VTech Telecommunications 80-4258-05 SPP-ID970 Cordless Telephone User Manual RF Module Theory of Operation

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Document ID	107274
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Document Description	RF Module Theory of Operation
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Document Type	User Manual
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Date Submitted	2000-06-30 00:00:00
Date Available	2000-08-31 00:00:00
Creation Date	2000-06-30 06:37:57
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Document Title	RF Module Theory of Operation

VTECH ENGINEERING CANADA LIMITED
TITLE
PDL MK3 with U2768 RF Theory Of Operation
MODEL
Sony 970
PDL MK3 with U2768
RF Module
THEORY OF OPERATION
Note: All rights reserved. No part of this publication may be reproduced or transmitted in any form or by
any means. The only controlled copy of this document is kept in electronic form on the VTECH computer
network. All printed copies are uncontrolled documents and are not updated. This Procedure is the
property of VTECH Engineering Canada Ltd., and shall be returned upon request
Prepared By
Title
Checked By
Title
John Wang
RF Engineer
Sarkis Teghararian
RF Development Manager
Reference:
Revision: 0
This document is proprietary to VTECH ENGINEERING CANADA LTD.
Specifications are preliminary and are subject to change without notice.
Page: 1 of 29
107274
VTECH ENGINEERING CANADA LIMITED
TITLE
PDL MK3 with U2768 RF Theory Of Operation
MODEL
Sony 970
Revision History
Revision No
Reference:
Description
Initial Release
Page
All
Revision: 0
This document is proprietary to VTECH ENGINEERING CANADA LTD.
Specifications are preliminary and are subject to change without notice.
Effective Date
June 9, 2000
Page: 2 of 29
107274
VTECH ENGINEERING CANADA LIMITED
TITLE
PDL MK3 with U2768 RF Theory Of Operation
MODEL
Sony 970
Table of Contents
RF SECTION OVERVIEW .....................................................................................4
1.1
Introduction .................................................................................................................................................4
1.2
Overview of changes for U2768...................................................................................................................4
1.3
Frequency Tables ......................................................................................................................................5
1.3.1
Handset Frequencies ...............................................................................................................................5
1.3.2
Base Frequencies ....................................................................................................................................6
1.4
Block Diagram.............................................................................................................................................7
RF SECTION DETAILED OPERATION..................................................................8
2.1
Antenna Section .........................................................................................................................................8
2.1.1
Antenna ...................................................................................................................................................8
2.1.2
SAW Duplexer ........................................................................................................................................8
2.2
Receive Section..........................................................................................................................................8
2.2.1
LNA ........................................................................................................................................................8
2.2.2
RX Mixer ................................................................................................................................................8
2.2.3
RX VCO..................................................................................................................................................8
2.2.4
RX Synthesizer........................................................................................................................................8
2.2.5
IF Amplifier Stage...................................................................................................................................9
2.2.6
IF Filtering..............................................................................................................................................9
2.2.7
Demodulator............................................................................................................................................9
2.2.7.1
RSSI Comparator ................................................................................................................................9
2.3
Transmit Section........................................................................................................................................9
2.3.1
TX Amp ..................................................................................................................................................9
2.3.2
TX VCO..................................................................................................................................................9
2.3.3
TX Synthesizer/PLL................................................................................................................................9
Reference:
Revision: 0
This document is proprietary to VTECH ENGINEERING CANADA LTD.
Specifications are preliminary and are subject to change without notice.
Page: 3 of 29
107274
VTECH ENGINEERING CANADA LIMITED
TITLE
PDL MK3 with U2768 RF Theory Of Operation
MODEL
Sony 970
1 RF Section Overview
1.1
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 provides a link between the microphone and receiver 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 922.45 MHz, and the
frequency at which the base transmits to the handset is centered around 903.8 MHz. 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.
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.
1.2
Overview of changes for U2768
The U2768 RF Module design is based on the original U2765 design. The U2768 contains:
1.
2.
3.
4.
5.
6.
All sections of U2765 IC
LNA Front End
RX VCO operating at 900MHz
Dual PLL (900MHz/450MHz)
PLL Demodulator.
Replaces the original mixer with an Image Reject Version
These sections remove a considerable amount of discrete circuits. The Baseband interface and test point
locations are kept the same as to minimize the differences between the modules.
This module also removes the interstage SAW filter. For us to maintain and improve the pager rejection,
we had to shift the Base RX frequencies. The updated table is shown below.
