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The present invention relates generally to integrated circuit switching
circuitry, and, more particularly, to a filter switching circuit for a
dual-bandwidth receiver for switching between a narrowband filter and a
wideband filter to permit alternate reception of either a narrowband
signal or a wideband signal.
A communication system for transmitting information between two locations
is comprised, at minimum, of a transmitter and a receiver interconnected
by a transmission channel upon which an information signal may be
transmitted. A radio communication system, one particular type of
communication system, is comprised of a transmitter and a receiver
interconnected by a radio-frequency channel.
To permit transmission of an information signal upon the radio-frequency
channel, the information signal is impressed upon a radio-frequency
electromagnetic wave by a process referred to as modulation. The
radio-frequency electromagnetic wave is of a frequency within a range of
frequencies defining the radio-frequency channel.
The radio-frequency electromagnetic wave is referred to as a carder wave,
and the carder wave, once modulated by the information signal, is referred
to as a modulated, information signal. The modulated information signal
occupies a frequency bandwidth centered at, or close to, the frequency of
the carrier wave, and is transmitted through free space upon the
radio-frequency channel to transmit thereby the information between the
transmitter and the receiver.
Various modulation techniques have been developed to modulate the
information signal upon the radio-frequency electromagnetic wave. For
example, four of such modulation techniques are amplitude modulation (AM),
frequency modulation (FM), phase modulation (PM), and complex modulation
(CM).
A receiver which receives a modulated information signal, once transmitted
over the radio-frequency channel, includes circuitry to detect, or to
recreate otherwise, the information signal modulated upon the
radio-frequency electromagnetic wave. This process is referred to as
demodulation, and the receiver contains demodulation circuitry to
demodulate the received, modulated information signal. Typically, the
receiver circuitry additionally includes down conversion circuitry to
convert downward in frequency the radio-frequency, modulated information
signal to permit proper operation of the demodulation circuitry. Some
receiver circuitry additionally contains up conversion circuitry to
convert upwardly in frequency the radio-frequency, modulated information
signal to permit proper operation of the demodulation circuitry.
As many different modulated information signals may be simultaneously
transmitted by a plurality of transmitters at a plurality of different
frequencies over many radio-frequency channels, a receiver further
contains tuning circuitry to demodulate only those signals received by the
receiver which are of certain desired frequencies. Such tuning circuitry
typically comprises filter circuitry having passbands for passing signals
only within certain bandwidths. The receiver down conversion circuitry, up
conversion circuitry (if present), and the receiver demodulation circuitry
may additionally contain filter circuits to prevent passage of undesired
signals.
The broad range of frequencies at which modulated information signals may
be transmitted is referred to as the electromagnetic frequency spectrum.
The electromagnetic frequency spectrum is divided into frequency bands,
and the frequency bands are divided into channels (referred to as
transmission channels)upon which the modulated information signals may be
transmitted. Regulation of radio-frequency communication in certain
frequency bands of the electromagnetic frequency spectrum minimizes
interference between simultaneously transmitted signals.
For example, in the United States, portions of a 100 MHz band of the
electromagnetic frequency spectrum (extending between 800 MHz and 900 MHz)
are allocated for radiotelephone communication. Such radiotelephone
communication may, for example, be effectuated by radiotelephones utilized
in a cellular, communication system. Existing radiotelephones contain
circuitry to generate and to receive simultaneously modulated information
signals.
The infrastructure required to form a cellular, communications systems is
comprised of numerous base stations which are positioned at spaced-apart
locations throughout a geographical area. Each of the base stations
contains circuitry to receive and to transmit modulated information
signals. Reception and transmission of modulated information signals to
and from radiotelephones in the vicinity of individual ones of the base
stations permits two-way communication therebetween.
To permit communication between a radiotelephone positioned at any location
throughout the geographical area and at least one of the base stations
forming the infrastructure of the cellular, communication system requires
careful selection of the locations at which the base stations are
positioned. Once suitably positioned, each base station defines a specific
geographic portion, referred to as a "cell" of the geographical area.
