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TECH ENGINEERING CANADA LIMITED
V
TIT LE PDL MK3 with U2768 RF Theory O f Operat ion
MODEL Sony 970
PDL MK3 with U2768
RF Module
THEORY OF OPERATION
Note: All r i ghts r eser ved. No par t of thi s publ ication may be r eproduced or transm i tted in any f or m or by
any means. The only controlled copy of thi s docum ent i s kept i n electronic f or m on the VTECH computer
network. All pr inted copies are uncontrolled docum ents and ar e not updated. This Proc edur e i s the
property of VTECH Engineeri ng Canada Ltd., and shall be r eturned upon r equest
Prepared By John Wang
Title RF Engineer
Checked By Sarkis Teghararian
Title RF Development Manager
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Specifications are preliminary and are subject to change without notice.
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TECH ENGINEERING CANADA LIMITED
V
TIT LE PDL MK3 with U2768 RF Theory O f Operat ion
MODEL Sony 970
Revision History
Revision No Description Page Effective Date
0 Initial Release All June 9, 2000
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TECH ENGINEERING CANADA LIMITED
V
TIT LE PDL MK3 with U2768 RF Theory O f Operat ion
MODEL Sony 970
Table of Cont ents
1 RF SECTION OVERVIEW .....................................................................................4
1.1 Introduction
1.2 Overview of changes for U2768...................................................................................................................4
1.3 Frequency Tables
1.3.1 Handset Frequencies...............................................................................................................................5
1.3.2 Base Frequencies ....................................................................................................................................6
1.4 Block Diagram
.................................................................................................................................................4
......................................................................................................................................5
.............................................................................................................................................7
2 RF SECTION DETAILED OPERATION..................................................................8
2.1 Antenna Section
2.1.1 Antenna...................................................................................................................................................8
2.1.2 SAW Duplexer........................................................................................................................................8
2.2 Receive Section
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
.........................................................................................................................................8
..........................................................................................................................................8
2.3 Transmit Section
2.3.1 TX Amp ..................................................................................................................................................9
2.3.2 TX VCO..................................................................................................................................................9
2.3.3 TX Synthesizer/PLL................................................................................................................................9
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Specifications are preliminary and are subject to change without notice.
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TECH ENGINEERING CANADA LIMITED
V
TIT LE PDL MK3 with U2768 RF Theory O f Operat ion
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 betw een the handset and base RF boards. The RF receiver and tr ansmitter circuitry
essentially provides a link between the microphone and receiver in the handset to t he telephone line in the
base set. I n this way the phone perf or ms 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 t he base transmits to the handset is centered ar ound 903. 8 MHz. The data rate w hich
is modulated onto the RF carrier is 48 kbps. On the receiver side, the data is extracted and then
conver t ed back into the original audio signal.
The follow ing sect ion will outline the t ra nsmit f req uencies used as w ell a s the c orr esp onding LO f re quency
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 o f chang es f or U2768
The U2768 RF Module design is based on the original U2765 design. The U2768 cont ains:
1. All sections of U2765 IC
2. LNA Front End
3. RX VCO operating at 900MHz
4. Dual PLL ( 900M Hz/450MHz)
5. PLL Demodulator.
6. Replaces the original mixer with an I mage Reject Version
These sect ions r emove a considerable amount of discr ete circuits. The Baseband inter face and test point
locations are kept the same as to minimize the differences betw een the modules.
This module also removes the interst age 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.
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TIT LE PDL MK3 with U2768 RF Theory O f Operat ion
MODEL Sony 970
1.3 Frequency Tables
This section outlines the RF frequencies and corr esponding channel number s. The handset uses a high
side LO while the base uses a low side LO t o down-convert the incoming signal.
1.3.1 Handset Frequencies
Channel Tra nsmit Receive RX LO
0 920.25 902.3 913
1 920.4 902.45 913.15
2 920.55 902.6 913.3
3 920.7 902.75 913.45
4 920.85 902.9 913.6
5 921 903.05 913.75
6 921.15 903.2 913.9
7 921.3 903.35 914.05
8 921.45 903.5 914.2
9 921.6 903.65 914.35
10 921.75 903.8 914.5
11 921.9 903.95 914.65
12 922.05 904.1 914.8
13 922.2 904.25 914.95
14 922.35 904.4 915.1
15 922.5 904.55 915.25
16 922.65 904.7 915.4
17 922.8 904.85 915.55
18 922.95 905 915.7
19 923.1 905.15 915.85
20 923.25 905.3 916
21 923.4 905.45 916.15
22 923.55 905.6 916.3
23 923.7 905.75 916.45
24 923.85 905.9 916.6
25 924 906.05 916.75
26 924.15 906.2 916.9
27 924.3 906.35 917.05
28 924.45 906.5 917.2
29 924.6 906.65 917.35
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TIT LE PDL MK3 with U2768 RF Theory O f Operat ion
MODEL Sony 970
1.3.2 Base Frequencies
Channel Tra nsmit Receive RX LO
0 902.3 920.25 909.55
1 902.45 920.4 909.7
2 902.6 920.55 909.85
3 902.75 920.7 910
4 902.9 920.85 910.15
5 903.05 921 910.3
6 903.2 921.15 910.45
7 903.35 921.3 910.6
8 903.5 921.45 910.75
9 903.65 921.6 910.9
10 903.8 921.75 911.05
11 903.95 921.9 911.2
12 904.1 922.05 911.35
13 904.25 922.2 911.5
14 904.4 922.35 911.65
15 904.55 922.5 911.8
16 904.7 922.65 911.95
17 904.85 922.8 912.1
18 905 922.95 912.25
19 905.15 923.1 912.4
20 905.3 923.25 912.55
21 905.45 923.4 912.7
22 905.6 923.55 912.85
23 905.75 923.7 913
24 905.9 923.85 913.15
25 906.05 924 913.3
26 906.2 924.15 913.45
27 906.35 924.3 913.6
28 906.5 924.45 913.75
29 906.65 924.6 913.9
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TIT LE PDL MK3 with U2768 RF Theory O f Operat ion
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 t o incorporat e t he differ ent tr ansmit and receive f r equencies.
