| United States Patent |
4,785,302 |
|
Ma
, et al.
|
November 15, 1988
|
Automatic polarization control system for TVRO receivers
Abstract
In a TVRO earth station having an antenna for receiving incoming satellite
signals and polarizing means associated with the antenna for adjusting the
relative alignment of the antenna orientation and the polarization of the
incoming signals, an automatic polarization control system comprising
means for producing an electrical control signal for controlling the
polarizing means to adjust the relative alignment, means for detecting the
noise level in the satellite signals received by the antenna, and means
responsive to the detected noise level for adjusting the control signal,
and thereby adjusting the polarizing means, to minimize the detected noise
level.
| Inventors: |
Ma; John Y. (Milpitas, CA), McCracken; David H. (San Jose, CA), Weiss; Steven (Los Gatos, CA), Houston, III; Albert C. (Santa Cruz, CA) |
| Assignee: |
Capetronic (BSR) Ltd.
(Kowloon,
HK)
|
| Family ID:
|
26293321
|
|---|
| Appl. No.:
|
06/792,780 |
|---|
| Filed:
|
October 30, 1985 |
|---|
| Current U.S. Class: |
342/362 ; 342/352; 342/358 |
| Current International Class: |
H01Q 1/12 (20060101); H01Q 21/24 (20060101); H01Q 021/06 () |
| Current CPC Class: |
H01Q 1/1257 (20130101); H01Q 21/245 (20130101) |
| Field of Search: |
342/352,357,358,362,75,361,363 343/909,754
|
References Cited [Referenced By]
U.S. Patent Documents
Other References
Butts, "A Frequency Reuse K-Band 60-Foot Antenna System for the TDRSS Ground Segment," pp. 25.2.1-25.2.5, 6/80.. |
Primary Examiner: Blum; Theodore M.
Assistant Examiner: Cain; David
Attorney, Agent or Firm: Rudisill; Stephen G.
Claims
We claim:
1.
In a TVRO earth station having an antenna for receiving incoming
satellite signals and polarizing means associated with the antenna for
adjusting the relative alignment of the
antenna orientation and the polarization of the incoming signals, an
automatic polarization control system comprising
means for producing an electrical control signal for controlling said polarizing means to adjust said relative alignment,
means for detecting the noise level apart from the signal level in the satellite signals received by the antenna, and
means responsive to the detected noise level for adjusting said
control signal, and thereby adjusting said polarizing means, to minimize
the detected noise level.
2. The TVRO earth station of claim 1 wherein said polarizing
means comprises a ferrite polarizer which adjusts the polarization of
the incoming satellite signals relative to the antenna orientation to
selectively determine which polarization of
the incoming signals is actually received by the antenna.
3. The TVRO earth station of claim 1 wherein said antenna
includes a feed horn containing a probe, and said polarizing means
comprises means for rotating said probe to selectively determine which
polarization of the incoming signals is actually
received by the antenna.
4. The TVRO earth station of claim 1 which includes a tuner for
selecting the signals received from a particular satellite transponder,
and a demodulator for demodulating the selected signals, and wherein
said noise-detecting means receives the
video output from the demodulator for detecting the noise level in the
signals received from the selected transponder.
5. The TVRO earth station of claim 1 which includes means for
determining the minimum detected noise level for a selected channel and
storing a value representing the level of said control signal
corresponding to the minimum detected noise
level.
6. The TVRO earth station of claim 5 which includes means
responsive to each channel selection for adjusting said control signal
to a level corresponding to said stored value.
7. The TVRO earth station of claim 1 which includes means for
adjusting said control signal through a predetermined range of values,
and thereby adjusting said polarizing means through a predetermined
range of settings.
8. In a TVRO earth station having an antenna for receiving
incoming satellite signals and polarizing means associated with the
antenna for adjusting the relative alignment of the antenna orientation
and the polarization of the incoming signals,
an automatic polarization control system comprising
means for producing an electrical control signal for controlling said polarizing means to adjust said relative alignment,
means for detecting the noise level apart from the signal level in the satellite signals received by the antenna,
means for adjusting said control signal through a predetermined
range of values, and thereby adjusting said polarizing means through a
predetermined range of settings, and
means for determining the minimum noise level detected during
the adjustments of said control signal and storing the control signal
value corresponding to said minimum noise level.
