Motorola ASTRO Spectra Instruction Manual page 44

Theory of Operation: Audio Interface Board
Transmit Audio Paths
Control and Data Paths
22
Speaker audio is available at Accessory Connector 2, pins 14 and 15
(REMOTE_RX+ and REMOTE_RX–, respectively) for use with Digital Remote
Control models. Speaker audio level is factory set but can be adjusted using
Customer Programming Software. An array of resistors capable of dissipating
10 watts provides a 16 Ω impedance for the desksets. To enable this audio
output, S101-7 must be ON.
REMOTE_RX– (SPKR_LO) and REMOTE_RX+
!
(SPKR_HI) should never be grounded. If they are
grounded, this will damage the radio.
C a u t i o n
There are three main paths for transmit audio. All of these paths pass through
transmit audio summing amplifiers (U101 pins 8, 9, and 10). The output of the
summing amplifier (U101 pin 8) following AC-coupling capacitor C101 is fed
into the ASTRO radio microphone input at J1 pin 23 and J1 pin 48. The level
of this signal should be 90 mVrms relative to a 1.0 kHz tone at 60% FSD.
Local microphone audio passes through the control head to J2 pin 30. The
microphone audio is biased and passed through the summing amplifier with
unity gain. This path has an impedance of 600 Ω .
A differential transmit path is available at Accessory Connector 2 pins 3 and 11
(TX+ and TX–, respectively) with an impedance of 1.0k Ω . This signal is
combined at U101 pins 2 and 3 into a linear signal at U101 pin 1 and then
routed through the transmit audio summing amplifier with a gain of +6 dB.
A linear transmit audio path is provided at Accessory Connector 2 pin 17
(TX_AUD). This transmit path has an impedance of 600 Ω and unity gain
through the transmit summing amplifier.
Transmit audio taken from the MIC_HI line is routed to the tape recorder output
based on the jumper position at header J14. TX audio is routed to the tape
recorder output at approximately 90 mVrms when the jumper is in the A-B
position. If the jumper is moved to the B-C position, TX audio is not routed to
the tape recorder output. (The TX and RX audio measurements were made
relative to a 1 kHz tone at 60% FSD.)
Transmit audio is routed to the VU meter/clock (option L114) with a gain of
+14dB.
The Audio Interface board is transparent to all control and data lines except the
data bus lines, MONITOR*, EMER (Emergency), and SPKR_UNMUTE. The
amount of gain developed by U105 is determined by the current requirements
of the remote desksets. Each op-amp on the BUS+, BUS-, and BUSY lines
compares the voltage drop between the + and – inputs. The resulting amplified
signal is further amplified by the transistor following each op-amp.
MONITOR* is presented to the radio with S101-6 OFF. See Table 1 on page 9.
The monitor function is available at the control head.
EMER (Emergency) is available at Accessory Connector 3 pin 13. When
triggered, the radio will broadcast an emergency signal (refer to the ASTRO
6881086C22-B