 |
Claims  |
|
|
We claim:
1. In a signal receiving station having a receiving antenna for receiving a
first transmitted signal which includes one or more information carrier
frequencies and a pilot frequency which is separated in frequency from
each one or more carrier frequencies by a respective predetermined amount,
and a convertor means for converting down proportionately the frequencies
in the first transmitted signal to a baseband signal,
a system for tracking and correcting any frequency drift and dispersion of
the carrier frequencies and pilot frequency in the baseband signal,
comprising
one or more carrier tuner means, each for receiving the baseband signal and
each including
means for deriving from the baseband signal an IF carrier signal
representing a respective carrier frequency,
carrier mixer means for mixing the IF carrier signal and a first local
oscillator signal to produce a resultant IF information signal,
means for demodulating the resultant IF information signal to recover
transmitted information, and
pilot tuner means for receiving the baseband signal and including
means for deriving from the baseband signal an IF pilot signal representing
the pilot frequency,
pilot mixer means for mixing the IF pilot signal and the first local
oscillator signal to produce a resultant pilot signal,
local oscillator means responsive to a first control signal level for
developing the first local oscillator signal having a frequency
corresponding to the first control signal level and for supplying the
first local oscillator signal to the carrier mixer means and pilot mixer
means to compensate for any frequency drift or dispersion of frequency of
the IF carrier signals and IF pilot signal, and
frequency discriminator means for detecting frequency drift and dispersion
of the resultant pilot signal from a predetermined center frequency and
for supplying to the local oscillator means the first control signal whose
level identifies the frequency drift and/or dispersion, of the resultant
pilot signal to thereby cause the local oscillator means to produce the
first local oscillator signal which, when supplied to the carrier mixer
means and pilot mixer means, compensates for any frequency drift and
frequency dispersion occurring in the IF carrier signals and IF pilot
signal from their operating frequencies, the frequency discriminator means
including a first frequency discriminator means for generating the first
control signal, the first control signal comprising a coarse control
signal and a digital frequency discriminator means operatively connected
to the first frequency discriminator means, the digital frequency
discriminator means including a crystal oscillator means which is
operatively connected to the a frequency detector for adjusting one or
more operating frequencies of the carrier tuner means to track changes in
the information carrier frequencies.
2. A system as in claim 1 wherein said frequency discriminator means
includes
a first signal source for producing a first reference frequency signal,
a second signal source for producing a second reference frequency signal
which is 90 degrees out of phase with the first reference frequency
signal,
a first mixer for mixing the resultant pilot signal and the first reference
frequency signal to produce a phase error control signal,
a second mixer for mixing the resultant pilot signal and the second
reference frequency signal to produce a first frequency error signal,
differentiation means for receiving the first frequency error signal and
for producing a signal representing the derivative of the first frequency
error signal,
multiplier means for generating the product of the phase error signal and
the derivative signal to produce a frequency error control signal, and
switch means for selectively supplying the phase error control signal and
frequency error control signal to the local oscillator means to switch
from coarse tuning to fine tuning in response to when information of at
least a predetermined accuracy is obtained from the carrier frequencies.
3. A system as in claim 2 further including a loop filter and integrator
means coupled between the switch means and the local oscillator means for
producing the first control voltage signal for supply to the local
oscillator means to control the frequency of the first local oscillator
signal.
4. A system as in claim 3 wherein said switch means includes means for
supplying the frequency error control signal to the loop filter in
response to a first switch control signal, and for supplying the phase
error control signal to the loop filter in response to a second switch
control signal, said system further including
control means responsive to initial receipt of the demodulated resultant IF
information for supplying the first switch control signal to the switch
means, and for supplying the second switch control signal to the switch
means when the demodulated resultant IF information is determined to be
accurate, and
means for supplying demodulated resultant IF information from the
demodulating means to the control means.
5. A system as in claim 4 wherein the digital frequency discriminator means
includes the crystal oscillator means for developing an output reference
frequency signal, and means for developing a DC output voltage whose value
is proportional to the difference between the frequency of the local
oscillator means and the crystal oscillator means, wherein said convertor
means includes at least one phase lock loop circuit which includes a
convertor oscillator for producing a convertor oscillatory signal, and a
mixer for mixing the transmitted signal and the convertor oscillatory
signal to produce the baseband signal, said control means being adapted to
adjust the frequency of the convertor oscillatory signal in response to
the DC output voltage to thereby reduce the frequency difference between
the local oscillator means and crystal oscillator means.