9. The TVRO earth station of claim 8 wherein said polarizing
means comprises a ferrite-core polarizer which adjusts the polarization
of the incoming satellite signals relative to the antenna orientation to
selectively determine which
polarization of the incoming signals is actually received by the
antenna.
10. The TVRO earth station of claim 8 wherein said antenna
includes a feed horn containing a probe, and said polarizing means
comprises means for rotating said probe to selectively determine which
polarization of the incoming signals is actually
received by the antenna.
11. The TVRO earth station of claim 8 which includes a tuner
for selecting the signals received from a particular satellite
transponder, and a demodulator for demodulating the selected signals,
and wherein said noise-detecting means receives the
video output from the demodulator for detecting the noise level in the
signals received from the selected transponder.
12. The TVRO earth station of claim 8 which includes means for
determining the minimum detected noise level for a selected channel and
storing a value representing the level of said control signal
corresponding to the minimum detected noise
level.
13. The TVRO earth station of claim 12 which includes means
responsive to each channel selection for adjusting said control signal
to a level corresponding to said stored value.
14. The TVRO earth station of claim 1 which includes a
demodulator for receiving the incoming satellite signals and producing a
video baseband signal which is supplied to said noise-level-detecting
means, and said noise-level-detecting means
detects the noise level at a frequency above that of the video
information in said baseband signal.
Description
BACKGROUND OF THE INVENTION
This invention relates generally to TVRO receivers for the
reception of a wide range of satellite TV signals and, more
particularly, to an automatic polarization adjustment system for
automatically aligning an earth station antenna with the plane
of polarization of the particular channel to which the TVRO receiver is
tuned at any given time.
In a TVRO system, the satellite signals are received by an
antenna (usually a paraboloidal dish) and converted to a lower "1st IF"
frequency at the antenna site. This conversion may be effected by a
down converter, which converts only a single
channel to the 1st IF frequency, or a block converter, which converts
all channels of a common polarity to a 1st IF block of frequencies
ranging from 950 to 1450 MHz. This entire block of frequencies is then
fed via coaxial cable to the receiver, which
selects a particular channel for viewing and/or listening. In the
receiver, the 1st IF signals are converted to a 2nd IF frequency range
which traditionally has been centered at 70 MHz in most TVRO systems.
The signals transmitted by a satellite have a designated
polarization determined by the mode of excitation at the transmitting
antenna prior to propagation through space. For efficient reception,
and maximum video and/or audio quality, the
receiving antenna subsystem of the TVRO must be properly aligned with
the polarization of the received signals.
To minimize interference among signals from satellite
transponders using adjacent frequency bands, the signals from
transponders using alternate 20-MHz frequency bands have a first
polarization, e.g., horizontal, while the transponders using the
intervening frequency bands have a polarization orthogonal to the first,
e.g., vertical. The frequency-modulated signals transmitted in
adjacent channels almost always overlap each other, and thus it is
important to have the receiving antenna precisely
aligned with the polarization of the desired channel in order to avoid
interference from adjacent channels. The signals from local terrestrial
microwave links also overlap the frequency bands of the satellite
channels, and thus precise polarization
alignment of the selected signals and the receiving antenna can also
help reduce terrestrial interference (commonly referred to as "TI").
Furthermore, proper orientation of an earth station antenna for
reception of polarized signals from one satellite does not mean that the
same orientation will provide optimum reception from other satellites.
There are numerous satellites in
geostationary orbit today, and these satellites are azimuthally spaced
from one another to avoid interference. Thus, as an earth station
antenna is swept across the array of geostationary satellites, the
polarization planes of the different satellites
are slightly skewed relative to each other due to the azimuthal spacing
of the satellites. As a result, the optimum plane of polarization of
the earth-based antenna varies from satellite to satellite.
To properly align the antenna subsystem and the polarization
plane of the incoming signals from any given satellite transponder, TVRO
systems normally include polarizers which can adjust the relative
alignment of the polarization of the incoming
signals and the orientation of the antenna. One type of polarizer
mechanically rotates the small probe that is included in the feedhorn of
most earth station antennas, by means of a small servomotor which is
powered by either the indoor receiver or an
antenna positioner. A second type of polarizer adjusts the polarization
of the incoming signal electronically, by changing the voltage applied
to a coil wound around an electromagnetic ferrite core located at the
throat of the feedhorn.
SUMMARY OF THE INVENTION
It is a primary object of the present invention to provide an
improved TVRO receiver which automatically and reliably optimizes the
relative alignment of the antenna orientation and the polarization of
the particular satellite signal selected by
the tuner.
