Paging technology: Pager development, testing Part 4–Three stages of pager development require various types of tests to verify performance. Here are some of the steps and the test equipment used in manufacturing and in quality verification prior to ship

Figure 1 below is a block diagram of a pager with receiving circuitry, digital decoding circuitry and devices that alert the subscriber and display a message.

The receiving section demodulates the data from the RF carrier and passes the data to the decoding section. The decoder then looks at the data and decides whether the information being received is for the subscriber. If so, that information is displayed on the pager, and the subscriber is alerted by the selected means–beep, vibration or flashing light.

Circuitry to “wake-up” the pager is most likely contained in both sections. Because the pager has a battery-saving mode, and depending on the paging protocol, the pager only “looks” for paging information during a particular point in time (as with Motorola Flex protocol and ERMES) or the signal being detected is what “wakes up” the pager (as with POCSAG and Golay sequential code).*[*Flex is short for flexible wide-area synchronous protocol.ERMES stands for European radio message system.POCSAG stands for Post Office Code Standardisation Advisory Group.Golay sequential code is named after Marcel Jules Edouard Golay (b. 1902), a physicist who constructed the code about 1955.]

The receiving section consists of several components. (See Figure 2 on page 24.) First, an antenna receives the RF information. Oscillators, mixers, filters and amplifiers translate the RF signal to the baseband frequency. This translated signal is detected, and a serial stream of digital bits is presented to the decoding section.

The decoding section processes the information from the receiver. (See Figure 3 on page 24.) The microprocessor is the heart of this section. The microprocessor, with its surrounding circuitry, checks the incoming bit stream and determines whether it contains a message and what to do with it.

Pager development There are three basic stages to pager development. These are research and development (R&D), manufacturing and service. (See Figure 4 on page 28.)

R&D workers design the pager and test a variety of circuit design characteristics. At this stage, the test objective is to thoroughly characterize all of the circuitry. Measurements must be made to verify performance, especially under varied environmental conditions such as temperature, humidity and shock. Typically, high-performance test equipment is selected for R&D work. The objective of production testing is to verify that the product and processes are consistent. Meeting this objective results in high-quality shipments while minimizing production costs and meeting shipment and business goals.

A competitive pager manufacturer must always be looking for ways to reduce the production cost per pager. Testing must be fast, reliable and economical. The test equipment selected for the job must have the necessary measurement accuracy and repeatability to verify correct pager operation.

For pager tests in manufacturing, equipment ranges from basic tools to dedicated, one-box testers. Parametric testing in production verifies the characteristics that vary from pager to pager. These are typically RF- and analog-dependent characteristics in the pager’s receiving section and any process-dependent characteristics.

The final, functional testing is performed to assure that the end-user receives an operational pager. Characteristics that do not vary from pager to pager do not need to be tested in production. These characteristics include those that have been thoroughly tested and proven during the design phase. They include most of the pager’s decoding functions–protocol decoding verification, for example. Testing function in production provides no information about the production process.

Because a pager’s receiver is intended to be selective and because received signal levels can be fairly low, its filters must be tuned to a precise center frequency. This tuning may be performed by a network or spectrum analyzer.

The oscillators must be tuned and aligned so that a precise intermediate frequency is achieved. Using the most basic tools, this test can be performed with a signal generator, encoder, frequency counter and voltmeter.

The pager must operate over a wide power range. Of course, the most important power test is operation at a low receive power level. Subscribers expect to receive their paging messages wherever they may be, and a sensitive pager is desirable. A sensitivity test can be performed with a signal generator and encoder. To accurately measure sensitivity without interference from other RF signals, the pager must be isolated. The use of a shielded room, isolation chamber or transverse electromagnetic mode (TEM) cell is highly recommended. (See Figure 5 above right.)

A shielded room may be large enough for one or more test stations. An isolation chamber encloses the pager and test fixture. A TEM cell can be sized for pager testing. These are all recommended ways to achieve isolation. The differences among the three are their prices and the amount of space they require. In final testing, before the pager is shipped, all of its basic functions should be tested. Does it beep? Does it vibrate? Does the light flash? Does it display the message correctly? The necessary test can be performed with a signal generator and an encoder. This installment ends the article series.

Intelligibility is the most important factor in mobile radio communication. Unfortunately, when radio communication is attempted in noisy environments, intelligibility is often lost. Low-frequency noise is most detrimental to intelligibility because, more than mid- and high-frequency noise, it masks other sounds including speech.

Active noise reduction (ANR) is the electronic manipulation of sound waves to reduce noise and enhance sound quality. One ANR method, the “anti-noise” approach, is the acoustic coupling of a noise wave with its exact mirror image. ANR is the only effective method of reducing low-frequency noise and is available today in “anti-noise” headsets for mobile radios that deliver unmatched clarity.