6. A system as in claim 5 wherein said digital frequency discriminator
means further includes
the frequency detector means for producing on a first output a first
digital output pulse train if the frequency of the local oscillator means
is higher than the frequency of the reference frequency signal, and for
producing on a second output a second digital output pulse train if the
frequency of the local oscillator means is lower than the frequency of the
reference frequency signal, the width of the pulses in each pulse train
being proportional to the magnitude of the difference between the
frequencies, and
operational amplifier means for developing the DC output voltage whose
value indicates which of the local oscillator means frequency and
reference signal frequency is higher and indicates an amount such local
oscillator means frequency and such reference signal frequency differ from
each other.
7. In a satellite transmission system in which information signals are
relayed by satellites to ground receivers, said signals including a
plurality of carrier frequencies which include at least one or more audio
carrier frequencies containing audio information, and/or data carrier
frequencies containing data information, and a pilot frequency separated
by predetermined frequency differences from the audio carrier frequencies
and/or the data carrier frequencies, a ground information signal receiver
system comprising
receiving means for receiving an information signal relayed by a satellite,
means coupled to the receiving means for down-converting the frequencies in
the information signal to a baseband signal containing frequencies
corresponding to the audio and/or data carrier frequencies and pilot
frequency, and separated in frequency by amounts proportionate to the
separation of the carrier and pilot frequencies in the information signal,
a plurality of carrier tuner means, each for receiving the baseband signal
and deriving therefrom an IF carrier signal containing information
corresponding to the information contained in a respective one of the
carrier frequencies, and each including
carrier mixer means for mixing the IF carrier signal and a first
compensating oscillatory signal to produce a resultant information signal,
and
means for demodulating the resultant information signal to recover the
information contained therein, and
pilot tuner means for receiving the baseband signal and deriving therefrom
an IF pilot signal whose frequency is separated from the frequencies of
the IF carrier signals proportionately to the separation of the pilot
frequency from the audio and/or data carrier frequencies in the
information signal, said pilot tuner means comprising
pilot mixer means for mixing the IF pilot signal and the first compensating
oscillatory signal to produce a resultant pilot signal,
compensating oscillator means responsive to a first voltage signal for
producing the first compensating oscillatory signal having a frequency
determined by the level of the first voltage signal, and for supplying the
first compensating oscillatory signal to the carrier mixer means and the
pilot mixer means,
frequency detector means for detecting the frequency of the resultant pilot
signal and any frequency drift or frequency dispersion of the resultant
pilot signal from a predetermined center frequency, the frequency detector
means comprising a first frequency discriminator means for generating the
first control signal and a digital frequency discriminator means for
adjusting one or more operating frequencies of the carrier tuner means to
track changes in the information carrier frequencies, including a crystal
oscillator means, the frequency detector means producing a signal
representing the frequency drift or frequency dispersion of the resultant
pilot signal, the signal representing the frequency drift or frequency
dispersion being applied to a switch means and used for adjusting of the
compensating oscillator means to track small changes in the either of the
resultant pilot signal and the IF pilot signal, and
means coupled to the switch means for supplying to the compensating
oscillator means the first voltage signal whose level is a function of the
detected frequency of the resultant pilot signal, so that the compensating
oscillator means produces the first compensating oscillatory signal which,
when mixed with the IF carrier signals and IF pilot signal, counters
frequency drift and frequency dispersion occurring in the IF carrier
signal and IF pilot signal signals by the amount of drift and dispersion
detected in the resultant pilot signal.
8. A system as in claim 7 further including an information signal
transmitter comprising
one or more modulators, each for modulating audio or data information onto
a corresponding carrier signal having a predetermined center frequency,
a pilot signal oscillator means for producing a pilot signal having a
frequency separated by predetermined frequency differences from the
carrier frequencies,
local oscillator means for producing a local oscillatory signal whose
frequency varies in accordance with a dispersal waveform,
generator means for supplying a periodically varying dispersal waveform to
the local oscillator means,
a signal mixer means,
means for supplying to the signal mixer means the carrier signals, pilot
signal, and local oscillatory signal to produce a composite signal with
audio and/or data carrier frequencies, and a pilot frequency which are
dispersed over a predetermined range,
means for up-converting the frequencies in the composite signal to an
information signal, and
transmitting means for transmitting the information signal to the
satellite.
