BARRETT 2061 OPERATING AND TECHNICAL MANUAL
6
Transmitter keying (VOX)
For a normal link, the VOX in the 2061 is set up to key the transmitter. The VOX responds to signals on the
telephone line and operates the keying circuits when voice is present. Once the VOX has tripped, it stays
active for a short time following the loss of audio to prevent dropout on pauses between syllables. This
delay is called VOX hang time.
The VOX in the 2061 is generated within the DSP and is extremely sensitive and reliable.
Adjusting VOX sensitivity
Where non-standard or poor quality telephone lines are used for connection to the 2061, adjustment of the
VOX sensitivity may be necessary. This is achieved by adjusting the “VOX Sensitivity” pot rear of the unit
using a small Philips head screwdriver. If the VOX sensitivity is too low (transmitter does not key reliably on
voice), rotate the pot clockwise. If the VOX sensitivity is too high (transmitter keys on noise other than
spoken words), rotate the pot anti-clockwise
Technical description
The basic blocks of the 2061 include a phone line interface, a transceiver interface, a hybrid,
microprocessor and speaker amplifier. The connect switch and operator override and their corresponding
LED's are to be found on the front panel and are connected to the main PCB via a loom connector (P7).
The phone line interface involves a line connect relay (RL1) which diverts the phone audio to either the
hybrid, when the unit is connected, or to the handset. A second relay (RL2) has the function of placing both
the handset and the hybrid on line during operator override, where the operator talks to both parties. A
gyrator (Q4, Q5 etc) is situated between the line connect relay and the hybrid and is used for looping the
line thereby informing the exchange that the line has been seized in the “CONNECT” mode.
The hybrid (U5) interfaces the phone line to the transceiver and has the primary function of canceling the
reflected transceiver line audio. The hybrid is connected to the line via a simple resistor network and audio
transformer. The transmit audio input and received line audio output connect the hybrid to the transceiver
via a filter and various balancing/unbalancing audio amplifiers (U1, U2, U3). Adaption to the line is initiated
by a microprocessor and takes around one second. During this time the transceiver line audio is muted via
a FET (Q2). The algorithm is continuously adaptive, always trying to null the transmitted audio reflected by
the line. A trimpot (POT1) is required to set the transceiver audio to the hybrid so that it is at an acceptable
level for a caller. The second trimpot (POT2) sets the hybrid audio to transceiver and is also very important
for setting the VOX level. Although setup in the factory this can be later adjusted after installation to cater
for various line conditions.
The received line audio is converted to balanced audio for interfacing to the transceiver. One of these
balanced outputs is passed through a precision rectifier (U3) and converted to DC for processing by a PIC
microprocessor (U8). Thus the VOX state including the hang time can be determined and transceivers
PTT line activated accordingly. Other functions of this microprocessor are to read the state of the switches,
drive the state LED's, activate the relays and to initiate the hybrid balance sequence.
The speaker amplifier (U9) provides the operator with a mixed signal of radio operator and caller audio.
Volume control is provided by a potentiometer mounted on the back (P6). To prevent acoustic feedback a
muting switch (Q6) is automatically activated when the operator bypass is enable.
Summary of Contents for 2061
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