Although numerous modulated information signals may be transmitted
simultaneously at different transmission frequencies (i.e., over different
radio-frequency channels), each modulated information signal, during
transmission thereof, occupies a finite portion of the frequency band
(i.e., each modulated information signal, during transmission thereof,
occupies a radio-frequency channel). Overlapping of simultaneously
transmitted, modulated information signals in the same geographical area,
whether by transmission of these signals at the same frequency or by
frequency drift of one or more signals, is impermissible as interference
between overlapping signals can prevent detection of either of the
transmitted modulated information signals by a receiver.
To permit such overlapping, the frequency band (which, as mentioned
hereinabove, extends between 800 and 900 MHz) allocated for radiotelephone
communication in the United States, is divided into 30 KHz channels. Such
channel spacing over portions of the frequency band forms a channelized
communication system. A first portion, extending between 824 MHz and 849
MHz of the frequency band, is allocated for the transmission of modulated
information signals from a radiotelephone to a base station. A second
portion, extending between 869 MHz and 894 MHz of the frequency band is
allocated for the transmission of modulated information signals from a
base station to a radiotelephone.
Other channelized communication systems for radiotelephone communications
are similarly defined in other countries. For instance, in Japan, the
frequency band allocated for radiotelephone communications is divided into
25 KHz channels.
Increased usage of cellular, communication systems, both domestically and
in other countries, has resulted, in many instances, in the full
utilization of every radio-frequency, transmission channel of the
frequency band allocated for cellular, radiotelephone communication. Other
frequency bands allocated for other uses of the electromagnetic frequency
spectrum are oftentimes similarly fully utilized.
As a result, various proposals have been made to utilize more efficiently
the frequency band allocated for radiotelephone communication to increase
thereby the information transmission capacity of a cellular,
radiotelephone communication system. Proposals have been similarly made to
use more efficiently other frequency bands allocated for other uses of the
electromagnetic frequency spectrum.
A modulated information signal formed by any of the above-mentioned
modulation techniques, is defined by the frequency of the electromagnetic
wave upon which the information signal is modulated. The modulated
information signal is, however, spread-out over a band of frequencies
centered at, or close to, the frequency of the carrier wave. The band of
frequencies over which the modulated, information signal is spread is
referred to as the bandwidth of the signal. The bandwidths of the
radio-frequency transmission channels into which the frequency band
allocated for cellular communication is divided, must be small enough such
that simultaneously transmitted modulated information signals over
adjacent radio-frequency transmission channels do not overlap. The
transmission channels must additionally be wide enough to permit a certain
amount of frequency drift of these signals transmitted over the
transmission channels. That is, the channel spacing defining the
transmission channel bandwidths must be great enough to permit frequency
drift of simultaneously transmitted signals on the adjacent channels in
which one, or more, of the signals exhibit frequency drift.
As previously mentioned, in the United States, the frequency band allocated
for cellular communications extending between 800 and 900 MHz is divided
into numerous 30 kilohertz channels. Transmitter circuitry of transmitters
which transmit the signals upon the transmission channels generate signals
which are somewhat smaller than the channel bandwidth. The channel
bandwidth is wide enough to permit simultaneous transmission of signals on
adjacent channels even when there is significant frequency drift (as a
percentage of the bandwidth of a transmitted signal) of the signals
transmitted upon the adjacent channels. Other cellular communication
systems similarly define radio-frequency transmission channels of
bandwidths to permit simultaneous transmission of signals upon adjacent
transmission channels even when there is significant frequency drift of
the transmitted signals.