9. In a signal receiving station having a receiving antenna for receiving a
first transmitted signal which includes one or more information carrier
frequencies and a pilot frequency which is separated in frequency from
each one or more carrier frequencies by a respective predetermined amount,
and a convertor means for converting down proportionately the frequencies
in the first transmitted signal to a baseband signal,
a system for tracking and correcting any drift and dispersion of the
carrier frequencies and pilot frequency in the baseband signal, comprising
one or more carrier tuner means, each for receiving the baseband signal and
each including
means for deriving from the baseband signal an IF carrier signal
representing a respective carrier frequency,
carrier mixer means for mixing the IF carrier signal and a first local
oscillator signal to produce a resultant IF information signal,
means for demodulating the resultant IF information signal to recover
transmitted information, and pilot tuner means for receiving the baseband
signal and including
means for deriving from the baseband signal an IF pilot signal representing
the pilot frequency,
pilot mixer means for mixing the IF pilot signal and the first local
oscillator signal to produce a resultant pilot signal,
local oscillator means responsive to a first control signal level for
developing the first local oscillator signal having a frequency
corresponding to the first control signal level and for supplying the
first local oscillator signal to the carrier mixer means and pilot mixer
means to compensate for any drift or dispersion of frequency of the IF
carrier signals and IF pilot signal, and
frequency discriminator means for detecting frequency drift and dispersion
of the resultant pilot signal from a predetermined center frequency and
for supplying to the local oscillator means the first control signal whose
level identifies the drift and/or dispersion of the resultant pilot
signal, to thereby cause the local oscillator means to produce the local
oscillator signal which, when supplied to the carrier mixer means and
pilot mixer means, compensates for frequency drift and frequency
dispersion occurring in the IF carrier signals and IF pilot signal from
their operating frequencies, the frequency discriminator means including:
a first signal source for producing a first reference frequency signal,
a second signal source for producing a second reference frequency signal
which is 90 degrees out of phase with the first reference frequency
signal,
a first mixer for mixing the resultant pilot signal and the first reference
frequency signal to produce a phase error control signal,
a second mixer for mixing the resultant pilot signal and the second
reference frequency signal to produce a first frequency error signal,
differentiation means for receiving the first frequency error signal and
for producing a signal representing the derivative of the first frequency
error signal,
multiplier means for generating the product of the phase error signal and
the derivative signal to produce a frequency error control signal, and
switch means for selectively supplying one of the phase error control
signal and frequency error control signal to the local oscillator means
when information of at least a predetermined accuracy is obtained from the
carrier frequencies. |
|
 |
|
 |
Claims |
|
|
|
Description |
|
|
BACKGROUND OF THE INVENTION
This invention relates to a satellite transmission and receiving system in
which audio and data information, among others, may be transmitted and
received more efficiently and economically while complying with regulatory
agency signal strength limitations.
Transmission of information such as audio, data, video, and the like via
communication satellites has become commonplace in today's information
technology society. In carrying out such transmission, it is desirable to
use the strongest signal allowable so that the transmitted signal may be
more accurately received by the least expensive equipment possible, i.e.,
using the smallest satellite receiving dish possible. On the other hand,
the stronger the transmitted signal, the greater likelihood there is that
such signal could interfere with other signals on the same satellite
having closely related frequencies, signals located in nearby space but
directed to or coming from other satellites, and signals transmitted from
ground stations directly to other ground stations. In addition, most
countries have established maximum acceptable satellite signal strengths
for signals in given frequency bands. Oftentimes, these maximum limits
make impossible or, at least, impractical, the use of smaller satellite
receiving dishes since such dishes are not capable of accurately receiving
signals whose strengths are under the allowed maxima. Of course, small
satellite receiving dishes are desirable because they are easy to install
and align, less aesthetically offensive, and are much lower in cost than
larger more conventional satellite receiving dishes.
