900 MHz Cordless Phone Base Unit Operational Description VTech Telecommunications Ltd

VTech Telecommunications Ltd 900 MHz Cordless Phone Base Unit

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900 MHz DIGITAL
Single line ADPCM
cordless phone with CIDCW
Model: VT1910C
THEORY OF OPERATION
January 29, 1996
PRC: 001R
REVISION 1.0
FILE: PDLPRT15.DOC
THIS DOCUMENT IS PROPRIETARY TO:
VTECH ENGINEERING CANADA LTD.
TABLE OF CONTENTS
SECTION 1 RF SECTION OVERVIEW...........................................................................1
1.0 Introduction..................................................................................................................................................... 1
1.1 Frequency Tables ............................................................................................................................................ 2
1.1.1 Handset....................................................................................................................................................... 2
1.1.1 Base............................................................................................................................................................ 2
1.2 Block Diagram................................................................................................................................................. 3
SECTION 2 RF SECTION DETAILED OPERATION......................................................4
2.1 Antenna Section................................................................................................................................................ 4
2.1.1 Antenna....................................................................................................................................................... 4
2.1.2 Duplexer..................................................................................................................................................... 4
2.1.3 RX, TX Bandpass Filters............................................................................................................................ 4
2.2 Receive Section ................................................................................................................................................. 6
2.2.1 RX Amps and SAW Filter.......................................................................................................................... 6
2.2.2 RX Mixer.................................................................................................................................................... 7
2.2.3 RX VCO and LO Buffer............................................................................................................................. 7
2.2.4 RX Synthesizer........................................................................................................................................... 7
2.2.5 IF Amplifier Stage...................................................................................................................................... 8
2.2.6 IF Filtering.................................................................................................................................................. 8
2.2.7 Demodulator, Data comparator, Mute Comparator .................................................................................... 8
2.3 Transmit Section............................................................................................................................................... 9
2.3.1 TX Amp...................................................................................................................................................... 9
2.3.2 TX VCO ..................................................................................................................................................... 9
2.3.3 TX Synthesizer/PLL................................................................................................................................... 9
SECTION 3 BASEBAND SECTION..............................................................................10
3.0 General Description ...................................................................................................................................... 10
3.1 Detailed Functional Description................................................................................................................... 10
3.1.1 Protocol Functions.................................................................................................................................... 10
3.1.2 Audio Functions ....................................................................................................................................... 10
3.1.3 System Control Functions......................................................................................................................... 10
SECTION 4 COMPLETE SYSTEM DESCRIPTION......................................................13
4.0 General Description for Handset................................................................................................................. 13
4.0.1 RF / ASIC Interface.................................................................................................................................. 13
4.0.2 Handset Serial Buss Connections ............................................................................................................. 13
4.0.3 Other Handset Connections...................................................................................................................... 13
4.1 General Description for Base ....................................................................................................................... 13
4.1.1 RF / ASIC Interface.................................................................................................................................. 13
4.1.2 Base Serial Buss Connections .................................................................................................................. 13
4.1.3 Other Base Connections ........................................................................................................................... 13
Theory of Operation Ver 1.0
VTECH ENGINEERING CANADA LTD. Page 1
Section 1 RF Section Overview
1.0 Introduction
The basic function of the base and handset RF sections is to provide a full
duplex wireless link between the handset and base sections of the
telephone. This is accomplished by setting up two simultaneous
communications links between the handset and base RF boards. The RF
receiver and transmitter circuitry essentially provide a link between the
microphone and speaker in the handset to the telephone line in the base
set. In this way the phone performs exactly as a corded phone, except
without the cord.
The frequency at which the handset transmits to the base is centered
around 926.55 MHz, and the frequency at which the base transmits to the
handset is centered around 903.8 MHz. Unlike a normal 46/49 MHz
cordless phone, which directly modulates audio (voice) onto the RF
carrier, a phone using the Mark III module first digitizes the audio signal
and then modulates high speed data onto the RF carrier. The data rate
which is modulated onto the RF carrier is 48 kbps. On the receiver side,
the data is extracted and then converted back into the original audio
signal.
It is important to note that the synthesizer / prescaler IC only operates up
to 500 MHz. The VCO's therefore oscillate at roughly 450 MHz. The 900
MHz signals needed for the transmit and receive sections are generated
from the second harmonic of the VCO frequencies.
The following section will outline the transmit frequencies used as well as
the corresponding LO frequency which is used for the receiver. This is
followed by the Block diagram and a block by block functional description
of the modules.
