GAI-Tronics ITR1000-001 Model ITR1000-001 Basic Tone Remote Deskset with Line Select User and Installation Manual - Manual (Page 26)

Theory of Operation ITR1000-001 Basic Tone Remote Desk Set
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The microcontroller selects the active microphone based on the state of the hookswitch. Microphone
audio is scaled through U12d and gated to the compressor circuit consisting of U11, Q8, and associated
components. Microphone audio at TPC1, the compressor output, is notch-filtered to ensure that voice
components in the 2175 Hz range do not interfere with LLGT. Microphone audio at TPN, the notch
filter output, is then gated to amplifier U12c where it can be summed with LLGT. POT4 adjusts the line
output level. The output of U12c is gated to line driver U14 and transmitted over the wire line.
Each control tone frequency is synthesized as a square wave by microcontroller U1. This signal is then
scaled to the correct amplitude at inverting amplifier U19a. By means of outputs PA5 and PA6, the
microcontroller selects either high level, function tone level, or low level. The scaled square wave is
smoothed into a sine wave by filter U19d. The resulting tone is gated either to U12c (if it is a control
tone) or U13b (if it is a volume beep).
Speaker Volume Control
The actual speaker output level adjustment is done internally within power amp U15. The control circuit
for this provides a precise control voltage connected to pin 5 of U15. The microcontroller examines the
VOLUME
Up/Down button presses and provides control pulses to electronic pot U9. U9 provides a
variable reference voltage into the divider resistors (R156, R157, R158, R159) connected to U13 pin 12.
Speaker muting voltage is also connected to this divider through R154. If the dc volume control voltage
at U15 pin 5 is below 0.3 V dc, the speaker amplifier switches to the mute mode. A dc volume control
voltage greater than 1.6 V dc results in maximum gain of the amplifier U15.
Power Supplies
The power supplies for the desk set operate in the linear series-pass mode. Programmable Zener U7
serves as reference for the 5 V digital and analog power supplies. The reference voltage is 5.00 V dc,
+/-0.1 V dc measured at U7 pin 1. Op-amp U8a controls the series pass element Q12 to provide 5 V dc
at the 5 VD source point at C60, C61. The 5 VA supply is low-current and is fully implemented by U8b.
The VOS power supply, used to power the op-amp rails, is regulated to be approximately 11.2 V dc. The
voltage level of the VOS supply is approximately 1.2 V above the 10 V (nominal) CMOS switch power
supply. The reference for VOS is the 5 VD output. The divided output sample (through R178, R179,
and R180) is compared to the reference at Q14 which serves as the control element for the series pass
transistor Q13.
Switching regulator U20 is configured as a “boost regulator” in order to generate an 18 V source from
the 12 V input. The 18 V source provides the reference for the line driver amplifier U14. A voltage
divider (R5, R189, R114, and R162) sets the output voltage. The RC network of C85 and R191 is used
for frequency compensation.
Line Select
Depressions of the
F4/LS
button cause a high at U6-5. This causes the collector of Q18 to toggle, thus
activating or deactivating relay K1 and selecting Line 1 or Line 2. A high at U6-5 selects Line 2, a low at
this pin selects Line 1. Diode D21 absorbs the reverse EMF that occurs when the transistor is switched
off.