Telecommunications – Transmitter and receiver at same station – Radiotelephone equipment detail
Utility Patent
1999-03-31
2001-01-02
Tsang, Fan (Department: 2746)
Telecommunications
Transmitter and receiver at same station
Radiotelephone equipment detail
C455S063300, C455S078000, C455S082000
Utility Patent
active
06169912
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to wireless communications, and more particularly to signal cancellation to eliminate the duplexer of a cordless phone.
BACKGROUND OF THE INVENTION
Wireless communications are frequently preferred by consumers and businesses because of the portability afforded. While cellular telephones allow use from almost anywhere, cordless phones permit use around the home and home-office via the same low cost telephone lines as wired phones. Cordless telephones, which continue to improve in quality, are one of the more successful consumer products of our times. Fierce competition ensures that prices remain as low as possible.
Cordless phones of the CTO type provide full-duplex communication by simultaneously using two channels of RF spectrum.
FIG. 1
shows that channels are used in an up-link frequency band and in a down-link frequency band. While standards vary, in many countries ten or twenty-five two-way channels are reserved for use by cordless telephones at about 49 MHz.
The modulation is analog narrow band frequency modulation (NBFM), with a deviation of +/−5 KHz. The higher-frequency up-link band is used by the portable phone handset to transmit to the base station, while the lower-frequency down-link band is used by the base station to transmit to the portable handset. Channels are spaced about 15-40 KHz apart. Down-link channel
12
is one of 10-25 channels within the down-link bank. Down-link channel
12
transmits from the base station while up-link channel
10
transmits to the base station. Full-duplex communication is possible using separate channels
10
,
12
.
For 25-channel-pair phones, the down-link band is about 7.3% wide, but is separated from the up-link band by a stop band of only 3.8%.
The power of the transmitted signal is limited by each country. In the USA, the Effective Radiated Power is limited to −15 dBm. Antennas which are much shorter than ¼-wave must be employed for the user's convenience, especially for the handset station. The circuitry needed to feed such antennas is in practice very lossy; therefore, a signal about 15 dB higher (0 dBm) is typically applied by the transmitter to the antenna feed circuitry. Part of this lossy circuitry is a series loading coil, which cancels the large capacitive reactance presented by a short antenna.
These many design constraints, together with cost and size limits make construction of cordless phones a challenge. Precise circuits and multiple stages are used to meet the requirements.
Duplexer used to Attenuate Transmitted Signal to Receiver—
FIG. 2
A single antenna is used for both the transmitted and received signals. Thus, it is necessary to provide antenna-coupler circuitry, which allows efficient coupling of the transmitted signal from the transmitter to the antenna and efficient coupling of the received signal from the antenna to the receiver. It is also critically important to sufficiently reduce the amount of transmitter signal power reaching the receiver, in order to prevent overloading of same. Circuitry which provides this isolation in the form of selectivity takes advantage of the different frequency bands used to transmit and receive. The tight spacing between the up-link and down-link bands makes this selectivity critical. Selectivity is improved by the use of a module of discrete components known as a duplexer. Duplexers now cost about 50 cents even with the high volumes produced.
FIG. 2
illustrates the coupling and selectivity functions provided by a typical duplexer. Antenna
20
is used both to transmit a signal from transmitter
18
and to receive a signal to be processed by receiver
16
. Since portable phones must be compact in size, a full-sized antenna is not desirable. Coil
22
is placed in series with antenna
20
to provide an inductive reactance which is equal to the capacitive reactance presented by the undersized antenna, thus allowing usable amounts of antenna currents to flow.
Duplexer
14
passes the transmit signal from transmitter
18
to coil
22
and antenna
20
with only about a 4 dB (decibel) loss. The receive signal from the remote base station or handset is captured by antenna
20
and fed to duplexer
14
. Owing to cost compromise, duplexer
14
attenuates the receive signal by about 5 dB before passing it to receiver
16
. Selectivity is provided by duplexer
14
despite the close proximity of the up-link and down-link bands; as the transmit signal from transmitter
18
is attenuated by about 40 dB when measured at the input to receiver
16
. This large attenuation of the transmit signal represents a considerable feat, since both signals are present on the single antenna
20
. Additionally, the transmit signal is much stronger than the receive signal, even when the remote unit is only a few feet away.
FIG. 3
is a schematic diagram of a typical duplexer for a cordless phone. Tuned filters are used to select the receive signal and attenuate the transmit signal before the signal from the antenna is input to the receiver RX. The transmit signal is typically 40 dB stronger than the receive signal, even when the remote unit is only a few feet away.
The transmit signal is applied to a tap of inductor coil
122
of duplexer
14
which provides an approximate 100-ohms impedance match to the transmitter output. Coil
122
and capacitor
124
form a parallel tuned L/C circuit to ground, in order to be resonant over the transmit frequency band. External series coil
22
and antenna
20
are connected to duplexer
14
between coils
132
,
136
and between capacitors
134
,
138
. Coil
132
and capacitor
134
form a parallel tuned series trap resonant at the receive band. This trap prevents the transmitter output circuit and the parallel tuned circuit consisting of coil
122
and capacitor
124
from loading down (i.e. presenting a low impedance to ground) the receive signal coming from antenna
20
and loading coil
22
.
Two separate parallel tuned, series connected traps are provided by coil
136
and capacitor
138
, and by coil
142
and capacitor
144
. These traps are resonant at separate portions of the transmit frequency band and thus tend to block the transmitter signal from entering the receiver port RX. These traps also prevent loading down of the transmit signal by the receiver input circuitry and by the parallel tuned circuit consisting of capacitor
126
and coil
128
. For 10-channel phones, the second trap of coil
142
and capacitor
144
can be deleted, but this second trap is helpful for the 25-channel base station phones because of the split down-link transmitter-band segments. The second trap is tuned to the additional segment used in North America while the first trap is tuned to the standard transmitter-band segments.
Coil
128
and capacitor
126
form a parallel tuned circuit to ground. It is resonant with the receive band, thus further blocking transmitter signals from entering the receiver. Coil
128
is tapped farther up than coil
122
to provide several thousand-ohm impedance matching to the receiver's RF amplifier transistor input. Each trap in the duplexer attenuates the desired signal by only a few dB, but the undesired signals by 20 to 30 dB.
The five coils
122
,
132
,
136
,
142
, and
128
are relatively large, bulky components. The coils and capacitors must be manufactured, tested, and inventoried, then individually assembled into a manufactured, shielding enclosure. The three traps must each be adjusted during production. It is desired to eliminate these discrete components and subassembly in order to reduce cost and space requirements.
FIG. 4
is a block diagram of a typical cordless phone using a duplexer antenna coupler. Controller
50
has many functions, including the generation of tones and signals for the phone line for a base station, or control of power and other features on the handset. Crystal
76
is used to generate a clock for controller
50
, which can send commands to the remote unit by applying very low frequency signals to audio processing amplifie
Auvinen Stuart T.
Jackson Blane J.
Pericom Semiconductor Corp.
Tsang Fan
LandOfFree
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