GAI-Tronics ITA2000A ITA2000A Series Tone Remote Adapter with Scanning User and Installation Manual - Manual (Page 39)

35 03/13
Theory of Operation
Technical Overview
U1 performs various tasks requested by the firmware which reconditions the signal, such as 2175 Hz
guard tone filtering, line auto-leveling, tone generation, tone filtering, tone decoding, and 2-wire audio
hybrid control. Radio RX audio and TX audio from the remote line is digitized in codec U20 and U21 to
allow U1 to perform digital signal processing. After U1 reconditions the digital signal, the signal is
passed to the codec, where it is converted back to analog at the internal reference level of –6 dBm.
Hardware gain circuitry is used to adjust the signal for the desired audio output level external to the unit.
The ITA2000A has seven configurable logic inputs; U8 provides the status of these inputs to U1, where
U1 processes the data and the end result is output by U11. An internal hardware jumper (JU2-JU14) can
configure each output, DL0-DL7, for a logic high or logic low. U11 drives DL0-DL7 low; when the
jumper is in the B position this provides an active low output, when the jumper is in the A position the
signal drives a transistor applying B+ to the output providing an active Hi output.
The ITA2000A takes the external audio input signals it receives and uses gain stages to produce an
internal reference level of −6 dBm. This internal reference level is then used as nominal input/output
level for codecs U20 and U21.
Transmit Audio
Audio from the phone line enters the unit at J1-3 and J1-4 and passes through transformer T2 and then
buffered by amplifier U24c. The output of U24c is fed into digital pot U22; the DSP U1 adjusts U22 for
an output (internal reference) level of –6 dB. The reference level is input at pin 14 of codec U21. The
digital output of U21 is fed to U1 DSP. The digital signal is then converted back to analog and output at
pin 3 of codec U20 at the internal reference level of –6 dB.
For radio mic TX, the reference level is adjusted with pot 1 and amplifier U16 to accommodate an output
range of 32 to 800 mV ac into a 560-ohm load. For TX audio to the accessory, the reference level is
adjusted with amplifier U17 for an output of 300 mV ac into a 560-ohm load.
Radio Audio
Radio RX audio enters the unit at J2-3 and J2-11 and is converted to a single-ended signal by op-amp
U18. Pot 2 (RX audio sensitivity) is used to adjust the output of U18a for the internal reference level of
–6 dBm. This reference level is output to the desk mic connector. For accessory RX audio, the reference
level is adjusted with amplifier U17 for an output of 300 mV ac into a 560-ohm load.
For telephone line output, the reference level is passed to U20 where it is converted from analog to
digital, the signal is then reconditioned by U1 DSP and converted back to analog by U21and output at
U21-3 at the reference level of –6 dBm. The output of U21-3 is passed to amplifier U23 where pot 3
(line out) adjusts the output to accommodate for a range of −15 dBm to +10 dBm into a 600-ohm load at
the telephone line output J1-3 and J1-4.