Introduction; Functional Description - BRUEL & KJAER 5998 Service Manual

Chapter 5 — Theory of Operation

Introduction

5.1 Introduction
To make troubleshooting easier, some knowledge of the principle of operation might
be helpful. This section contains a description to block level of the principle of
operation for the Type 5998.
5.2

Functional Description

The block diagram
shows the basic configuration where a transmitter microphone is coupled acoustical-
ly to a receiver microphone. The system uses single frequency sinusoidal signals.
Four sequential measurements are made to determine the voltage ratio between the
receiver and the transmitter; direct measurement of Ch. A and Ch. B outputs, and
two reference measurements to determine the gain difference.
Transmitter Microphone (Channel B)
The transmitter microphone is used as a sound source and is driven by the genera-
tor signal applied to the microphone housing via a 6dB amplifier. The amplifier's
low output impedance helps to minimise the influence of electromagnetic fields and
hum.
The current through the reference capacitor (4.7 nF) is the same as the microphone
current. Since one end of the capacitor is grounded, the voltage across it can be
used directly to determine the transmitter microphone current.
The unity gain amplifier in the Transmitter Unit has a very low output-impedance
suitable for driving cables.
Receiver Microphone (Channel A)
The receiver microphone detects the sound pressure produced by the transmitter
microphone within the coupler cavity volume. The unity gain amplifier has a very
low output-impedance suitable for driving cables.
Signal Routing within Type 5998
Inside Type 5998 the signals passes through a high-pass filter with selectable low-
frequency roll-off frequencies (1 Hz, 20 Hz, 100 Hz). Selection is done via the front
panel or the IEEE–488 bus. The output amplifiers have variable gain (0 to 50 dB in
10 B steps). Gain selection is done via the front panel or the IEEE–488 bus, and is
the same for both channels.
The output voltages appear on their respective output terminals. Moreover, Chan-
nel A has a switch multiplexer allowing other signals to appear at its output termi-
nal. This allows all voltages to be sequentially measured using a voltmeter
connected to the Output terminal.
5 − 2
(section 5.3) shows a reciprocity calibration system. The figure
Reciprocity Calibration Apparatus Type 5998
Service Manual
Brüel & Kjær