A number of modulation and transmission methods have been developed for
transmitting signals via satellites some of which have attempted to
overcome the signal strength limitation problem. The most commonly used
method, however, known as the single carrier per channel (SCPC) method,
generally does not overcome this problem and so the larger, more expensive
satellite receiving dishes must be used with the method. In the SCPC
method, the frequency bandwidth available for transmission of signals is
divided into carriers, each having a bandwidth different from the
bandwidths of the other carriers and each being assigned a "center
frequency" located in the center of the carrier bandwidth. Each source of
information such as audio information, data information, video
information, etc., is considered a "single channel" and is modulated onto
a respective one of the carriers, and each carrier carries only the
information of its respective channel. Among the advantages of the SCPC
method are the flexibility in the assignment of frequency and bandwidth
and the allocation of power to each particular carrier, and generally
lower power requirements. The major disadvantages of the SCPC method is
the need for frequency stability in the reception of the transmitted
signal and this generally requires the use of high stability (and high
cost) crystals located in constant temperature ovens, and the use of
phase-locked oscillators located on the satellite receiving dish. Also, as
already mentioned, the limitations imposed on transmitted signal strength
generally eliminates the use of small size satellite receiving dishes with
the SCPC method.
One approach to overcoming the signal strength limitation problem is the
so-called "spread spectrum" method. This method allows for the
transmission of a very strong signal by moving the energy of the signal
rapidly among different frequencies at the transmitting end. In this
manner, the average signal strength is spread among a number of
frequencies and therefore is maintained below the maximum allowed.
However, this approach is expensive, requiring high cost receiving
equipment, and only a few satellite carriers necessary with this method
can be accommodated by a satellite transponder.
Another approach to overcoming the signal strength limitation problem is
referred to as the FM-FM method and allows many carriers to share a common
transmitter and satellite transponder. In particular, a number of
frequency modulated carriers are combined into a common signal and then
this common signal is modulated again onto a transmitted carrier. For
example, a number of FM audio and frequency shift keyed (FSK) data signals
may be multiplexed and modulated onto a wide band transmitted FM carrier.
Among the disadvantages of this method is that all signals must originate
from the same point since only one transmitter may be used and thus
operators must deliver their audio or data information to the transmitter
site; also, an entire satellite transponder or significant portion
thereof, must be allocated for the method.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a satellite communication
system capable of transmitting a variety of information such as audio,
data and video information, via satellites to small satellite receiving
dishes.
It is another object of the invention to provide such a system which meets
the signal power density limitations established by regulatory agencies
for satellite transmission.
It is a further object of the invention to provide such a system in which
minimal error or loss of transmitted signal occurs at the receiver.
It is an additional object of the invention to provide such a system which
may utilize low cost conventional components.
It is also an object of the invention to provide such a system in which
drift or dispersal of frequencies in a transmitted signal may be
accurately tracked and compensation made therefor in an efficient and
reliable manner.
It is still another object of the invention to provide such a system which
requires the use of relatively small and inexpensive transmitters.
The above and other objects of the invention are realized in a specific
illustrative embodiment of a satellite communication system which includes
a transmitting station and a receiving station for respectively
transmitting and receiving information via a satellite. The transmitting
station has a plurality of information modulators, each for modulating
information from a corresponding audio, data, video, etc. information
source (or channel) onto an information carrier signal having a
predetermined center frequency. Also included in the transmitting station
is a pilot signal oscillator for producing a pilot signal having a
reference frequency separated by predetermined amounts from all
information carrier frequencies which, in turn, are separated in frequency
from one another by predetermined amounts. A local oscillator is provided
for producing a local oscillatory signal whose frequency varies in
accordance with the application thereto of a dispersal waveform. A
dispersal waveform generator supplies a time varying voltage dispersal
waveform to the local oscillator to cause the local oscillatory signal
frequency to periodically vary or "disperse." The information carrier
signals, pilot signal and local oscillatory signal are all supplied to a
signal mixer which produces an intermediate frequency composite signal
with information carrier frequencies and a pilot frequency which
synchronously vary or "disperse" over a predetermined range. The composite
signal is passed to a bandpass filter which selects either the sum or
difference frequencies produced by the mixer process, for supply to an
up-convertor which increases the frequencies to produce an information
signal for transmission to a relay satellite.
The receiving station includes a satellite receiving dish for receiving
information signals relayed by the satellite, and a down-convertor, which
may consist of several stages, for converting the frequencies in the
information signals to a baseband signal containing frequencies
corresponding to the information carrier frequencies and pilot frequency,
separated in frequency by amounts proportionate to the separation of the
information carrier and pilot frequencies in the original composite
signal. Also included are a plurality of carrier tuners, each for
receiving the baseband signal and deriving therefrom an IF carrier signal
containing information corresponding to information contained in a
respective one of the original information carrier signals. Each of the
carrier tuners includes a carrier mixer for mixing the IF carrier signal
and a compensating oscillatory signal to produce a resultant information
signal, and a demodulator for demodulating the resultant information
signal to recover the information contained therein.