Theory of Operation Ver 1.0
VTECH ENGINEERING CANADA LTD. Page 2
1.1 Frequency Tables
This section outlines the RF frequencies and corresponding channel
numbers. The handset uses a high side LO while the base uses a low
side LO to down-convert the incoming signal.
1.1.1 Handset
Channel Transmit Receive RX LO
1 925.05 902.3 913.0
2 925.35 902.6 913.3
3 925.65 902.9 913.6
4 925.95 903.2 913.9
5 926.25 903.5 914.2
6 926.55 903.8 914.5
7 926.85 904.1 914.8
8 927.15 904.4 915.1
9 927.45 904.7 915.4
10 927.75 905.0 915.7
1.1.1 Base
Channel Transmit Receive RX LO
1 902.3 925.05 914.35
2 902.6 925.35 914.65
3 902.9 925.65 914.95
4 903.2 925.95 915.25
5 903.5 926.25 915.55
6 903.8 926.55 915.85
7 904.1 926.85 916.15
8 904.4 927.15 916.45
9 904.7 927.45 916.75
10 905.0 927.75 917.05
Theory of Operation Ver 1.0
VTECH ENGINEERING CANADA LTD. Page 3
1.2 Block Diagram
Both the handset and base RF sectionss follow the same block diagram
shown below with only minor changes to incorporate the different transmit
and receive frequencies.
SAW
FILTER
MIXER
SYNTHESIZE
R
10.7 MHz
CERAMIC
DATA
O/P
18.25 MHz
REFERENCE
3 LINE DATA
BUS
RX L.O.
DUAL
PRESCALER/
457 MHz
Fin
Fin V-tune
V-Tune
2nd RX
AMP
10.7 MHz
CERAMIC
MC13156
2 POLE
CERAMIC
2 POLE
CERAMIC
1st R
X
AMP
LOW
PASS
TX O/P
AMP
IF AMP
TX Data
Figure 1. RF Section Block Diagram
As can be seen by the block diagram, there are several important
input/output signals which are necessary for operation of the RF section
(this does not include the separate supply lines for both TX and RX
sections). An 18.25 MHz reference is present for use in the frequency
synthesizers. The accuracy of this 18.25 MHz input will affect the transmit
and receive frequencies. In order to ensure proper operation of the RF
sections, the 18.25 MHz reference signal must be at least 500 mV in
amplitude. Also present is the 3-line serial data bus on which data is
transferred to the synthesizers to set both the transmit and receive
frequencies.
The modulation input allows digital data to be modulated directly onto the
TX carrier. The Data output is the demodulated signal after being filtered
and shaped by a comparator. The data is then sent to the AMD ASIC
where the original voice signal is reconstructed.
The RF section performs a single down-conversion of the incoming RF
signal to 10.7 MHz where it is demodulated and sent to the AMD ASIC.
The transmit section directly modulates the carrier.
The following section explains the individual blocks in the RF section in
detail. All reference to part numbers correspond to the handset
schematic.
Theory of Operation Ver 1.0
VTECH ENGINEERING CANADA LTD. Page 4
Section 2 RF Section Detailed Operation
2.1 Antenna Section
2.1.1 Antenna
The antenna is a device which allows effective conversion of energy from
air to the RF circuitry. The antennas used are a retractable 1/2 wave with
2.5 dB gain relative to an isotropic radiator and a fixed 1/4 wave antenna
with roughly 0 dB gain relative to an isotropic radiator. The duplexer and
filters which follow the antenna, require a 50 ohm match to operate
properly. The antenna is not matched to 50 ohms and requires a simple
microstrip matching network to achieve this. If a network analyzer is
attached to the BFA connector after disconnecting the duplexer, the
antenna match may be measured. In order to achieve a good 50 ohm
match, one must be careful not to obstruct the antenna as any object near
the antenna will affect its impedance.
2.1.2 Duplexer
The Duplexer ensures that the two bandpass filters do not interact with
each other. It accomplishes this by making each filter see a high
impedance from the opposite filter in its own passband. This is necessary
to ensure that both filters work effectively when connected together. If the
Duplexer were not present, mismatches from one filter would cause the
passband of the other to be distorted and this would degrade
performance.
The Duplexer itself is simply composed of two microstrip and discrete
filters which shift each filters out of band match to a high impedance. To
ensure that the Duplexer is operating correctly, the match looking into the
filters from the BFA connector may be measured. To do this it is
necessary to remove the 0 ohm resistor which connects the antenna to
the Duplexer. A return loss of approximately 15 dB should be measured
for both the TX and RX bands.
