Motorola ASTRO Digital Spectra, Instruction Manual

Page: 48 / 114

Theory of Operation: Tone Remote Control Board
26 6881086C22-B
contains an MF10 dual switched capacitor filter and a dual op-amp for
summing purposes. The notch filter is formed by summing together the high-
pass and low-pass outputs of the filter IC. The MF10s on both HY1 and HY2
require a high-frequency clock input that is derived from a divider circuit (U127
and U129) and the MPU “E” clock. The output of the clock divider circuit
(FILTER CLK) is a 110.4 kHz square wave that is used by the MF10s to create
the filters. The purpose of the notch filter is to notch out the guard tone
frequency area of the voice spectrum prior to transmission across the wireline.
The receive notch filter has a gain of 0 dB at 1 kHz. At the 2175 Hz notch
frequency the response is -35 dB, relative to 1 kHz. To allow the slight drifting
of the guard tone frequency due to wireline translation, the filter must
guarantee 30 dB of attenuation at ±5 Hz from the center frequency. The filter
has a “Q” of 3.8 in order to remove all voice components in the guard tone
frequency range before reaching the wireline. Without this protection, false
guard tone detects by the TRC board would be inevitable, since the audio
leaving the TRC board is up to 20 dB higher than that arriving to make up for
wireline attenuation.
The output of the notch filter passes through another muting gate (U114 gate
2). This audio path is always closed when the Consolette is receiving. Local
microphone audio and trunking tones also share this audio path to the console.
During local PTT this gate can be used to mute local audio on the receive path
(see “Transmit Audio Path” on page 26).
Receive audio is also routed from U116 gate 1 to buffer amplifier U106 op amp
4 (MOB_RX). The output of this buffer drives the TAPE OUT line, which is used
for external recording purposes.
The line-adjust circuit (U111 op amp 3 and R545) allows the notched audio
level to be adjusted via R545. By varying R545, you can adjust the audio level
to the line. This allows you to compensate for line losses to obtain the desired
audio levels. The adjusted audio is then sent through an audio-shaping filter
(U111 op amp 4). This filter is a unity-gain, low-pass filter with a corner
frequency of 3 kHz.
The output of the line audio-shaping filter is split into opposing phases in the
line driver circuit (U111 op amps 1 and 2, Q114 and Q115), where the receiver
audio is applied to the line transformer and the phone line. With two-wire
selected, the line driver audio is also applied to the input of the transmit audio/
tones detection path through T100-6. A portion of the line driver audio is also
fed into the input amplifier of the transmit audio/tone detection path, via U111
op amp 2 and S101-7,8; this audio is approximately 180° out of phase with the
received audio at T100. This is done in order to, at least partially, cancel the
receive audio so that a tone from a control console may be more easily
detected in the presence of receiver audio. With four-wire selected, this
cancellation is not necessary and S101-7 should be open and S101-8 should
be closed.
Transmit Audio Path
Wireline audio originating from a console to be transmitted over the air is
applied to the wireline interface network. The wireline interface network
consists of T100 (two-wire operation), T1610 (four-wire operation), U110, and
other discrete components. The purpose of this circuitry is to match the
impedance of the TRC board to that of the wireline (T100, T1610, S101), block
any DC components between the TRC board and the wireline (C502, C501),
and provide protection from surges on the line (E1, E2).