Reference:
Revision: 0
This document is proprietary to VTECH ENGINEERING CANADA LTD.
Specifications are preliminary and are subject to change without notice.
Page: 4 of 29
107274
VTECH ENGINEERING CANADA LIMITED
TITLE
PDL MK3 with U2768 RF Theory Of Operation
MODEL
Sony 970
1.3
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.
1.3.1
Handset Frequencies
Channel
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
Reference:
Transmit
920.25
920.4
920.55
920.7
920.85
921
921.15
921.3
921.45
921.6
921.75
921.9
922.05
922.2
922.35
922.5
922.65
922.8
922.95
923.1
923.25
923.4
923.55
923.7
923.85
924
924.15
924.3
924.45
924.6
Receive
902.3
902.45
902.6
902.75
902.9
903.05
903.2
903.35
903.5
903.65
903.8
903.95
904.1
904.25
904.4
904.55
904.7
904.85
905
905.15
905.3
905.45
905.6
905.75
905.9
906.05
906.2
906.35
906.5
906.65
RX LO
913
913.15
913.3
913.45
913.6
913.75
913.9
914.05
914.2
914.35
914.5
914.65
914.8
914.95
915.1
915.25
915.4
915.55
915.7
915.85
916
916.15
916.3
916.45
916.6
916.75
916.9
917.05
917.2
917.35
Revision: 0
This document is proprietary to VTECH ENGINEERING CANADA LTD.
Specifications are preliminary and are subject to change without notice.
Page: 5 of 29
107274
VTECH ENGINEERING CANADA LIMITED
TITLE
PDL MK3 with U2768 RF Theory Of Operation
MODEL
Sony 970
1.3.2
Base Frequencies
Channel
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
Reference:
Transmit
902.3
902.45
902.6
902.75
902.9
903.05
903.2
903.35
903.5
903.65
903.8
903.95
904.1
904.25
904.4
904.55
904.7
904.85
905
905.15
905.3
905.45
905.6
905.75
905.9
906.05
906.2
906.35
906.5
906.65
Receive
920.25
920.4
920.55
920.7
920.85
921
921.15
921.3
921.45
921.6
921.75
921.9
922.05
922.2
922.35
922.5
922.65
922.8
922.95
923.1
923.25
923.4
923.55
923.7
923.85
924
924.15
924.3
924.45
924.6
RX LO
909.55
909.7
909.85
910
910.15
910.3
910.45
910.6
910.75
910.9
911.05
911.2
911.35
911.5
911.65
911.8
911.95
912.1
912.25
912.4
912.55
912.7
912.85
913
913.15
913.3
913.45
913.6
913.75
913.9
Revision: 0
This document is proprietary to VTECH ENGINEERING CANADA LTD.
Specifications are preliminary and are subject to change without notice.
Page: 6 of 29
107274
VTECH ENGINEERING CANADA LIMITED
TITLE
PDL MK3 with U2768 RF Theory Of Operation
MODEL
Sony 970
1.4
Block Diagram
Both the handset and base RF sections follow the same block diagram shown below with only minor
changes to incorporate the different transmit and receive frequencies.
10.7MHz
10.7MHz
MIXER
LNA
LIM1
FM PLL
Demod
LIM2
RX
Audio
V-Tune
RSSI
900MHz.
RX_LO.
DUAL PLL
450MHz
SAW DUPLEXER
Fin
Loop
Filter
TEMIC U2768B
V-Tune
18.4MHz
REFERENCE
TX
PAD
TX_LO.
3 LINE DATA BUS
TX O/P
AMP
TX Data
Figure 2.3 RF Section Block Diagram
As can be seen by the block diagram, there are several important input/output signals that are necessary
for operation of the RF section (this does not include the separate supply lines for both TX and RX
sections). An 18.4 MHz reference is present for use in the frequency synthesizers that built inside the
U2768. The accuracy of this 18.4 MHz input will affect the transmitting and receiving frequencies. In order
to ensure proper operation of the RF sections, the 18.4 MHz reference signal must be at least 500 mV in
amplitude. Also present is the 3-line serial synchronous data bus on which data is transferred to the
synthesizers to set both transmit and receive frequencies.
In the transmit direction, modulation input allows digital data to be modulated directly onto the TX carrier.