The receiving station further includes a pilot tuner for receiving the
baseband signal and deriving therefrom an IF pilot signal. Finding the
pilot signal (and its frequency) will allow finding the carrier signals
since the frequency separation between the carrier signals and pilot
signal is know. The pilot tuner includes a pilot mixer for mixing the IF
pilot signal and the compensating oscillatory signal to produce a
resultant pilot signal, a compensating oscillator responsive to a voltage
signal for producing a compensating oscillatory signal having a frequency
determined by the level of the voltage signal, and for supplying the
compensating oscillatory signal to the carrier mixers and the pilot mixer.
The pilot tuner further includes a frequency detector for detecting the
frequency of the resultant pilot signal and any frequency drift or
dispersion of the resultant pilot signal from a predetermined center
frequency, and a controller coupled to the frequency detector for
supplying to the compensating oscillator a voltage signal whose level is a
function of the detected frequency of the resultant pilot signal, so that
the compensating oscillator produces a compensating oscillatory signal
which, when mixed with the IF carrier signals and IF pilot signal,
counters any frequency drift and dispersion occurring in such signals by
the amount of drift and dispersion detected in resultant pilot signal.
In effect, a dispersal signal is utilized at the transmitting station to
spread the energy of the information carrier signals and the pilot signal
over a wider bandwidth to thereby maintain the signal strength within
regulatory limits. In carrying out such dispersal, the center frequency of
the information carrier signals and the pilot signal are moved
synchronously at a gradual rate so that at the receiving station, the
pilot signal can be found and the frequency relationship between the pilot
signal and the information carrier signals established. In other words,
the pilot tuner tracks the pilot signal frequency dispersion (and the
drift which may occur with temperature changes, component deficiencies,
etc.) and signals the carrier tuners to similarly track and follow the
dispersion (and drift) of the information carrier center frequencies. This
allows narrow bandwidth signals to be tracked and demodulated to recover
the desired information.
Although the system is designed to intentionally disperse the frequencies
of the transmitted signals and then track such dispersion at the receiving
station, the system is also useful for simply tracking drift which may
occur in the transmitted and received signal, even if no dispersion is
intentionally produced.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, features and advantages of the invention will
become apparent from a consideration of the following detailed description
presented in connection with the accompany drawings in which:
FIG. 1 is a schematic diagram of a satellite communication system
transmitting station made in accordance with the principles of the present
invention;
FIG. 2 is a schematic diagram of a satellite communication system receiving
station made in accordance with the principles of the present invention;
and
FIG. 3 is a schematic diagram of one illustrative embodiment of the
dispersal tracking circuit of the pilot signal tracking tuner of FIG. 2.
DETAILED DESCRIPTION
Referring to FIG. 1, there in shown a plurality of information sources such
as audio sources 4 (which might be compact disk players or tape playback
equipment) and data sources 8 (which might be computers, news wire
services or facsimile equipment). Other information sources such as video
could also be provided and accommodated and the showing in FIG. 1 of the
audio and data sources is only to illustrate that one or more information
sources may be accommodated with the present system.
Audio and data information, in the form of electrical signals, are supplied
to respective audio modulators 12 and data modulators 16 where the
information is modulated onto respective carrier signals, each of whose
center frequency differs from the center frequency of every other carrier
signal. For example, the audio and data carrier center frequencies could
be 7, 6.8, 6.6, 6.4, etc. MHz. The modulators 12 and 16 could be any type
of conventional modulator such as FM (frequency modulation), FSK
(frequency shift keying), PSK (phase shift keying), and quadriture PSK, or
other continuous carrier signal modulator.
Also provided in the transmitter of FIG. 1 is a pilot signal oscillator 20
for producing a pilot signal having a frequency which is spaced from the
center frequencies of the information carrier signals by predetermined
amounts and this might, for example, be set at a frequency of 7.2 MHz. As
will become clear later on, the pilot signal is used as a reference
frequency signal to enable a receiving station to locate and track the
pilot signal and thus locate and track the carrier frequency signals
containing desired information. The modulated carrier frequency signals
and the pilot signal are all supplied to a mixer 24 which is conventional
in design as is the pilot signal oscillator 20.