Another ANR method, which does not account for frequency, is adaptive speech filtering. This is the elimination of noise from speech and other transmitted and received signals and can be integrated into communications devices.

The technology An ANR anti-noise system includes a microphone, a tiny computer (signal processor) and a speaker. The microphone picks up the signature of the undesired sound and transmits it to the signal processor. The signal processor analyzes the wave signature, creates its inverse (called “anti-noise”) and, via the speaker, hurls it back at the original sound wave. If it were possible to achieve a perfect coupling of the opposing waves, the result would be absolute silence. Most frequently, because perfection is not achievable, a substantial noise reduction results.

Noise Noise can be defined as unwanted sound. Sound is the result of pressure changes in air caused by vibration or turbulence. The amplitude of these pressure changes is the sound level (expressed as decibels or dB), and the rate of speed at which the pressure changes occur is the frequency (expressed as cycles per second or Hertz). Because the decibel scale is logarithmic, a small increase in decibels represents a large increase in sound energy. For example, an increase of 3dB represents a doubling of sound energy, and a 10dB increase represents a 10-fold increase. To the ear, a 10dB increase is perceived as a doubling in loudness.

Low-frequency noise Prolonged exposure to noise, including low-frequency noise, is known to have detrimental psychological and physiological effects including: #fatigue, anxiety and depression. #loss of concentration and productivity. #headaches, high blood pressure and hearing loss.

Low-frequency noise interferes with communication because it masks other sounds. Scientific studies have shown that an increase in the low-frequency component of background noise correlates to a decrease in speech intelligiblity. In environments where there is an abundance of low-frequency noise, people express the ability to sense another speaking, but not to understand what is being said. Low-frequency noise from engines, motors and fans dominates ambient sound in most industrial settings. Oral communication in these environments is often crucial, making low-frequency noise reduction necessary.

Passive noise control Passive noise control uses sound-dampening materials to absorb noise and vibration energy and to control its propagation. The use of sound-absorbing and rigid materials to reduce noise levels is effective with high-frequency sound. Below 500Hz, the cost, weight and mass of passive sound attenuation often make it ineffective or impractical. Therefore, another technique for noise control is required.

Active noise reduction Although active anti-noise systems work across the full range of sound frequencies, most commercial systems treat low-frequency sounds–represented by buzzes, hums, booms and rumbles. Low-frequency waves are long, they travel extensive distances undiminished, and they can easily penetrate passive barriers.

The length of a sound wave is determined by dividing its speed by its frequency. For example, a 100Hz sound wave travelling at the typical 1,200 feet per second for sound in air has a 1/2-foot-long wavelength. When the peak of a noise wave is intersected by the trough of an anti-noise wave, the noise wave is completely cancelled. The sound waves are said to be exactly 180 degrees out of phase. Some degree of cancelling is achieved even when the waves are not perfect mirror images–180 degrees out of phase.

Figure 1 shows the relationship in time of a noise signal, an anti-noise signal and the residual noise that results when they meet. Note that the active noise cancellation removes a significant portion of the noise energy from the environment instead of just masking the noise.

In most environments, noise exists in a wide frequency range. To attain the optimum level of noise reduction, it often is necessary to apply both active and passive methods.

Applications for communications *Active headsets–One of the simplest, yet most effective applications of ANR can be found in headsets. ANR headsets provide as much as 20dB of background noise reduction in the low frequencies and, depending on the style, also provide a passive component for mid- and high-frequency noise reduction. ANR headsets provide a level of intelligibility and clarity that standard headsets cannot, because they reduce the low-frequency noise which most interferes with intelligibility.

Photos 1 and 2 on page 30 show ANR headsets. Models are available in both an open-back style for environments where low-frequency noise dominates and a closed-back style for high-noise environments.

*ANR telephones and adaptive speech filters — The most-often-used communications tool is the telephone. Using various applications of ANR, this tool can be greatly improved. By integrating electronics into a telephone handset or headset, low-frequency environmental noise can be reduced at the user’s ear. This is beneficial when conversing on phones in noisy environments including factories, on telemarketing or trading floors and outdoors.

Dynamically adaptive speech filters represent another method of improving communications. In addition to the reduction in ambient environmental noise, dynamically adaptive speech filters continuously clean transmitted and received signals, attenuating background noise from speech.

This technology can now be cost-effectively integrated into two-way, mobile and AM/FM radios; telephone handsets and headsets; cellular phones; and even telephone networks to reduce environmental noise, static, line hum and other in-wire interference. Figure 2 above shows speech patterns, with and without noise, using this type of filtering.

The future The key to communication is intelligibility, and noise from the environment, as well as line hum, static and other in-wire interference, is a major detriment to intelligibility. Only advanced technologies can address these noise problems, and as the world becomes a noisier place, ANR will become increasingly important for successful communication.