As commercially-viable methods and apparatus for reducing signal bandwidth
of transmitted signals, and for better minimizing frequency drift of the
transmitted signals are developed and implemented, the bandwidths of the
radio-frequency transmission channels upon which the signals are
transmitted may be reduced. A reduction in the bandwidths of the
radio-frequency transmission channels permits a greater number of
transmission channels to be defined for a frequency band allocated for a
particular use. In the particular instance of the frequency band allocated
for cellular communications in the United States, a reduction in the 30
KHz bandwidth defining each radio-frequency transmission channel therein,
would result in a corresponding increase in the number of transmission
channels which could be defined within the allocated frequency band. For
instance, by reducing the size of the bandwidths of the transmission
channels from 30 KHz to 15 KHz would result in a doubling of capacity of a
cellular communication system within a particular geographical area.
Similarly, a reduction in the size of the bandwidths of the transmission
channels of the frequency band allocated for radiotelephone communications
in Japan from 25 KHz to 12.5 KHz would similarly result in a doubling of
capacity of the Japanese cellular communication system.
Such a reduction in transmission channel bandwidths, however, requires
alteration of the infrastructure, (i.e., base stations) as well as the
radiotelephones utilized in such a cellular communication system. Because
such an alteration of the infrastructure necessitates significant capital
expenditures, only those cellular communication systems which are
presently, or are anticipated to be, fully utilized need to be altered to
permit greater numbers of radio-frequency transmission channels to be
defined. However, to permit operation of a radiotelephone in both existing
cellular communication systems and cellular communication systems altered
to increase the capacity, thereof, the radiotelephones must contain
circuitry to permit operation thereof in either an existing system or an
altered system.
What is needed, therefore, is a radiotelephone construction which permits
operation thereof in both a conventional, cellular communication system,
and a cellular communication system of increased capacity.
SUMMARY OF THE INVENTION
It is, accordingly, an object of the present invention to provide a filter
switching circuit permitting reception of a signal of either a first
bandwidth, or a second bandwidth.
It is a further object of the present invention to provide a filter
switching circuit for a radiotelephone to permit reception of both a
wideband signal and a narrowband signal.
It is yet a further object of the present invention to provide a
radiotelephone selectively operable to receive either a wideband signal or
a narrowband signal.
In accordance of the present invention, therefore, a filter switching
circuit for switching between at least two filters wherein a first of the
filters is operative to pass signal portions of a signal within a first
bandwidth, and a second of the filters is operative to pass signal
portions of a signal within a second bandwidth. The filter switching
circuit is operative to form an output signal indicative of signal
portions within a bandwidth of a desired one of the filters, and is
comprised of a drive circuit for supplying isolated drive signals to each
of the filters, and a switch for switching between the filters such that
signal portions of the desired one of the filters forms the output signal.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be better understood when read in light of the
accompanying drawings in which:
FIG. 1 is a graphical representation of a typical modulated information
signal graphed as a function of frequency;
FIG. 2 is a graphical representation of several adjacent transmission
channels of a frequency band formed of a portion of the electromagnetic
frequency spectrum;
FIG. 3A is a graphical representation, similar to that of FIG. 2, but
further illustrating the simultaneous transmission of modulated
information signals upon adjacent transmission channels of a conventional,
cellular communication system;
FIG. 3B is a graphical representation, similar to that of FIG. 3A, but
illustrating the simultaneous transmission of modulated informations
signals upon adjacent channels of a cellular communication system of
increased capacity;
FIG. 4A is a simplified, block diagram of the filter switching circuit of a
preferred embodiment of the present invention;
FIG. 4B is a simplified, block diagram of the filter switching circuit of
an alternate, preferred embodiment of the present invention;
FIG. 5 is a simplified circuit schematic of the filter switching circuit of
the preferred embodiment of the present invention;
FIG. 6 is a block diagram of a radiotelephone of the present invention in
which the filter switching circuit of FIGS. 4-5 forms a portion thereof;
and
FIG. 7 is a flow diagram representing the method of the preferred
embodiment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Turning first to the graphical representation of the FIG. 1, a modulated
information signal, referred to generally by reference numeral 10, is
plotted as a function of frequency. The level, i.e., amplitude, of the
signal 10, scaled in terms of volts on ordinate axis 14, is graphed as a
function of frequency, scaled in terms of hertz, on abscissa axis 18.