A second input to the mixer 24 comes from a frequency modulated local
oscillator 28 which produces a local oscillatory signal of significantly
higher frequency than the frequencies of the carrier signals or pilot
signal such as, for example, 77 MHz. Coupled to the oscillator 28 is a
dispersal wave form generator 32 which supplies to the oscillator a
time-varying voltage waveform preferably of low frequency such as, for
example, 20-30 Hz. Although a variety of waveform shapes could be
utilized, a modified triangular shape such as shown at 36 is preferred
since, at the receiving end, it facilitates the location and tracking of
received signal frequencies as will be further explained hereafter. The
dispersal waveform generator 32 might illustratively, comprise a digital
to analog convertor driven by a digital input signal generated by a
digital signal generator included within the dispersal waveform generator.
The dispersal waveform 36 supplied to the local oscillator 28 causes the
oscillator to vary or sweep the frequency of its output signal over some
predetermined range about its center frequency such as, for example, plus
and minus 100 KHz.
The mixer 24 mixes the local oscillatory signal, the modulated information
carrier signals, and the pilot signal to produce sum and difference
frequency signals which are then supplied to a band pass filter 40 which
filters out the signal which composes the sum of the frequencies and
passes the signal which composes the difference of the frequencies (but
either the sum or the difference signal could be used). An up-convertor 44
further converts and increases the frequency of the signal to a level
suitable for satellite transmission and the signal is then supplied to an
amplifier 48 which amplifies the signal and supplies it to a satellite
dish transmitter 52 for transmission to a satellite.
All of the individual components of the transmitter of FIG. 1 are
conventional and, well-known, but they have been combined and used in a
novel manner as will be further evident hereafter.
Referring to FIG. 2, there is shown one illustrative embodiment of
receiving equipment for receiving information signals relayed by a
satellite, such equipment including a satellite receiving dish 104 which
receives the relayed information signals and supplies them to an amplifier
108 which amplifies the signals and, in turn, supplies them to a
down-convertor 112. The down-convertor 112 converts down the frequencies
in the received signal to an intermediate lower frequency range such as
950 to 1450 MHz, while maintaining the relative separation between the
pilot frequency and the carrier frequencies. The satellite receiving dish
104, amplifier 108, and down-convertor 112 are all conventional devices.
Each down-converted signal is then supplied to a transponder tuner 116
where the signal is further down-converted to a lower frequency range and
where the desired frequency band, referred to as the baseband signal, is
selected for passing onto the next stage in the receiving station.
Illustratively, the baseband signal range is 4-8 MHz. The transponder
tuner 116 is shown to include two stages for down conversion of the
frequencies but there could be as many stages as desired. Each stage
includes a mixer such as mixer 120 which receives the down-converted
signal (after amplification by amplifier 124) from the down-convertor 112,
and also receives an oscillatory signal from an oscillator 128 whose
output is maintained stable by a phase lock loop circuit 132. The mixer
120 mixes the two input signals and supplies the resultant signal to an
amplifier 136 and then to a band pass filter 140 where the desired
frequency band is selected and supplied to another mixer 144 for further
mixing and down-converting of the frequency range.
At the transponder tuner stage in most prior art systems, generally great
expense is expended to provide equipment and components to maintain signal
accuracy and stability. With the present invention, the transponder tuner
stage may utilize low cost components since the next stage of the
receiving station, as will next be described, provides for compensation of
frequency drift and dispersal which may be present in the received
signals.
Coupled to the transponder tuner 116 for receiving the baseband signal
produced thereby are one or more audio carrier tuners 150, one or more
data carrier tuners 154, and a pilot signal tracking tuner 158. As will be
described in detail, the pilot signal tracking tuner 158 locates, locks
on, and tracks the pilot signal frequency and provides signals to the
audio carrier tuners 150 and data carrier tuners 154 to similarly lock
onto and track respective information carrier signals to enable
demodulation of the signals and recovery of the transmitted information.
In effect, the pilot signal tracking tuner 158 counters any dispersion or
drift of frequency present in the pilot signal and information carrier
signals so that each carrier channel is confined to its normal narrow
bandwidth. The signal-to-noise ratio is thereby improved because the
higher power (allowed by dispersal) is concentrated in a narrow bandwidth
after dispersal tracking is achieved.
