RF Field-Service Monitor Spans Gap between Analog and Digital

Two trends that have been evident in test instrumentation for several years are miniaturization and digitalization. Test equipment that once would have filled a couple of 12-foot racks can now be packed into a unit the size of a briefcase. And digital technology is largely responsible for this dramatic change.

When then is one of the leading manufacturers of communications test gear adding an oscilloscope (an analog device) to the front panel of its AM/FM field-service monitor?

When engineers at San Jose, California-based Cushman Electronics decided to design a new field-service monitor, human design considerations were paramount. The engineers concentrated on the physical and psychological problems users had with the monitors. The physical problems were straightforward: matters of size, weight and the number of pieces of equipment that service technicians were obliged to carry into the field. The psychological problem was more subtle. Paradoxically, the digital technology that enabled the engineers to solve the physical problems was the very source of the psychological problem that had to be overcome.

In radio testing, more than in any other field of electronics, the technology has gotten ahead of the people who use it. While digital techniques were making test equipment smaller, lighter and less expensive, they were simultaneously creating a psychological gap between the service technician and the equipment.

Radio testing is and has been an analog discipline since long before the first digital circuits were invented. Whereas analog problems can and indeed do have digital solutions, the present need of the communications industry is to take optimal advantage of the many qualified RF technicians who are currently keeping mobile and marine communications gear on the air, rather than to train a new generation of technicians in the mysteries of RF. Thus, the decision was made to use digital technology to miniaturize the equipment and make that technology transparent to the operator by providing familiar analog controls and indicators.

Most radios work on a system whereby several users share a repeater. Transmissions are coded, so that each receiver gets only the communications that are intended for it. In servicing, several encoders are typically needed to deal with both analog and digital encoder signals. The encoder is an electronic key that is used to unlock the radio's audio circuits.

In addition, pager transmitters use various kinds of codes to work with different kinds of pagers. There are more than a million possible digital codes for pagers. The technician's encoder box has to simulate most, if not all of them, in order to emulate and display pager formats. The problem is further complicated by the fact that large users buy different kinds of pagers at different times and the encoding format changes as time goes on. If a technician were testing pagers made by various manufacturers, several stand-alone encoders would be needed.

Specifications for the Monitor

The first task was to draw up a list of specifications for the new monitor. Its size was to be such that it could fit easily under the seat of an airplane or helicopter or into a backpack. The weight was to be less than 25 pounds, and it had to be rugged and built to last.

Cushman engineers decided that service technicians shouldn't have to carry more than one piece of equipment. This meant that the monitor should somehow be capable of generating a wide array of tone and data formats for selective signaling over voice-grade communication channels. This included single tone (CTCSS), tone-dial interrupt, two-tone sequential paging, dual-tone multi-frequency, mobile telephone service, improved mobile telephone service, 5/6-tone pager, remote base station, digital-coded squelch (CDCSS) and digital-pager display formats.

They also decided that the unit should have a built-in oscilloscope and a fully protected signal-generator output/RF input. The receiver was to be capable of detecting two microvolts at the antenna input (a condition required by today's corporate users). The unit should also have AM diagnostic capability for use in global markets.

These design goals were met in the Cushman CE-7010 FM/AM Field Service Monitor. Its built-in universal signal encoder can emulate all of the common tone formats, and its built-in oscilloscope can measure voice peaks and can analyze auido characteristics.

Large fleet owners will welcome the capability--and portability--of this latest addition to Cushman's line of communications test equipment. The service technician using the CE-7010 can perform a full test of all functions without removing the radio from the vehicle, nor does the technician have to have the vehicle to the shop.

The universal tone-signal generator used in the CE-7010 works with pages as well as with mobile radios. The monitor emulates everything from simple two-tone sequential codes to the most-sophisticated digital display pager codes, including formats that are specific to various manufacturers.

Monitor Minimizes Hazards

There is scarcely an RF technician alive who has not at one time or another ruined an expensive monitor by accidently keying directly into its RF input port. The design of the CE-7010 minimizes this hazard. The monitor can tolerate up to 150 watts of RF input for two minutes without damage. This is in contrast to similar units that will tolerate only 100 watts, and then only for 10 seconds or so. The unit generates a high-level, fully protected RF output signal of +3 dBm.

Test equipment like the CE-7010 has many applications. Fleet service operators, radio and pager manufacturers, public-safety organizations, state and federal agencies, delivery services, aircraft and marine radio services all have need for FM/AM field-service monitors that are portable, rugged and reliable.

No one can say for sure what the instrumentation of the future will look like, just as no one 30 years ago could have foreseen the remarkable changes in equipment size and weight that would come as a result of the semiconductor revolution. One thing is certain however, the prudent corporate user, now and in the future, will want to take advantage of the best human and technological resources available.

COPYRIGHT 1985 Nelson Publishing
COPYRIGHT 2004 Gale Group