2.1.3 RX, TX Bandpass Filters
The RX and TX bandpass filters provide two functions. The first is to
effectively pass the correct frequencies to the RX and TX sections. It is
important especially for the RX section that these filters have a low
insertion loss in order to ensure a low front end noise figure. These filters
are also designed to provide > 25 dB rejection for the opposite band. This
means that the transmit carrier will be attenuated by at least 25 dB before
entering the receive section of the phone. A plot of the low band filter is
shown below.
Theory of Operation Ver 1.0
VTECH ENGINEERING CANADA LTD. Page 5
0 dB
10 dB/div
925-928MHz Reject Band
902-905MHz Passband
815 101
5
Figure 2 Low band Ceramic filer response
For this filter the insertion loss is less than 3dB at 902 to 905MHz while
the 925-928MHz band has >25dB attenuation. This filter is used for the
RX filter in the handset or the TX filter in the base. The high band filter
characteristic is shown below. This filter is used for the handset TX filter
and base RX filter.
0 dB
10 dB/div
925-928MHz Passband
902-905MHz Reject Band
815 101
5
Figure 3 High band ceramic filter response
Theory of Operation Ver 1.0
VTECH ENGINEERING CANADA LTD. Page 6
2.2 Receive Section
2.2.1 RX Amps and SAW Filter
The purpose of the first RX amp is to provide enough gain that the noise
figure of the RX section is fixed to as low a value as possible. It must
provide a good 50 match to both the RX bandpass filter and the SAW
filter. This amplifier must also have good power handling capability due to
the limited filtering which precedes it. The design employs a collector
inductor to improve the output power capability of the transistor. This form
of matching also ensures that the gain of this stage is not too wide band
further improving its performance by allowing it to effectively reject signals
which are far out of its passband.
Directly following the first RX amp is the SAW filter. This filter is
responsible for the bulk of the filtering in the receive section. It provides
more than 40dB of image rejection and TX carrier suppression. The
insertion loss of this filter is relatively high due to its SAW implementation.
It has an insertion loss of less than 5 dB, typically 4 dB. An amplifier is
required before this SAW filter to keep the noise figure low. If it were not
present, the noise figure of the phone would increase by the 4 dB loss
associated with the SAW filter.
The second RX amp provides a limited amount of gain. Its main function
is to ensure that the mixer sees a good wideband match. Measuring the
RX gain from the BFA connector to the output of this amplifier will produce
results as shown in Figure 4 below.
0 dB
10 dB/div
875 97
5
925-928MHz Passband
902-905MHz Reject Band
Figure 4. RX Front end Response
Theory of Operation Ver 1.0
VTECH ENGINEERING CANADA LTD. Page 7
2.2.2 RX Mixer
The function of the mixer is to combine the incoming signal with a LO
signal in order to convert the desired signal to the 10.7 MHz IF frequency.
The mixer used for this task is a dual gate FET (CF739R). The LO and
RF signals are placed on the gates of the FET and the IF signal is coupled
off of the drain. The FET provides conversion gain along with adequate
power handling characteristics. Both the RF and LO ports are shorted to
ground by rectangular microstrip inductors. These inductors provide a
high impedance at both the RF and LO frequencies while presenting a
very low impedance at the IF frequency. The mixer is followed by an
emitter follower which converts the high impedance output of the mixer to
a 330 ohm output suitable for directly driving the IF ceramic filters. The
gain for the pair (mixer and follower) is about 8 dB (50 ohms in, 330 ohms
out).
2.2.3 RX VCO and LO Buffer
The RX VCO is a Colpitt's type oscillator operating at about 450 MHz with
a frequency selective network tuned to about 900 MHz on the collector.
The frequency of oscillation is controlled by a varactor diode in the tank
circuit connected to the base of the transistor. This diode is connected to
the loop voltage from the RX synthesizer. Rough tuning is achieved with
a variable chip cap. This capacitor is used to center the tuning voltage to
ensure reliable operation over a wide temperature range and also to
compensate for variances in component values.
The 450 MHz LO for the PLL is coupled off of the emitter of the VCO
transistor. This is lightly coupled to ensure that the VCO is not loaded by
the PLL. The LO Buffer isolates the PLL from the VCO preventing the TX
VCO from interfering with the RX VCO and vice versa. The 900 MHz RX
LO signal for the Mixer is coupled off the collector of the VCO transistor.