Signal 10 is representative of a modulated information signal formed by any
of the previously mentioned modulation techniques; namely, an AM, FM, PM,
or CM technique. Other modulation techniques may generate signals of
different shapes. While a signal formed by a particular one of the
modulation techniques may vary somewhat in amplitude, shape, and/or
bandwidth, the energy of signal 10 is typically centered about a center
frequency, f.sub.c, of a particular frequency. As illustrated, signal 10
is symmetrical about vertically extending line 22, shown in hatch, which
is defined by the center frequency fc. Signal 10 is of a frequency
bandwidth of a magnitude indicated by arrow 26.
To properly determine the information content of a received signal, such as
signal 10, a receiver should be of a construction to permit reception of
the signal with minimum practical distortion. Filter circuitry which forms
a portion of the receiver forms a filter passband for passing the desired,
received signal. However, a receiver typically receives not only the
desired signal (here represented by signal 10), but, additionally, other
signals, such as signal 28 which is representative of, e.g., a spurious
noise signal or an adjacent channel signal, located at other frequencies,
including frequencies close to the bandwidth of frequencies encompassed by
signal 10. Ideally, the receiver filter circuitry is of a passband to pass
the desired signal in undistorted form, and all signals other than the
desired signal are filtered, or otherwise blocked. However, the
transmitted signal is sometimes susceptible to frequency drift which
causes an alteration of the upper and lower frequency boundaries defined
by the signal bandwidth. Therefore, the passband of the receiver filter
circuitry must be great enough to permit passage of the transmitted
signal, while also allowing for changes in frequencies caused by frequency
drift of the transmitted signal. The passband of the receiver filter
circuitry should be great enough to pass the received signal while
accomodating frequency drift. By increasing the passband of the receiver
filter circuitry, however, other signals, here represented by signal 28,
are passed, thereby resulting in less than ideal signal recreation by the
receiver. Decreasing the passband of the receiver filter circuitry would
reduce the amount of noise passed by the receiver filter circuitry, but
would increase the possibility that the desired signal would be truncated,
or otherwise be not passed in undistorted form.
Turning now to the graphical representation of FIG. 2, a portion of a
frequency band representative of a portion of the frequency band allocated
for cellular communications is illustrated. Similar to the graph of FIG.
1, the ordinate axis, here axis 30, is scaled in terms of volts, and the
abscissa axis, here axis 34, is scaled in term of hertz.
As mentioned previously, the frequency band allocated for cellular
communications in the United States is divided into 30 KHz channels upon
which a modulated information signal may be transmitted. FIG. 2
illustrates five of such transmission channels, here referred to by
reference numerals 38, 42, 46, 50 and 54. It is to be noted that, while
the frequency demarcations of abscissa axis 34 correspond to the United
States cellular communication standard, virtually any channelized
communication system may be similarly described with appropriate
substitution of frequency demarcations. For instance, the Japanese
cellular communication system may be illustrated by scaling abscissa axis
34 in terms of 25 KHz demarcations rather than the 30 KHz demarcations
which correspond to the United States standard.
As illustrated with respect to the United States cellular system, the
vertical lines are spaced at 30 KHz intervals to represent boundaries
between adjacent ones of the transmission channels 38-54. Each
transmission channel 38-54 is of a 30 KHz bandwidth, and modulated
information signals, such as signal 10 of FIG. 1, may be transmitted
simultaneously upon individual ones of the transmission channels 38-54 as
long as the signals transmitted upon adjacent ones of the channels are not
of bandwidths to overlap, or do not otherwise interfere, with signals
simultaneously transmitted upon adjacent channels.