SAW DUPLEXER
LNA
TX O/P
RX_LO.
MIXER
TX
PAD
V-Tune
Loop
Filter
10.7MHz
450MHz
900MHz.
Fin
LIM1
DUAL PL L
V-Tune
TX_LO.
TX Data
10.7MHz
LIM2
FM PLL
Demod
TEMIC U2768B
RX
Audio
RSSI
18.4MHz
REFERENCE
3 LINE DATA BUS
Figure 2.3 RF Section Block Diagram
As can be seen by t he block diagram, there are several impor tant 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 af fect the transmit ting and rec e iving frequencies. I n o rder
to ensure proper operation of the RF sections, the 18.4 MHz r eference 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 bot h transmit and receive f r equencies.
In the transmit direction, modulation input allows digital data to be modulated directly onto the TX carrier.
In the receive direct ion, t he re ceiver RF signal w ill dire ctly go t o t he U2768 chip. After LNA and Mixer, the
RF signal w ill be convert ed to a 1 0.7 MHz IF sig na l. The IF signal w ill be amplifie d by LIM 1, L IM2 and
demodulat ed by FM PLL Demod. The demodulated s ignal is sent to Baseband section.
The following sections explain in detail the individual blocks in the RF section module. All references to part
numbers cor r espond to the handset s chematic.
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TIT LE PDL MK3 with U2768 RF Theory O f Operat ion
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 retr actable 1/2 wave with 2.5 dB gain relative t o an isotropic radiator and a fixed 1/4
wave antenna with roughly 0 dB gain r elative to an is otropic radiat or. The duplexer and f ilters 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, t he antenna match may be measured. In order to achieve
a good 50 ohm matc h, one must be car ef ul not to obstruct the antenna as any object near t he antenna w ill
affect its impedance.
2.1.2 SAW Duplexer
The SAW Duplexer consists of t wo bandpass filters to pr ovide the necessary isolation between t ransmit
and rec eive sect ions and to pass the cor rect frequencies to transmit and r eceive 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 r et urn loss of approximately 15 dB should be measur ed f or 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 t o provide enough gain at a low noise f igure so that
the receiver can pick up the signal correct ly. I t 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 t he mixer is t o combine the incoming s ignal 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
connect s 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 oscillat e at f requency 8 70-970M Hz. This VCO o ut put
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 bot h the TX and RX sides. The Synthesizers receive channel information
from the embedded microprocessor in the AM D ASIC via the ser ial buss. It also requires a stable 18. 4
MHz r ef er ence clock that is also supplied by the AMD ASIC.
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TIT LE PDL MK3 with U2768 RF Theory O f Operat ion
MODEL Sony 970
A passive loop filter is employed to connect the synt hesizer of the internal VCO. This tuning voltage may
be observed fr om test point RX_LOOP_V on the bottom side of the PCB. The loop filter cutoff fr equency
is set to about 1 kHz to allow relatively fast power-up times (<15ms).
2.2.5 IF Amplifier Stage
There ar e two stages of IF amplification in the U2768, namely LIM1and LIM2. LIM 1 and LIM2 provide the
enough gain for the signal. Both of the LIM1 and LIM2 r equire external 10.7 MHz cer amic IF filter s.
2.2.6 IF Filtering
The choice of 10.7 MHz as an IF f requency, allows the us e of relatively inexpensive filters. Tw o ce ramic
filters are used to achieve the desired adjacent channel suppres sion. Tw o 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 demod ulat e the FM s ig nal t o the analog signal
and this analog s ignal is sent t o the baseband. The quadrature voltage can also be observed at the ATE
test point connector. This volt age should nominally be 1. 5 V f or bot h 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 t he necessar y 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 f ield str ength measur ed at 3 met er s.
2.3.2 TX VCO
TX VCO o s c illa tes a t 450MHz and the transistor Q 1 output circuitry resonates at 2nd harmonic, which is the
TX frequency. The TX VCO is also FSK modulated by t he transmit data t hrough a second varactor in the
tank (25kHz peak). The data to be modulated is first f iltered and then t he amplit ude 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 wor king at 450M Hz. 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 c r itical (< 60ms) .
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