Comparing Analog and Project 25 Test Philosophies

PART III

To build an understanding of how digital technology will change testing, it is useful to first contrast digital with analog testing philosophies. In analog wireless systems the information is conveyed by modulation in the form of AM or FM, which changes the amplitude or frequency of a carrier signal in a linear way.

In a digital system, the analog signal representing the voice is passed to a vocoder, which converts the voice signal to a digital data stream using a defined algorithm. This algorithm varies from system to system, but results in a data stream in the order of 4-10kbytes/s being transmitted.

Analog Testing. Measurement technology for analog systems relies on the signal being present for sufficient time to make the measurement. In the case of the receiver SINAD this may be in the order of several seconds. The test equipment required ranges from power meters, modulation analyzers, audio analyzers, spectrum analyzers and signal generators.

Transmitter Testing.

When testing the transmitter of analog systems, five measurements are standard: 1) Power Output; 2) Transmitter Spurious; 3) Frequency; 4) Modulation; and 5) Distortion.

Receiver Testing.

In receiver testing, we inject a low level carrier modulated in a manner appropriate to the demodulator under test, e.g. FM, at a known rate, say 1 kHz. We examine the audio output of the receiver in order to separate the original test tone (1 kHz) from any distortion and noise that the receiver added to the original test signal. We can equate the ratio of the two separated signals to some test threshold, and manipulate the receiver’s circuitry to optimize the ratio such that the distortion and noise portion is minimized.

Digital Testing.

Traditional test strategies have primarily focused on the parametric performance of the radio terminal, where measurements such as power, frequency, modulation and sensitivity are the primary indicators of performance. The open standard concept adopted for Project 25 introduces some new variables into the testing equation which relate to the interoperability of equipment sourced from different manufacturers supporting the standard.

Project 25, like its analog forebears, is an FDMA (Frequency Domain Multiple Access) system and it produces continuous signals when the radio is keyed. Therefore some of the more complex measurement techniques required for TDMA (Time Domain Multiple Access) systems such as TETRA are greatly simplified. Several key differences, however, distinguish the test required to determine system performance.

Digital Transmitter Testing.

A digital modulator imparts information to an assigned carrier by adjusting the carrier’s power, phase, or frequency, among a small dictionary of possibilities. Since there is no quantitative amount involved with digital modulation, no adjustments are generally required in the modulation path. Instead, it is usually sufficient to examine the quality of the modulation, or how good is the modulator at making its adjustments to the carrier. This is measured in percent, or some other quality score against a goal. When the goal is not met, one looks for a defective component that is causing the problem.

Digital Receiver Testing

Except for its decision circuit, digital receivers are remarkably similar to their analog cousins but they are tested in rather different ways. In the analog case, we find the SINAD procedure to be efficient, as it tests the whole receiver path in one pass.

Receiver sensitivity is determined by measuring its ability to recover data transmitted to it. The measurement referred to as receiver BER, or Bit Error Rate, measures the ratio of the bits received correctly as a percentage of the total bits transmitted. BER = 0.02 is an indication of a better receiver then one with BER = 0.2

Channel coding is a baseband process in most digital radio systems that deliberately adds carefully contrived redundancy to a symbol stream, which the receiver can use to repair a limited amount of damage to a recovered symbol stream. For those radios that protect all the user traffic with redundancy bits (which are also encoded into symbols), we loose the ability to detect individual bit errors. Instead, the frames or blocks of bits (symbols) that can not be repaired are marked, by the receiver itself as having suffered an error. FER = 0.10 is worse then FER = 0.00. The second result indicates no frames were received with uncorrected errors.

Increasingly, all of the baseband processes in digital radios are realized in software, and they either work, and work well; or they don’t work at all. It is not wise to spend valuable technician time verifying these functions.

 

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