the type of data collected.) The data showed good consistency and repeatability, which produces confidence in the results. For instance, the blue line shows the curve fitted to the backlash and compliance data. The length of the vertical line at zero-load is reported as backlash. The slope of the lines fitted to the observations is the stiffness. This is a conservative method for determining such values. The normal manual four-point test would have given both lower backlash and compliance numbers. The four-point test uses two torques that are just a little higher than breakaway torque in each direction and two torques that are a quarter to one half of the full load torque.

Converting from manual to automated testing: The first use of an automated system involves debugging because the test system, as well as the tested device, may have problems. An advantage to starting this system was that previous manually collected results were available for reference. Problems with the test system are seen quickly. For the first test, some manual measuring devices were used in parallel with the new test equipment. This either verified the results or showed problems. For example, an incorrect scaling factor was quickly detected and corrected. This illustrates one principle of successful testing: Check the calibration of the test equipment before running the tests.