Sony SPP-930, Руководство по обслуживанию

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BASE UNIT SECTION
3.1 Introduction
This Machine is a 900MHz analog cordless telephone. The phone was developed with low cost and high performance in mind. The phone
minimizes costs by using analog speech transmission and by utilizing a number of design innovations that reduce production costs. The
speech transmission differs from our digital phones in that digital speech encoding and decoding does not occur in speech transmissions. This
means that elaborate software algorithms and additional hardware are avoided in the design.
This Machine base unit is a single PC board design. All RF, audio, MCU control and power management components are placed on a single
PC board. The use of a single PC board reduces costs associated with ATE testing, handling and assembring. This document describes the
technical aspects and operation of this Machine base unit.
3.2 Telephone Line Interface
The analog this Machine low cost phone’s telephone line interface couples audio and line signaling to and from the telephone line while
isolating the phone from the telephone line. The interface provides basic 2-wire to 4-wire conversion for the audio and facilitates pulse
dialing and ring detection. Isolation is achieved by inductively coupling audio through a transformer and by coupling line signaling through
opto-couplers. The isolation is necessary for the phone to meet the FCC 1.5kV high-pot requirement. The interface also provides protection
from high voltage transients and surge currents.
3.2.1 Line Protection and Filtering
The audio signal from the central office or PABX is carried by the telephone line (tip and ring) to the phonejack. The two lines are
also used to carry the ring signal (40 - 150 VRMS, 15.3 - 68Hz) and various line signaling (i,e., DTMF, dial tone, etc,). After the
phone jack, inductors are installed to reduce the effect of EM fields that could find its way onto the telephone line. The inductors also
serve to minimize any undesired RF emissions that could escape from the phone and onto the telephone line. Inductors are also
installed near the power jack to isolate the power line from similar high frequency signals.
A fuse is installed in the telephone loop to limit the loop current to no greater than 250mA. A varistor is used to limit the voltage
across the line interface when a high voltage transient appears on the telephone line (i.e., lightning strikes). Further high voltage
protection (1.5kV) is afforded by the audio transformer’s isolation and high voltage spark gaps found in the layout of the PC board.
The voltage spark gaps are designed to arc across at about 2kV allowing the phone to undergo the 1.5kV hi-pot test while preventing
component damage from higher voltages.
3.2.2 Ring Detect
The ring signal across the tip and ring is detected by an AC coupled opto-coupler. Zener diodes are used to set the threshold detection
voltage and to meet the no-ring response/impedance requirement for EIA-470. Resistors limit the current flow into the opto-coupler
and maintain the necessary ringing impedance as specified in EIA-470. The diode across the opto-coupler input provides a discharge
path for the coupling capacitor during negative ring cycles.
On the opto-coupler output side, a pull up resistor is used to set the transistor’s collector current (i.e., sensitivity control) when a ring
signal is detected. The output of the opto-coupler is connected to the MCU where the ring signal is analyzed for validity. A typical
ringing pattern from the central office is one second ON and four seconds OFF. The presence of a ringing signal at the base is
indicated by flashing the “In Use” LED.
3.2.3 Pulse Dialing or Off-Hook Switch
For pulse dialing (8-11 PPS, 58-64% break, interval 53-80 ms), an opto-coupler is used to make and break the telephone line loop. In
order for the phone to function normally irrespective of the polarity on the tip and ring, a diode bridge is used to ensure the potential
on the collector of the opto-coupler is positive with respect to its emitter. The opto-coupler input is connected to the MCU from
which the required state of the opto-coupler is controlled. For pulse dialing, the opto-coupler is simply pulsed off and on at the
appropriate rate. To set the phone off-hook, the opto-coupler is activated which closes the telephone loop. The off-hook condition is
indicated by turning on an LED on the base labeled “In Use”.
3.2.4 Speech Circuit
To minimize costs, a speech network IC is not used in this Machine design. Instead, an isolation transformer with supporting hard-
ware is used to provide all the speech network functions. The speech circuit provides line impedance matching, 2-wire to 4-wire
conversion and sidetone cancellation.
Matching (or return loss) is at its best when the termination impedance equals the source impedance. The effective impedance
looking into this Machine is a combination of the component impedances in the line interface. This effective impedance was derived
empirically by fine tuning the resistor across the audio transformer’s secondary. The speech circuit matches a line impedance of 600
Ω
(EIA-470 : 4.5.2.3) while the transmit, receive and sidetone frequency responses are set with a 900
Ω
line impedance (EIA-470 : 4.
l. 1 - 4.1.3).
The 2-wire to 4-wire (or 4 to 2) conversion is accomplished by transmitting audio to the transformer’s secondary via a uni-directional
buffer and receiving audio directly from the transformer’s secondary. The uni-directional buffer is a common emitter transistor
amplifier whose high impedance collector. which is connected to the transformer, prevents received audio from entering the transmit
path while allowing transmit audio to pass to the transformer.