Television – Monitoring – testing – or measuring
Reexamination Certificate
2000-10-27
2003-05-20
Kostak, Victor R. (Department: 2611)
Television
Monitoring, testing, or measuring
C455S245100, C455S237100, C725S149000, C375S345000, C348S678000
Reexamination Certificate
active
06567118
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to broadband communications, such as cable television systems, and more specifically to automatic gain control (AGC) circuits.
BACKGROUND OF THE INVENTION
A communication system
100
, such as a two-way cable television system, is depicted in FIG.
1
. The communication system
100
includes headend equipment
105
for generating forward signals that are transmitted in the downstream direction along a communication medium, such as a fiber optic cable
110
, to an optical node
115
that converts optical signals to radio frequency (RF) signals. The RF signals are further transmitted along another communication medium, such as coaxial cable
120
, and are amplified, as necessary, by one or more distribution amplifiers
125
positioned along the communication medium. Taps
130
included in the cable television system split off portions of the forward signals for provision to subscriber equipment
135
, such as set top terminals, computers, and televisions. In a two-way system, the subscriber equipment
135
can also generate reverse signals that are transmitted upstream, amplified by any distribution amplifiers
125
, converted to optical signals, and provided to the headend equipment
105
.
Communication systems, as depicted in
FIG. 1
, often include amplifiers to boost signal levels, and automatic gain control (AGC) circuits within the amplifier are typically used to monitor and control the gain of such amplifiers.
FIG. 2
is an electrical block diagram of a conventional amplifier
125
that includes an AGC circuit. In operation, the amplifier
125
receives a forward signal from the downstream path at an input port
205
. In conventional cable television systems, the forward signals have been predominately transmitted over analog channels. The forward signal is transmitted through one or more gain stages
210
for amplifying the forward signal. The amplified signal is then transmitted through a Bode circuit
215
that varies the signal level by attenuation. A final output gain stage
220
subsequently processes the forward signal, which is then transmitted to an output port
225
. The output of the final gain stage
220
is also coupled to an AGC circuit
230
that is used to further control the attenuation of the Bode circuit
215
in response to the signal level of the amplified forward signal.
FIG. 3
is an electrical block diagram of the conventional AGC circuit
230
of FIG.
2
. The AGC circuit
230
includes an input port for receiving the forward signal, which is also coupled to the amplifier output
225
of the final gain stage
220
. The AGC circuit
230
includes a band pass filter
305
and a gain stage
310
for filtering and amplifying, respectively, the pilot signal. The filtered signal is then transmitted to an AM detector
3
15
that demodulates the signal to recover an analog video signal. Next, the demodulated analog video signal is amplified through a video amplifier
320
.
A video peak detector
325
then samples the horizontal synchronization pulses of the demodulated video signal in order to establish a DC voltage that represents the peak carrier level. The peak detector
325
monitors the DC voltage of the horizontal synchronization pulses of the demodulated video signal to set and hold peak voltage values and to generate a peak voltage signal representative of the peak voltage values. This can be done, for example, by storing the DC voltage values in a capacitor-resistor network between synchronization pulses.
The DC voltage signal provided by the peak detector
325
is compared with a thermal reference level by the integrator
330
to verify that the gain of the amplifier gain stages has remained constant. If the compared peak voltage signal has dropped below or risen above the predetermined thermal reference level, a thermal reference voltage signal from the reference voltage circuit
335
will be provided at the output of the AGC circuit
230
until such time as the voltage level of the pilot carrier signal level again equals the reference voltage level.
Again referring to the forward input signal to the AGC circuit
230
, the input signal is filtered through the band pass filter
305
to allow a predetermined pilot carrier signal to pass. The pilot carrier signal is then demodulated and used, through comparison with the reference voltage level, to control the Bode circuit
215
(FIG.
2
). The frequency of the pilot carrier signal is generally determined by selecting a median point between the lowest channel and the highest channel in the forward frequency spectrum, although the frequency of the pilot carrier may vary as long as the band pass filter
305
and other device components are configured to process a carrier signal of the desired frequency.
Historically, cable television systems have transmitted only analog signals, so transmission and processing of an analog pilot carrier signal by conventional analog AGC circuits has worked well. The cable television industry, however, is migrating to transmission of digital signals, so a pilot carrier signal in the digital frequency spectrum may, in the future, be chosen for processing through the AGC circuit. These digital signals are generally QAM modulated, and QAM modulated digital signals cannot be accurately processed by prior art AGC circuits, such as the AGC circuit
230
shown in FIG.
3
. More specifically, the DC voltage values of the QAM modulated signals include complex, multi-level data having peak values at varying times and different rates, so peak detection in a conventional AGC circuit does not provide useful or accurate information.
In addition to the potential problem that the pilot carrier signal may be within the digital frequency spectrum, another problem is that cable operators may choose to change the frequency of the pilot carrier signal after field installation. More specifically, after the network
100
has been installed and is servicing subscribers through use of a particular pilot carrier frequency, channel frequencies may change; for example, the cable operators may want to bundle channels into different packages that may require some cable television channels to change in the frequency spectrum. Consequently, the frequency required for a new channel may be the same frequency as the pilot carrier frequency. Additionally, the operator may have ordered an incorrect pilot carrier frequency and installed the units before the error is realized. In order to change a preset pilot carrier frequency to a different frequency, the operator must locate each piece of equipment that includes an AGC circuit and physically replace it with an AGC circuit having a different pilot signal frequency. Replacing this amount of circuitry can be, however, extremely time consuming and expensive.
Thus, what is needed is an AGC circuit that has the agility and flexibility to tune and process numerous pilot carrier signals in either the analog or digital frequency spectrum.
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patent: 6054896 (2000-04-01), Wright et al.
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patent: 0495141 (1992-07-01), None
patent: WO 97/47093 (1997-12-01), None
Al-Araji Saleh
Patterson Dale
Quesenberry Paul
Barnhardt III Hubert J.
Couturier Shelley L.
Kostak Victor R.
Massaroni Kenneth M.
Scientific-Atlanta, Inc.
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