Another object of this invention is to provide an improved TVRO
receiver having an automatic polarization control system which can be
used with either mechanical or electronic polarizers.
A further object of the invention is to provide such an improved
TVRO receiver in which the automatic polarization control system does
not require any manual intervention or data input by the user.
Still another object of the invention is to provide such an
improved TVRO receiver in which the automatic polarization control
system can be easily and economically incorporated in an otherwise
standard receiver at a relatively low cost.
Other objects and advantages of the invention will be apparent
from the following detailed description and accompanying drawings.
In accordance with the present invention, an automatic
polarization control system is provided for a TVRO earth station having
an antenna for receiving incoming satellite signals and polarizing means
associated with the antenna for adjusting the
relative alignment of the antenna orientation and the polarization of
the incoming signals, the control system comprising means for producing
an electrical control signal for controlling the polarizing means to
adjust the relative alignment of the
antenna and the signal polarization, means for detecting the noise level
in the satellite signals received by the antenna, and means responsive
to the detected noise level for adjusting the control signal, and
thereby adjusting the polarizing means, to
minimize the detected noise level.
In a particularly preferred embodiment, the control signal is
adjusted through a predetermined range of values, thereby adjusting the
polarizing means through a predetermined range of settings, and the
control system includes means for
determining the minimum noise level detected during the adjustments of
the control signal and storing the control signal value corresponding to
the minimum noise level.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention and other objects and advantages thereof, may best
be understood by referring to the following description in conjunction
with the accompanying drawings, in which:
FIG. 1 is a functional block diagram of a conventional TVRO earth station;
FIG. 2 is a diagram of a demodulator for use in the system of FIG. 1;
FIG. 3 is a simplified block diagram of an automatic
polarization control system, embodying the present invention, for use in
the system of FIG. 1;
FIG. 4 is a graphical illustration of the polarization-rotating effect of a conventional ferrite polarizer;
FIG. 5 is a diagram of a preferred noise detector for use in the system of FIG. 3; and
FIG. 6 is a flow chart of a program for controlling the
microprocessor in the control system of FIG. 3 according to the
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Although the invention will be described with respect to certain
preferred embodiments, it will be understood that it not intended to
limit the invention to those particular embodiments. On the contrary,
it is intended to cover all alternatives,
modifications and equivalent arrangements as may be included within the
spirit and scope of the invention as defined by the appended claims.
Referring now to the drawings, in FIG. 1 there is shown a
functional block diagram of a TVRO earth station for the reception of
satellite signals. The system includes an antenna 11, which is
typically a paraboloidal dish equipped with a low
noise block (LNB) converter and related accessories and positioning
mechanisms, for capturing signals transmitted from orbiting satellites;
and a receiver system including a tuner 12, a demodulator 13, a video
processing and amplification section 14, and
an audio tuner 15.
The antenna 11 receives signals transmitted from the satellite
in the 4-GHz frequency band (3.7 to 4.2 GHz), and this entire block of
frequencies is converted to a 1st IF frequency range of 950 to 1450 MHz
by the block converter located at the
antenna site. The 1st IF signals are then sent via coaxial cable to the
tuner 12 which selects a particular channel for viewing and converts
the signals in that particular channel to a 2nd IF frequency range. The
2nd IF frequency range is preferably
high enough to permit the 2nd IF VCO frequencies to be above the 1st IF
block of frequencies, to prevent the VCO from interfering with the
desired signals. For a 1st IF frequency range of 950 to 1450 MHz, this
means that the center frequency of the
second IF frequency range must be at least 500 MHz. A particularly
preferred 2nd IF center frequency is 612 MHz.
FIG. 2 is a block diagram of a demodulator 13 for receiving the
2nd IF output of the tuner 12 in the TVRO system of FIG. 1. This
demodulator circuit includes a pair of conventional IF amplifiers 30 and
31 for receiving the 2nd IF signal from the
final amplifier 25 in the tuner 12. Both of these amplifiers 30 and 31
receive an automatic gain control (AGC) signal via resistor 32. From
the amplifier 31, the 2nd IF signal is passed through a filter 33 and on
to a conventional video detector 34
which demodulates the frequency-modulated signal to the baseband of the
original video signal (e.g., 0 to 10 MHz), producing a composite video
output signal. The 2nd IF filter 33 preferably has a pass band that is
only about 22 MHz wide; a pass band of
this width passes the essential video and audio information while
rejecting unwanted noise received by the antenna on the edges of the
selected channel.