In the receive direction, the receiver RF signal will directly go to the U2768 chip. After LNA and Mixer, the
RF signal will be converted to a 10.7 MHz IF signal. The IF signal will be amplified by LIM1, LIM2 and
demodulated by FM PLL Demod. The demodulated signal is sent to Baseband section.
The following sections explain in detail the individual blocks in the RF section module. All references to part
numbers correspond to the handset schematic.
Reference:
Revision: 0
This document is proprietary to VTECH ENGINEERING CANADA LTD.
Specifications are preliminary and are subject to change without notice.
Page: 7 of 29
107274
VTECH ENGINEERING CANADA LIMITED
TITLE
PDL MK3 with U2768 RF Theory Of Operation
MODEL
Sony 970
2 RF Section Detailed Operation
2.1
Antenna Section
2.1.1 Antenna
The antenna is a device that 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 that follow
the antenna, require a 50 ohm match to operate properly. The antenna is roughly 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 SAW Duplexer
The SAW Duplexer consists of two bandpass filters to provide the necessary isolation between transmit
and receive sections and to pass the correct frequencies to transmit and receive sections. 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 that connects the antenna to the
Duplexer. A return loss of approximately 15 dB should be measured for both the TX and RX bands.
2.2
Receive Section
2.2.1 LNA
U2768 first stage is LNA. The purpose of the LNA is to provide enough gain at a low noise figure so that
the receiver can pick up the signal correctly. It must provide a good 50 Ω match to the RX bandpass filter.
2.2.2 RX Mixer
The second stage of the U2768 is 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 output is
connected by 4:1 transformer, which has balance inputs and single output. The transformer output
connects to 10.7MHz ceramic filter. This ceramic filter has 330 ohm input and output impedance and its
bandwidth 150 kHz.
2.2.3 RX VCO
The RX VCO is provided by U2768 chip. This VCO oscillate at frequency 870-970MHz. This VCO output
internally connects to U2768 PLL while its loop filter should be connected externally.
2.2.4 RX Synthesizer
The U2768 also has Dual PLL for both the TX and RX sides. The Synthesizers receive channel information
from the embedded microprocessor in the AMD ASIC via the serial buss. It also requires a stable 18.4
MHz reference clock that is also supplied by the AMD ASIC.
Reference:
Revision: 0
This document is proprietary to VTECH ENGINEERING CANADA LTD.
Specifications are preliminary and are subject to change without notice.
Page: 8 of 29
107274
VTECH ENGINEERING CANADA LIMITED
TITLE
PDL MK3 with U2768 RF Theory Of Operation
MODEL
Sony 970
A passive loop filter is employed to connect the synthesizer of the internal VCO. This tuning voltage may
be observed from test point RX_LOOP_V 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 (<15ms).
2.2.5 IF Amplifier Stage
There are two stages of IF amplification in the U2768, namely LIM1and LIM2. LIM1 and LIM2 provide the
enough gain for the signal. Both of the LIM1 and LIM2 require external 10.7 MHz ceramic IF filters.
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
The U2768 contains FM PLL Demod. This demodulator will demodulate the FM signal to the analog signal
and this analog signal is sent to the baseband. The quadrature voltage can also be observed at the ATE
test point connector. This voltage should nominally be 1.5 V for both the base and handset when a signal
is center tuned.
2.2.7.1
RSSI Comparator
The U2768 provides an RSSI voltage that is proportional to the input signal level that is then sent to the
ASIC A/D pin.
2.3
Transmit Section
2.3.1 TX Amp
There is one transistor which provide the necessary gain for the transmit section. Transistor Q3 amplifies
the signal from the TX VCO. The output power is set through TX PAD (attenuation pad) such that we
guarantee less then 50mV/meter radiated field strength measured at 3 meters.
2.3.2 TX VCO
nd
TX VCO oscillates at 450MHz and the transistor Q1 output circuitry resonates at 2 harmonic, which is the
TX frequency. The TX VCO is also FSK modulated by the transmit data through a second varactor in the
tank (25kHz peak). The data to be modulated is first filtered and then the amplitude is set via a precision
resistor divider (R42/R46 both on handset and base) to set the deviation of the data modulation.
2.3.3 TX Synthesizer/PLL
The TX PLL is also provided by U2768. The TX frequency synthesizer is working at 450MHz. 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 (<60ms).
Reference:
Revision: 0
This document is proprietary to VTECH ENGINEERING CANADA LTD.
Specifications are preliminary and are subject to change without notice.
Page: 9 of 29
107274

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