To prevent such overlapping, or other interference, the signals transmitted
upon any of the transmission channels, should be of bandwidths (and center
frequencies) to maintain the signals within the boundaries defining the
respective transmission channels. Such control is required, not only to
prevent overlapping of simultaneously transmitted signals upon adjacent
transmission channels, but, additionally, because the passbands of the
receiver filter circuitry are of magnitudes corresponding to the
bandwidths of the transmission channels. Only those portions of a received
signal within the passband of the receiver are passed (i.e., only those
portions of a received signal transmitted within the boundaries of
specific transmission channels such as transmission channels 38-54 are
passed by the receiver).
FIG. 2 further illustrates signals 58 and 62 centered within transmission
channels and 38 and 42, respectively. Signals 58 and 62 are similar to the
modulated information signal 10 of FIG. 1, and are of bandwidths of
magnitudes less than the magnitudes of the bandwidths which define the
transmission channels 38 and 42, respectively. Signals 58 and 62 may be
transmitted simultaneously as long as the signals do not overlap with one
another.
Historically, the channel spacing determining channel bandwidths was
defined to ensure that transmitters utilizing commercially-viable
technology could transmit signals of bandwidths less than the bandwidths
of the transmission channels. Technical improvements, however, have
permitted the construction of commercially-viable transmitters which are
capable of transmitting signals of reduced bandwidths. Therefore,
commercially-viable transmitters may now be utilized to transmit signals
of smaller bandwidths than the bandwidths of signals transmitted by
previously-existing, commercially-viable transmitters. Commercially-viable
methods and apparatus for reducing the frequency drift of transmitted
signals have similarly been developed.
Positioned within channels 46 and 50 of FIG. 2 are signals 66 and 70,
respectively. Signals 66 and 70 are similar in shape to signal 10 of FIG.
1, but, as compared with signals 58 and 62 positioned within channels 38
and 42, are of significantly smaller bandwidths. Signals 66' and 70' are
positioned in channel 54 which are identical in shape and bandwidth with
signals 66 and 70 to illustrate that the signals may be formed to require
one half, or even less, of the bandwidth required of signals 58 and 62.
Because the bandwidth requirements of signals generated by transmitters of
newer constructions, represented in FIG. 2 by signals 66 and 70, are
significantly less than the bandwidth requirements of signals generated by
conventional transmitters, represented in FIG. 2 by signals 58 and 62,
significant portions of each channel of the frequency band allocated for
radiotelephone communications are unused. However, by re-defining the
bandwidths of the channels of the allocated frequency band to reduce
thereby the bandwidths of some, or all, of the channels, greater numbers
of channels may be defined over the allocated frequency band. By defining
greater numbers of channels, greater numbers of signals may be transmitted
simultaneously over the allocated frequency band, thereby increasing the
transmission capacity of the frequency band.
Turning now to the graphical representations of FIGS. 3A-3B, the increase
in channel capacity may be better illustrated. The graphical
representations of FIGS. 3A and 3B are similar to the graphs of FIGS. 1
and 2 and are plots of voltage, scaled in terms of volts on ordinate axes
74 of FIGS. 3A and 3B as functions of frequency, scaled in terms of hertz
on abscissa axes 78.
FIG. 3A represents five simultaneously transmitted signals 82, 86, 90, 94,
and 98 transmitted upon transmission channels 38, 42, 46, 50, and 54,
respectively, which form a portion of the frequency band allocated for
cellular communications. Signals 82-98 are of bandwidths similar to the
bandwidths of signals 58-62 of FIG. 2, and channels 38-54 of FIG. 3A
correspond in size with the similarly numbered channels of FIG. 2. FIG.
3A, thus, represents conventional transmission of signals over a system
defined to have conventional channel spacing.
