Wave transmission lines and networks – Pilot line-controlled systems
Reexamination Certificate
2000-10-02
2003-07-01
Summons, Barbara (Department: 2817)
Wave transmission lines and networks
Pilot line-controlled systems
C333S016000, C327S065000, C455S500000, C455S068000
Reexamination Certificate
active
06587012
ABSTRACT:
The present invention relates to methods for implementing the automatic slope and gain (ASG) control function in communications equipment, such as cable television distribution amplifier.
BACKGROUND OF THE INVENTION
Those skilled in the art know that the attenuation of radio frequency (RF) signals in coaxial cable is, to an extent, a function of the temperature of the cable. Accordingly, it has long been the practice in cable television systems to provide gain correction at certain amplifiers in a cascade, which is a “string” of amplifiers used to distribute signals to subscribers. Several methods of gain correction are used. The simplest gain correction method is known as thermal equalization. In this technique, the internal temperature of an amplifier station is sensed, for example by a thermistor, and the gain of the station is changed to approximate the change in cable loss that is assumed to result due to changes in ambient temperature, as represented by the change of temperature inside the amplifier. This technique is low in cost, but provides only for approximate compensation.
A more sophisticated method of controlling the amplifier gain involves sensing a signal level and adjusting the gain of the amplifier ahead of the sensing point to bring the level back to a standard, or expected, level. This is called an automatic gain control (AGC) system. The present invention is applicable to AGC systems, but present practice adds yet another level of sophistication called ASG. In ASG systems, it is recognized that the variation of attenuation with temperature exhibited by the coaxial cable is also a function of frequency. Accordingly, it is the practice to not only adjust the gain of the amplifier station according to the sensed signal level, but to adjust the gain vs. frequency characteristic (“slope”) of the station according to the sensed signal level.
Because the entire spectrum of signals on the cable is very wide (downstream transmissions in North America presently use frequencies from 54 to about 870 MHz.), it is common practice to sense only the level of one signal, called the “pilot” signal. Previous systems employed two pilot signals, one at the low end of the spectrum and one at the high end. Such systems were called dual pilot systems, but they are not common today due to reduced cascade lengths which resulted from employment of fiber optic transmission.
FIG. 1
is a block diagram of an amplifier station as they exist in the prior art.
FIG. 1
shows an input path for the incoming signal to the amplifier input. This signal is received by input diplexer
1
. Input diplexer
1
separates the signal into downstream (generally higher frequency) signals and reverse (usually lower frequency) signals. The reverse section of the amplifier usually does not employ ASG, but is merely shown for illustrative and complete diagram purposes, as will be appreciated by those skilled in the art. The remainder of this discussion will concentrate on the downstream path, shown at the top of the FIG.
1
.
Downstream signals from input diplexer
1
are applied to input equalizer and attenuator
2
, used for equalization of the signal. From the output of the input equalizer and attenuator
2
, the signal is received by input amplifier
3
. The input amplifier
3
is used to amplify the signal before downstream processing. The output of the input amplifier
3
is received by the interstage equalizer and attenuator
4
. The interstage equalizer and attenuator
4
is used, as required, to shape the static response of the amplifier station and its output is received by ASG attenuator
5
. The ASG attenuator
5
is used to apply the proper attenuation to the signal in accordance with its coordinating interstage amplifier
6
circuit. As explained above, the real circuit is usually more complex than just an attenuator. The real circuit usually includes components that shape the response such that there is more change in attenuation at high frequencies than at low frequencies. This is not shown here for clarity, but is well known by those skilled in the art.
Thus, from the attenuator
5
, the signal is applied to a second amplifier stage, interstage amplifier
6
for further amplification in accordance with the needs of the pilot signal level. From interstage amplifier
6
, the signal is sent to output splitter
8
, which divides the signal into two different output paths. One path allows the signal to be applied, to a directional coupler
16
, which takes a sample of the signal to the ASG detector
7
. The ASG detector
7
detects the level of the pilot signal. The output of ASG detector
7
is a control signal which adjusts the attenuation of attenuator
5
such that the level of the pilot signal as measured by ASG detector
7
remains constant, regardless of the level of the incoming signal, within reasonable limits.
The pilot signal, having received correction, is directed by the directional coupler
16
. The signal comes from directional coupler
16
and is applied to output amplifier A
9
. The output of output amplifier A
9
is then received by output diplexer A
10
. The diplexer having an input from the output amplifier A
9
, and a reverse flow path to the reverse channel.
The path, yet to be discussed, from output splitter
8
goes through output amplifier B
11
. From output amplifier B
11
, the signal is received by output diplexer B
12
. The output diplexer B
12
, as does output diplexer A
10
, separates the downstream and reverse signals as explained above.
Other amplifiers may have fewer or more outputs, and fewer or more amplification stages. Other variations on the architecture are well known to those skilled in the art. This block diagram is presented by way of example and not as a limitation to the applicability of the present invention.
FIG. 2
is a diagram showing an ASG detector known in the prior art. Note, signal flow in
FIG. 2
is from left to right, while in the ASG detector
7
of
FIG. 1
, the signal flow is in the opposite direction. Signal input from directional coupler
16
(also see
FIG. 1
) is supplied to an input amplifier
21
, used to amplify the input signal. The signal comes from the input amplifier
21
and is received by a bandpass filter
22
, which selects the one signal that is to be used as the pilot from the plurality of carriers on the cable. As is well known in the art, the bandpass filter has a upper and lower limit of frequencies it will pass, forming an output referred to as passband of frequencies, or simply passband signal. After filtering, the signal may be amplified again, if necessary, by RF amplifier
23
the output of RF amplifier
23
, is received by or detected by detector diode
24
and filter
25
. The voltage on filter
25
is proportional to the level of the pilot carrier. Filter
25
also serves to pass the AC component from the detector circuit to a reference (ground). The voltage across filter
25
, is compared with a reference voltage V
Ref
28
by loop amplifier
27
. The feedback loop of this comparator circuit, the loop compensation
26
, helps determine the dynamic response of the ASG loop, as is well understood by those skilled in the art. The output of loop amplifier
27
is supplied to ASG attenuator
5
(see
FIG. 1
) to control its attenuation. Thus, the ASG detector
7
and the attenuator
5
work to ensure that the level of the pilot signal remains constant at the output of the amplifier station of FIG.
1
.
According to the teachings of the present state of the art, the ASG detectors utilize a circuit similar to that shown in FIG.
1
. After the signal progresses through the input equalizer, amplifiers, and attenuators, the signal reaches the ASG detector. The prior state of the art teaches an ASG detector as shown in FIG.
2
.
The system shown in
FIG. 2
works, but has some limitations that are overcome by the present invention. The tuning of the ASG detector is set by the bandpass filter. This filter traditionally has been built with conventional inductors and capacitors (L-C filter) and recently there has
Farmer James
Hollabaugh John S.
Kenny John J.
Stover Calvin W.
ARRIS International, Inc.
Summons Barbara
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