Each audio and data carrier tuner includes a mixer, such as mixer 162 of
tuner 150, which receives the baseband signal and mixes it with a local
oscillator signal received from oscillator 166. The stability of the
output of oscillator 166 is maintained by a conventional phase lock loop
circuit 170 to which it is coupled. The mixer 162 mixes the two input
signals and supplies the sum and difference frequencies to a bandpass
filter 174 which, in turn, filters out either the sum or difference
frequencies and passes the other to a second mixer 178. The center
frequency of the frequencies passed to the mixer 178 might illustratively
be 10.7MHz. The mixer 178 mixes the signal received from the bandpass
filter 174 with a master local oscillator signal (which, for example,
might be 11.7 MHz) received from an oscillator 204 of the pilot signal
tracking tuner 158. It is this master local oscillator signal which allows
the audio carrier tuners 150 and data carrier tuners 154 to follow any
dispersion or drift present in the corresponding information carrier
signals to enable demodulation thereof. The mixer 178 mixes the two input
signals and supplies them to a bandpass filter 182 which selects the
desired mixing product, for example, a 1 MHz centered information carrier
signal. This signal is then supplied to an audio demodulator 186 where the
information contained in the corresponding information carrier signal is
recovered and supplied to a utilization device (not shown). Each of the
audio carrier tuners 150 and data carrier tuners 154 operate in the same
manner described above for processing respective information carrier
signals.
The baseband signal developed by the transponder tuner 116 is also supplied
to the pilot signal tracking tuner 158 and to a mixer 190 included
therein. The function of the pilot signal tracking tuner 158 is to locate
and track the transmitted pilot signal, and this is done with a first
stage mixer 190 and bandpass filter 194 which operate in the manner
similar to the first stage of the information carrier tuners. The bandpass
filter 194 passes the filtered intermediate frequency (IF) pilot signal to
a mixer 198 where it is mixed with the master local oscillator signal
supplied by the oscillator 204. As will be described momentarily, the
frequency of the oscillator 204 is continually adjusted to counter any
dispersion or drift occurring in the received information carrier signals
and pilot signal and, in effect cancel such dispersion or drift.
The mixer 198 supplies the mixing product to a bandpass filter 208 which
selects and passes the desired mixing product to a dispersal tracking
circuit 212. The dispersal tracking circuit 212 processes the received
signal and supplies a voltage to the oscillator 204 to vary the frequency
of the output of the oscillator as necessary to cancel any dispersion or
drift occurring in the received information carrier signals and pilot
signal.
The dispersal tracking circuit 212 is shown in detail in FIG. 3 to include
a quadricorrelator frequency discriminator 220. The quadricorrelator
frequency discriminator (QFD) 220 is of conventional design (see Gardner,
F. M., Phaselock Techniques, Wiley, New York, 1979, pp. 84-87) and
includes a mixer 224 for receiving and mixing the IF pilot signal from the
bandpass filter 208, and a reference frequency signal (e.g., 1 MHz)
received from a reference frequency generator 228. The mixing product is
supplied by the mixer 224 to a low pass filter 234 which passes a signal
having frequencies, for example, in the range of direct current (DC) to 25
KHz, which serves as a "phase error" signal. The IF pilot signal is also
supplied to a second mixer 238 of the QFD 220 which mixes it with a second
reference frequency signal, (e.g., also 1 MHz) ninety degrees out of phase
with the first reference frequency signal, supplied by a reference
frequency signal generator 242. The mixing product is supplied to a second
low pass filter 246 which passes a signal having frequencies, for example,
in the range of DC to 25 KHz to a differentiator 250. The differentiator
250 produces a signal representing the derivative of the output signal
from the low pass filter 246 and passes it to a multiplier 254 which
multiplies the derivitive with the output of the low pass filter 234 to
develop a "frequency error" signal. The frequency error signal contains a
DC component proportional to the frequency difference between the IF pilot
signal and the signal from the reference frequency generator 242.
The frequency error signal, provides a coarse tuning signal for initially
acquiring the pilot signal, and the phase error signal, provides a fine
tuning signal for precisely tracking the pilot signal. Both the frequency
error signal and phase error signal are supplied to a loop switch 258
which, based on its setting, passes either one or the other of the signals
to a loop filter 262. Initially the loop switch 258 wou | | |
