Sony SPP-930, Руководство по обслуживанию

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In order to achieve a long stand-by time, the handset conserves power by “sleeping” when not in use and occasionally “waking-up”. In the
“sleep” condition, the handset supplies power only to those circuits deemed essential for proper operation such as the microcontroller (MCU)
and memory. In the “wake-up” condition, in addition to the vital circuits, the handset powers the circuits that allow it to receive data. This
function is necessary to detect if the base requires the handset to act on a condition such as an incoming call. With this sleep/wake-up
sequence, this Machine handset is able to achieve an impressive seven day stand-by time.
4.2.1 Power Control
There are five power lines used in the handset. Four of the lines are supplied by a single 3VDC adjustable regulator and one line
comes straight off the battery. The power lines give the handset the flexibility of powering down circuits that are not needed to
minimize current consumption and to prevent audio signals from interfering with data and vice-versa. The MCU controls the switched
power lines through transistors switches.
The five power lines are labeled MCU_PWR, RX_PWR, TX_PWR, V_ANA and V_BAT. MCU_PWR is a full-time 3VDC regu-
lated line that is used to supply all of the handset circuits except for those in the RF module and microphone biasing. RX_PWR is a
switched in 3VDC regulated line that supplies the RF circuits associated with receiving and demodulating an incoming RF signal.
The TX_PWR is similar to the RX_PWR, but supplies the circuits associated with modulating and transmitting the RF signal.
V_ANA is also a switched 3VDC regulated supply and is used to bias the microphone. This supply is turned on with the TX_PWR
supply and is implemented to isolate noise from the RF section from the microphone circuits. And flatly, V_BAT is a direct line from
the battery and thus can vary from about 3VDC to 4VDC. The V_BAT supply is used to power the ringer which requires a good
voltage and current supply to operate properly. The ringer can also produce noise that can find its way onto its power supply so the
V_BAT line provides some isolation from the rest of the handset’s circuits.
4.2.2 Battery Maintenance and Low Voltage Detect
The battery is recharged via a cradle contact on the base. The handset has a corresponding charge contact at the bottom of the handset
chassis. The charge contact is protected from a short to ground by a diode placed in line with the battery connection. The diode
prevents the battery from discharging from the charge contact. Protection from ESD is afforded by a bypass capacitors installed at the
charge contact.
When the battery voltage drops below the minimum working voltage of the MCU, the phone will not function properly again if the
MCU is not properly reset. Therefore, circuits have been implemented to insure that the battery has sufficient charge for proper
operation.
The heart of the battery maintenance circuit and for that matter the power supply, is the adjustable regulator mentioned earlier. The
regulator features an integral reference to which battery charge is compared to. If the battery voltage drops below 3.3VDC a low
battery line is activated to inform the MCU. The latter action in turn causes the MCU to notify the user by activating the low battery
LED and by producing an audible tone. If the battery falls below 2.8VDC, the regulator turns off the power to prevent the handset
from being used while it operates improperly. A slight hysterics has been designed into the point where the low battery indicators are
turned off when charging. The low battery indicators are disable when the battery voltage exceeds 3.35VDC.
4.3 Audio Path
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.
Afire 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.
4.3.1 Receive Direction (from RF module to ear piece receiver)
The receive audio is transmitted from the base to the handset using frequency modulation (FM). The FM signal from the base enters
the handset’s RF module where the signal undergoes filtering, downconversion and finally demodulation. The baseband audio then
leaves the RF module via the demodulator IC at about -16dBv (for a deviation of +/- 25kHz). From the RF module, the audio is then
fed into a buffer amplifier where the audio is lowpass filtered and directed to both an audio channel and a data channel. The audio
undergoes this split in directions after the buffer amplifier because both data and audio share the same circuits upto and including the
buffer amplifier. The filtering at the buffer amplifier provided some rejection at higher frequencies (> 40kHz).
The buffer amplifier output is connected to an active third order lowpass filter with a - 3dB cut-off set at about 5.5kHz. The filter has
unity gain in its passband. The filtered audio is then passed to an active de-emphasis filter where de-emphasis occurs across the entire
audio band (300Hz to 3,400kHz) at a rate of 6dB/octave or 20dB/decade. After de-emphasis, the audio undergoes the expansion
process and is passed through a transmit audio level control. The level control which has a range of about 20dB, is used to set the