2.2.4 RX Synthesizer
The PLL and prescaler for both the TX and RX sides are now combined
into one IC. The Synthesizer receives channel information from the
embedded microprocessor in the AMD ASIC via the serial buss. It also
requires a stable 18.25 MHz reference which is also supplied from the
AMD ASIC.
A passive loop filter is employed to connect the synthesizer to the VCO.
This tuning voltage may be observed from test point Z1 on the bottom
side of the PCB. The loop filter cutoff frequency is set to about 1 kHz to
allow relatively fast power-up times.
Theory of Operation Ver 1.0
VTECH ENGINEERING CANADA LTD. Page 8
2.2.5 IF Amplifier Stage
There is only one stage of discrete IF amplification. Transistor Q5 is used
as an amplifier with 330 ohms input and output impedance. The rest of
the IF gain is provided by the FM demod IC discussed below.
2.2.6 IF Filtering
The choice of 10.7 MHz as an IF frequency, allows the use of relatively
inexpensive filters. Two ceramic filters are used to achieve the desired
adjacent channel suppression. Two different bandwidth filters are used,
230 kHz and 150 kHz, so that any shifting in the passband does not
narrow the bandwidth excessively.
2.2.7 Demodulator, Data comparator, Mute Comparator
This RF section uses a MC13156 FSK demodulator. It incorporates all
three of the above functions into a single IC.
The Quadrature voltage may be observed form test point [TBD]. This
voltage should nominally be 1.2 V when a signal is center tuned.
The data stream which comes out of the demodulator has a peak to peak
amplitude of approx. 0.5V. In order to be of use it is first filtered and
converted to a digital (0 to 5V) signal by using a comparator inside the IC.
Theory of Operation Ver 1.0
VTECH ENGINEERING CANADA LTD. Page 9
2.3 Transmit Section
2.3.1 TX Amp
There is one transistor which provide the necessary gain for the transmit
section. Transistor [TBD] amplifies the signal from the TX VCO. The
output power is set to the required -5 dBm (this is the power level at the
BFA connector required to guarantee less then 50mV/meter radiated field
strength measured at 3 meters).
2.3.2 TX VCO
The basic operation of the TX VCO is the same as the RX VCO, except
for one detail. The TX VCO is also FSK modulated by the transmit data
through a second varactor in the tank (25kHz peak to peak). The data is
first filtered and then the amplitude is adjusted through [TBD] to set the
deviation of the data modulation.
2.3.3 TX Synthesizer/PLL
The TX PLL is combined into one IC with the RX PLL. See above. The
loop filter cutoff frequency is about 100 Hz. This allows the data
modulation to include frequencies down to about 100 Hz. The power-up
time of the TX PLL is not critical.
Theory of Operation Ver 1.0
VTECH ENGINEERING CANADA LTD. Page 10
Section 3 Baseband Section
3.0 General Description
The AMD ASIC is a custom designed IC based on the core platform of the
AMD CT2 baseband chipset. This custom modification contracted by
VTech removes the protocol blocks associated with the CT2 TDD
architecture and replaces it with a VTech proprietary FDD protocol block.
The AMD ASIC performs virtually all the non-RF functions with the
exception of the line interface block on the base unit and the LCD driver
function on the handset. The base and handset ASIC’s are identical with
the exception of the ROM code which will be masked into the ASIC prior
to mass production.
3.1 Detailed Functional Description
The principle components are:
3.1.1 Protocol Functions
The protocol block conducts a signaling and a voice channel in the
transmit and receive directions. All data I/O are CMOS levels.
3.1.2 Audio Functions
1) CODEC. The CODEC transcodes analog voice signals and 32
kbps ADPCM data.
2) Audio Front End. The audio front end connects the analog voice
I/O pins to the CODEC.
3) Tone Ringer. The tone ringer produces digital square-wave ringing
tone signals for output on the RING pin.
4) Biasing. The biasing circuits establish precision currents and
voltage references to support audio and battery detection analog
operations.
5) DTMF Generator. The DTMF generator produces digitally-
generated tones for DTMF dialing and call progress tones.
3.1.3 System Control Functions
1) Microcontroller. An 8-bit 80C32T2 microcontroller executes the
program and controls the protocol logic and other hardware
configuration. It includes 12 Kbytes of mask-programmable ROM
and 256 bytes of RAM. It also includes an asynchronous serial
port.
2) Synchronous Serial Port. The serial port provides a synchronous
serial link to devices such as RF synthesizers, serial EEPROMs,
etc.
Theory of Operation Ver 1.0
VTECH ENGINEERING CANADA LTD. Page 11
3) Peripheral Ports. The peripheral ports are for general purpose I/O
functions. One port is designed such that any change of state
generates an interrupt for the key scanning function.
