Codan NGT 2010 RF Technical & Service Manual page 780

In this configuration, the resistive cell is in series with a 20 kΩ resistor and a 1 nF
capacitor C113. The resulting series network is connected in parallel to the input resistor
R124. Because the cell is in the input circuit, and not part of the feedback network used
in the amplifier IC103a described previously, minimum gain occurs when the cell has a
high resistance (DC current on pin 16 at zero). This has minimum effect on the AC gain
of the amplifier, therefore the gain is set by the feedback resistor R127 in conjunction
with input resistor R124. At this point the amplifier has a flat AC response, the first
amplifier (IC103b) has unity gain, while the second amplifier (IC104b) attenuates the
signal by 6 dB.
As the DC current on pin 16 of IC103b and IC104b is raised by the microprocessor-
controlled DAC IC2, the resistance of the cell reduces. However, due to the AC
reactance of the 1 nF capacitors C113 and C124, the gain increase is proportional to the
rise in frequency. For example, there is approximately 2 dB increase at 100 Hz, rising to
about 17 dB at 2 kHz for the first amplifier IC103b and, because of different resistor
values, about 11 dB for the second amplifier IC104b. The combination of the two
amplifiers has an overall minimum gain of –6 dB (×0.5) and a maximum gain of 28 dB
at 2 kHz (×25). At the highest gain setting, the gain continues to rise with frequency until
about 10 kHz, at which point it rolls off due to the influence of feedback capacitors C117
and C127.
Low-pass filter, audio gate and transmit audio bus
Drawings
As the equalisation amplifiers have a rising gain with frequency (except at minimum
gain), it is necessary to connect the output to a 3 kHz LPF to remove any high-frequency
noise that may result from the high gain in the equalisation amplifiers. IC105a and its
associated components form an active 3-pole LPF of conventional design.
At the output of the LPF, the audio is fed via an audio gate IC1 to the primary of
transformer T1. From the secondary, the signal connects to the Backplane PCB via P1
pins 32a and 32b (designated as Tx audio bus).
NGT Transceiver System Technical Service Manual
04-02541 and 04-02691
8571—Technical description
341