The AGC feedback loop includes an IF amplifier 36 which
amplifies the output of the filter 33 and supplies it to an AGC detector
37. The output of this detector 37 is passed through an AGC amplifier
38, which produces a signal strength meter
drive signal at a terminal 39. This signal strength meter is usually
located on the front panel of the TVRO receiver.
The illustrative demodulator also includes an IF amplifier 40
which receives the same input supplied to the video detector 34,
amplifies it, and passes it through a narrow passband filter 41. The
output of the filter 41 is passed through a
detector in the form of a diode 42. The signal passed by the diode 42
is smoothed by an amplifier 43 to produce a DC output voltage that can
be used to detect the presence of a signal near the center frequency of
the particular satellite channel to
which the receiver is tuned. Although this signal is not used in the
system of the present invention, it is useful to discriminate between
satellite signals and TI.
The output of the demodulator illustrated in FIG. 2 comprises
the baseband signals which range from DC to about 8.5 MHz; this includes
video information from about 15 KHz to 4.2 MHz, and subcarriers from
about 4.5 to 8.5 MHz. The video
information in these baseband signals is passed through the video
processing and amplification section 14 before being displayed on a
video monitor or television set, and the audio signals are passed
through an audio tuner and then on to one or more
speakers which convert the signals to audible sound.
In accordance with one important aspect of the present
invention, the polarizer is controlled by an automatic system comprising
means for producing an electrical control signal for controlling the
polarizer to adjust the relative alignment of the
antenna orientation and the polarization of the incoming signals; means
for detecting the noise level in the satellite signals received by the
antenna; and means responsive to the detected noise level for adjusting
the polarizer control signal, and
thereby adjusting the polarizer, to minimize the detected noise level.
The use of the noise level to adjust the polarizer control signal
provides an extremely rapid response to the incoming signals.
In the illustrative embodiment of FIG. 3, the video output of
the demodulator is fed to a noise detector 50 which produces a DC output
whose magnitude is proportional to the noise level in the video
baseband signal. This DC output is passed
through an analog-to-digital converter (ADC) 52 to a microprocessor 53.
Both the noise detector 50 and the program for controlling the
microprocessor 53 will be described in more detail below in connection
with FIGS. 5 and 6. The basic function of the
microprocessor 53 is to produce an output signal that can be used to
continually adjust the polarizer over a predetermined range, while at
the same time evaluating the noise level detected at successive settings
of the polarizer to determine the
polarizer setting which produces the minimum noise level. This setting
is then stored so that it can be retrieved to re-set the polarizer the
next time the same channel is selected.
The microprocessor output signal for controlling the polarizer
is supplied to a digital-to-analog (DAC) converter 54, whose analog
output signal is applied to a summing junction 55. This signal
represents the commanded voltage to be applied to
the coil of a conventional ferrite-core polarizer 56. The other input
signal to the summing junction 55 is a signal representing the actual
setting of the polarizer, as determined by the current flow through a
fixed resistor 57 connected between the
polarizer coil and ground. The summing junction 55 algebraically sums
these "command" and "actual" input signals, and produces a resulting
"error" output signal proportional to any difference between the two
input signals. Of course, whenever the
microprocessor produces a signal intended to produce a change in the
polarizer setting, there will be an immediate discrepancy between the
"command" signal and the "actual" signal, thereby producing a deliberate
"error" signal to change the setting of
the polarizer 56.
The "error" output signal from the summing junction 55 is passed
through an amplifier 58 to a polarizer driver circuit 59 which
generates a DC voltage at the level required to set the polarizer 56 to
the desired position represented by the
"command" signal from the microprocessor 53. A number of different
conventional circuits can be used for the driver 59, but it is preferred
to use a two-stage, collector-output, current-limited Class B amplifier
for this purpose.
As is well known, the ferrite-core polarizer 56 includes a wire
coil wound around an electromagnetic ferrite core. The polarizer
essentially acts as a controlling phase shifter and has a feed horn
arrangement for accepting the incoming satellite
signals and then passing them through the ferrite core. When a voltage
is applied across the coil by the driver 59, an electromagnetic field of
corresponding strength is set up around the ferrite core. This field
interacts with the electromagnetic
fields propagating through the core and rotates the plane of
polarization of the received signals to a predetermined angle
corresponding to the magnitude of the DC voltage applied to the coil by
the driver 59.