FIG. 3B represents ten simultaneously transmitted signals 100, 104, 108,
112, 116, 120, 124, 128, 132, and 136 upon channels 138, 142, 146, 150,
154, 158, 162, 166, 170, and 174. The vertically extending lines in the
Figure are representative of boundaries between adjacent transmission
channels. Transmission channels 138-174 occupy the same range of
frequencies as do transmission channels 38-54 of FIGS. 2 and 3A, but are
of bandwidths of one half of the bandwidths of transmission channels
38-54.
Comparison of FIGS. 3A and 3B illustrates that twice as many signals may be
simultaneously transmitted upon the system of FIG. 3B as that of FIG. 3A.
However, as previously mentioned, because alteration of existing
infrastructure of a cellular system to effectuate such a reduction in
transmission channel bandwidth (and commensurate increase of transmission
capacity) necessitates the expenditure of significant amounts of capital,
such alteration of the infrastructure is mandated only in systems which
are, or which are anticipated to be, fully utilized. Because both existing
systems and systems of increased capacity are to be utilized concurrently,
the systems must be compatible with one another. The channel spacing of
the system of FIG. 3B is compatible with the system of FIG. 3A as the
number of channels of the system of FIG. 3B is a multiple of the system of
FIG. 3A. A system in which the channels are of another multiple (such as,
for example, a multiple of three) would similarly define a system
compatible with existing systems.
Receiver circuitry forming a portion of a radiotelephone must be capable of
operation in either an existing cellular communication system, or in a
cellular communication system of increased capacity. Existing
radiotelephone receiver circuitry permits reception and accurate
demodulation of signals transmitted upon transmission channels of an
existing cellular communication system. Existing radiotelephone receiver
circuitry, however, is not operative to receive efficiently signals
transmitted upon a system of increased capacity. Therefore, a
radiotelephone construction which is operable in both an existing cellular
communication system, or in a cellular communication system of increased
capacity is required to permit efficient utilization of cellular
communication systems of increased capacity, as well as existing
communication systems.
As mentioned previously, a receiver typically includes filter circuitry
having a passband of a bandwidth corresponding to the bandwidth of the
transmission channel upon which the signal is transmitted. Because the
transmission channels of existing, United States, cellular communication
systems are of 30 KHz bandwidths, existing radiotelephones operable in the
United States system, therefore, typically contain filter circuitry having
passbands of nearly 30 KHz. Such a bandwidth, or the bandwidth of any
corresponding existing cellular communication system, will be referred to
hereinafter as a wideband bandwidth. For example, because the transmission
channels of the existing Japanese, cellular communication systems are of
25 KHz bandwidths, existing radiotelephones operable in the Japanese
system, therefore, typically contain filter circuitry having passbands of
nearly 25 KHz. Such a bandwidth is referred to as the wideband bandwidth.
To permit proper reception of a modulated information signal transmitted in
a communications system of expanded capacity, the receiver filter
circuitry should be of passbands of a decreased bandwidth (namely, the
passbands should be of bandwidths of one half, or some other fractional
multiple, of the magnitudes of the bandwidths of the wideband bandwidth of
the existing cellular communication systems). Such a reduced bandwidth, to
be referred to hereinafter as a narrowband bandwidth, not only prevent
passage of simultaneously transmitted signals transmitted upon adjacent
transmission channels, but, additionally, reduces the amounts of white
noise and other spurious signals. Filter circuitry having a passband of
the narrowband bandwidth cannot be utilized, however, when receiving a
modulated information signal transmitted upon a transmission channel of an
existing cellular communication system as the transmitted signal may be
partially, or wholly, outside of the passband of the narrowband, bandwidth
filter circuitry of the receiver.
Therefore, a radiotelephone construction operable to receive modulated
information signals transmitted upon transmission channels of either
existing cellular communication systems, or cellular communication systems
of expanded capacity, require two separate filter circuits. A first
receiver filter circuit forms a passband of a wideband bandwidth, and a
second filter circuit forms a passband of a narrowband bandwidth.
Alternate operation of either the first filter circuit or the second
filter circuit permits operation of a single radiotelephone in either an
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