4) Battery Level Detector. The battery level detector reports the low
battery condition to prevent misoperation when batteries are low. It
also emits a dead battery control signal which can be used to hold
the device in a disabled, low power state and a high level detection
for battery charging control.
5) Watchdog Timer. The watchdog timer protects the system from
errant software by periodically issuing a reset unless serviced by
software.
6) Wakeup Timer. The wakeup timer is a multivibrator controlled by
external passive components to effect a low power periodic wakeup
for call detection.
7) Interrupt Controller. The interrupt controller structures the various
interrupts for manageable service by the microcontroller.
8) Clock Generator. The clock generator creates required internal
timing signals from the crystal operating at 18.25 MHz. It also
generates a PLL reference at the crystal frequency.
9) Address decoder. The address decoder generates strobes
accessing selected address spaces in the device.
Theory of Operation Ver 1.0
VTECH ENGINEERING CANADA LTD. Page 12
ADPCM
CODEC
DTMF
GENERATOR
AUDIO
FRONT
RINGER
BIASING
80C32T2
Microcontroller
ROM: 12K bytes
INTERRUPT
CONTROLLER
AUDIO FUNCTIONS
SYSTEM CONTROL FUNCTIONS
TXBB
XTAL1
WAKEUP WAKEUP
TIMER
IREF
CFILT
AIN
AOP
AON
/INT0
/INT1
ALE
SERIAL
PORT SCLK
SDIN
SDOUT
/PSEN
P0[7:0]
P1[7:0]
P2[5:0]
P3[1:0]
PROTOCOL
FUNCTIONS
DETECT
PARALLEL
PORTS
P4[4:0]
BATTERY
FORMATTER
RXBB
RAM: 256 bytes
XTAL2
BATMON
MODE0
BAT
RING
GENERATOR
CLOCK
MODE1
P3[7:6]
SYNCHRONOUS
P5[4:0]
P6[5:0]
/RESET WATCHDOG
TIMER
CPUCLK
and END
SELECT
MODE
PLLREF
DEAD
DIGITAL
TST0
MON[3:0]
TST1
FRAME
Figure 5. AMD ASIC - Internal Structure Diagram
Theory of Operation Ver 1.0
VTECH ENGINEERING CANADA LTD. Page 13
Section 4 Complete System Description
4.0 General Description for Handset
4.0.1 RF / ASIC Interface
Basically the interface between the RF section for both base and handset
and the AMD ASIC is the same. There is the serial buss connection to
control the synthesizers, and there are separate controls for powering the
transmit and receive sections of the radio.
4.0.2 Handset Serial Buss Connections
Also connected on the serial buss is an EEPROM used to store CID
information and the handset security code. In addition, an LCD driver is
also tied onto the same serial buss and is used to control the LCD display
functions on the handset
4.0.3 Other Handset Connections
The handset microphone, receiver and alerter (ringer) are connected
directly to the AMD ASIC. General I/O is used for keyboard scanning as
well as to control other miscellaneous functions such as LCD and keypad
backlighting.
4.1 General Description for Base
4.1.1 RF / ASIC Interface
Basically the interface between the RF section for both base and handset
and the AMD ASIC is the same. There is the serial buss connection to
control the synthesizers, and there are separate controls for powering the
transmit and receive sections of the radio.
4.1.2 Base Serial Buss Connections
Also connected on the serial buss is an EEPROM used to store the base
security code as well as all the user programmed speed dial numbers.
4.1.3 Other Base Connections
The line interface section is discrete and is external to the AMD ASIC.
After the line interface section and hybrid, the transmit and receive audio
path are routed to the corresponding audio connections on the AMD
ASIC. A discrete Caller ID signal decoder IC is connected in the line
interface and the decoded data is routed to the AMD ASIC.
General I/O is used for keyboard scanning as well as to control other
miscellaneous functions such as LED illumination.
Download: 900 MHz Cordless Phone Base Unit Operational Description VTech Telecommunications Ltd
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Document ID63291
Application IDiJL5+s88eMZlVQRjV8YscA==
Document DescriptionRF THEORY
Short Term ConfidentialNo
Permanent ConfidentialNo
SupercedeNo
Document TypeOperational Description
Display FormatAdobe Acrobat PDF - pdf
Filesize76147
Date Submitted1999-10-12 00:00:00
Date Available1999-12-20 00:00:00
Creation Date2001-05-22 11:24:54
Producing SoftwareAcrobat Distiller 4.0 for Windows
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