The effect of changes in the DC voltage from the driver 59 on
the operation of the polarizer 56 is illustrated in FIG. 4, which is a
plot of the rotation of the signal polarization (in degrees) as a
function of the DC voltage applied to the
polarizer coil. As shown, zero voltage produces no phase shift, -18
volts rotates the signal 90.degree. in one direction, and +18 volts
rotates the signal 90.degree. in the opposite direction. In actual
ferrite-core polarizers which are commercially
available, the ferrite core is usually saturated at plus or minus 14
volts, which corresponds to .+-.75.degree. of signal rotation, so the
total practical range of signal rotation is about 150.degree..
The details of a preferred noise detector 50 are shown in FIG.
5. In this particular detector the video baseband signal from the
demodulator 13 is initially fed through a bandpass filter 60 which
preferably has a pass band that is about 500 KHz
wide centered at about 23 MHz, which is well above the video information
in the baseband signal. The 23-MHz center frequency also avoids
interference from 27-MHz CB signals, 21-MHz and 24.5-MHz ham radio
signals, and harmonics of the 4-MHz output of the
crystal oscillator in the tuner 12.
The output of the bandpass filter 60 is passed through a
conventional RF amplifier 61 to a detector in the form of a diode 62.
This diode 62 rectifies the AC output from the amplifier 61, and the
resulting signal is smoothed by passing it
through a DC amplifier 63 and a low pass filter 64. It is the smooth DC
output of the filter 64 that is applied to the ADC 52 in FIG. 3; as
explained previously, the magnitude of this DC signal will vary in
direct proportion to the noise level in the
video baseband output from the demodulator.
FIG. 6 is a flow chart of a preferred program for controlling
the microprocessor 53. This program is entered at step 101 where the
current value P.sub.C of a polarizer mode operator is initialized to a
current value of 1. This corresponds to a
zero voltage level at the polarizer, which produces no phase shift.
At step 102, the current value N.sub.C of the DC output of the
noise detector 50 is read to determine the noise level existing within
the particular signal to which the tuner is currently tuned. Step 103
then checks whether the operator P.sub.C
is at its initialized value, and if the answer is affirmative, the
current values N.sub.C and P.sub.C of the noise level and the polarizer
mode operator are stored at step 104 as values N.sub.L and P.sub.L
representing, respectively, the lowest measured
value of the noise level of the current signal, and the corresponding
mode operator at which that noise level was measured.
At step 105, which is reached by a negative response at step 103
or following step 104, the values N.sub.C and N.sub.L are compared. If
the current noise level value N.sub.C is found to be greater than the
lowest previously measured noise value
N.sub.L, as determined at step 106, the lowest noise value N.sub.L and
the corresponding polarizer mode operator value P.sub.L are restored at
step 107. But if the current noise level value N.sub.C is found to be
lower than the lowest noise value
N.sub.C, the current values N.sub.C and P.sub.C are stored as the new
values N.sub.L and P.sub.L at step 108.
From step 107 or 108, the system advances to step 109 to
determine whether the current value P.sub.C of the polarizer mode
operator is equal to the maximum value M. An affirmative answer at this
step indicates that the polarizer has been adjusted
through its entire range of settings, and thus the currently stored
value P.sub.C represents the polarizer setting that will produce the
lowest noise level. A negative answer at step 109 indicates that the
polarizer has not been adjusted through its
entire range of settings, and thus the current value P.sub.C of the
polarizer mode operator is to be incremented and steps 102 through 109
re-iterated. The incrementing of P.sub.C is effected at step 110, which
then returns the system to step 102.
It will be appreciated that each time the polarizer mode
operator value P.sub.C is incremented at step 110, the resulting output
signal from the microprocessor 53 changes the DC voltage output of the
polarizer driver 59 to change the setting of
the polarizer 56. The adjustment range of the polarizer is limited by
the saturation points of the ferrite core, but the sensitivity of the
control system is determined by the size of the increments, i.e., the
number of increments required to cover the
full range of the polarizer 56.
If the answer at step 109 is affirmative, the system advances to
step 111 where the polarizer setting is adjusted to the level
represented by the polarizer mode operator value P.sub.L corresponding
to the lowest noise level value N.sub.L. In
actual practice, instead of setting the polarizer directly to the best
mode P.sub.L, it is advisable to move upwardly in steps from the zero
setting to the selected setting in order to avoid hysteresis effects
inherent in the ferrite core of the
polarizer.
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