Sony SPP-930, Руководство по обслуживанию

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Sidetone cancellation is accomplished by taking the reflected transmit audio (the sidetone) and resistively combining it with out-of-
phase transmit audio. In a real-world situation, the match between the line interface and the telephone line is not perfect. This slight
mis-match results in some transmit audio being reflected back in the receive direction. The sidetone cancellation signal is created by
tapping off the signal found on the emitter resistor of the transistor amplifier which is inherently 180
°
out of phase with the transistor’s
collector (i.e., the sidetone source).
3.3 Power Management
This Machine base power circuits consists of DC power regulation and charging circuits for the handset and spare batteries. The base unit
operates on a regulated 5VDC power supply. The power is supplied to the regulator via a UL approved 9VDC, 300mA power adapter. During
normal operation, the base unit draws about 100mA of current (add 30mA with the spare battery and add 60mA when the handset is in the
cradle).
3.3.1 Power Supply
DC power is supplied to the base via a UL approved AC to DC power adapter rated at 9VDC, 300mA. The power from the adapter
is then regulated down to 5VDC. Filter capacitors are connected to both sides of the 5VDC regulator to ensure AC variations are
eliminated from the power lines. An LED is used to indicate the presence of the 5VDC supply.
3.3.2 Handset Charge Circuit
To reduce costs by keeping circuits simple, the handset charge circuit is designed to supply a charging current to a cradled handset
regardless of whether the battery is fully charged or not. This current varies with the charge on the battery and is limited to 0.1C or
10% of the batteries capacity by a limiting resistor.
In this Machine, the handset battery has a capacity of 600mA, thus the maximum charging current is set to approximately 60mA. The
specification of 0.1C allows a battery to be constantly charged without damaging the battery. The handset charge circuit components
have been selected to withstand shorting the charge contacts on the cradle. The handset charge circuit also provides a signal to the
MCU for cradle detection and an LED labeled, “Charge” , to indicate the on-cradle condition.
3.3.3 Spare Battery Charge Circuit
This Machine features a spare battery charger. The circuit for this charger is similar to that of the handset charger except that the
charging current has been limited to 0.05C or 30mA. The lower limit has been implemented to accommodate prolonged charging. As
with the handset charger, this charging circuit also activates an LED (labeled, “Spare Bat”) when a spare battery is placed in the
charger.
Note, the spare battery does not supply any power to the base circuits in the event of a power failure.
3.3.4 ESD Protection
The charge contacts for the spare battery and handset are vulnerable to ESD because they of their exposure to the outside world.
Since the contacts are connected directly to the base’s circuits. ESD can damage some of the base’s internal circuits if no protection
is implemented. Tuerefore, a number of measures have been taken to protect internal circuits from ESD damage.
Since the MCU is connected directly to a charge contact and its ground reference is connected to another, care must be taken to
prevent ESD from damaging the MCU and corrupting the ground reference. All charge contacts have LC filtering on them to bypass
ESD. Low voltage spark gaps (arc at – 200V based on 1kV/mm electric discharge through air) are also implemented in the PC board
layout between the charge contacts and a special ESD ground. This ESD ground channels any ESD discharge directly to the AC
adapter preventing discharges from entering the main circuits. A spark gap can also be found between the ESD ground and the
antenna.
3.4 Audio Circuits
Audio circuits are necessary to condition speech for RF transmission and reception. The conditioning includes amplification, filtering, pre-
emphasis/de-emphasis and compression/expansion, all of which ensures that the speech is received and transmitted with maximum clarity
and legibility.
Pre-emphasis/de-emphasis is used to improve signal-to-noise ratio which is, as a consequence of frequency or phase modulation, degraded
at high audio frequencies. Compression/expansion is also used to improve the perceived signal-to-noise ratio by reducing the noise vulner-
ability of low level signals. The compression process amplifies low level signals more than it does for high level signals. Thus, by compress-
ing the dynamic range of the audio before transmission, noise picked up during transmission has less of an effect on the low level signals.
After receiving the transmission, the expansion process maintains this improved signal-to-noise ratio while restoring the low level signals
back to their original levels.
The audio circuits are implemented around a compandor IC (TA31103F). The IC provides compression/expansion, amplification and muting
all in a clean, simple package. The IC is a good compromise between the parts cost, flexibility, size, and performance. Refer to the IC’s data
sheets for a detailed description of its operation.
3.4.1 Transmit Direction (from RF module to telephone line)
The transmit audio is transmitted from the handset to the base using frequency modulation (FM). The FM signal from the handset
enters the base’s RF module where the signal undergoes filtering, downconversion and finally demodulation. The baseband audio