Methods and filter for isolating upstream ingress noise in a bi-directional cable system

An upstream ingress filter (300) for isolating upstream ingress noise on a bi-directional cable system having an upstream band and a downstream band for carrying upstream signals and downstream signals, respectively. The upstream ingress filter (300) includes a DC switching element (310) for changing the DC voltage on the cable system at said cable access unit and a switchable notch filter (340) that switches ON and OFF in response to the DC switching element. The notch filter isolates upstream ingress noise by attenuating upstream signals in the upstream band when the upstream ingress filter is ON. Both the DC switching element (310) and the notch filter (340) are connected in parallel to the cable (301).

BACKGROUND OF THE INVENTION 
The present invention relates, in general, to telecommunication systems, 
and more particularly to isolating upstream ingress noise in a 
bi-directional cable system. 
Cable communication infrastructures typically comprise a hub servicing 
various nodes, such that one or more nodes are at the end of each spoke of 
the hub. Spokes are typically fiber optic cable. The fiber optic cable 
leads to a distribution portion of coaxial cable extending to individual 
subscribers such as homes, businesses, etc. The distribution portion at 
the end of a particular spoke is often divided into manageable subsets, 
for example 20 kilometer radius subsets, which are the individual nodes. 
The nodes typically include one or more subscriber drops that connect the 
individual subscribers to the cable communication system. 
Although cable infrastructures are electromagnetically sealed to prevent 
radio frequency noise from leaking into the cable, leakage can and usually 
does occur. When the leakage is from outside the cable system into the 
cable it is called "ingress" noise. Ingress noise can be generated in many 
ways such as, for example, two-way dispatch services, amateur radio 
transmission, various commercial, medical, or industrial electronic 
equipment, as well as ignition noise from combustion engines. 
Additionally, one very common and troublesome source or ingress noise is 
electromagnetic emissions at the cable subscriber's premise from electric 
motors in fans, carpet vacuums, hair dryers and the like. These emissions 
are often coupled onto the cable system cable via unterminated cable stubs 
in the subscriber's premise, the stubs tending to act as antennas. 
Bi-directional cable communication systems often comprise an upstream band 
and a downstream band for carrying upstream signals and downstream 
signals, respectively. Ingress noise can appear in either the upstream or 
downstream portion of the cable transmission frequency spectrum. Ingress 
noise may be continuous, intermittent, or sudden, and in the extreme case, 
catastrophic, rendering the information being transmitted on a particular 
carrier channel totally unusable. Ingress noise is normally isolated to a 
relatively narrow band, but a single incidence may cause interference to 
several carriers in the same subregion of the cable spectrum. Downstream 
ingress noise may be seen as distortion, snow, or other impairments to the 
television video or audio signal. Upstream ingress noise is especially 
troublesome to cable service operators because upstream ingress noise from 
any individual subscriber drop is additive with upstream ingress noise 
from other subscribers on the same cable branch. The ingress noise 
discussed above which results from electromagnetic emissions at the 
subscriber's premise is a significant source of this upstream ingress 
noise. It is desirable for a cable operator to be able to isolate (shut 
off) this sort of upstream ingress noise while minimally disturbing 
service to the subscriber premise which is the source of the noise. With 
the capability of isolating each particular subscriber premise noise, the 
cable operator can identify any chronic noise source subscriber and take 
corrective measures. 
There are presently at least two methods currently used by cable operators 
to isolate sources of ingress noise emanating from a subscriber's premise. 
When the signal is separated into upstream and downstream portions of the 
spectrum, one method requires that a diplex filter be installed at the 
point of the subscriber's access to the cable (sometimes refered to as a 
cable access unit) to physically separate the upstream and downstream 
bands onto two separate lines. Once the upstream and downstream paths are 
physically separate, a switch is connected in series with each line. In 
order to determine whether ingress noise is leaking into the upstream 
portion, the upstream switch is engaged to form an open circuit, 
effectively removing that cable access unit from the cable infrastructure 
so that a noise, or ingress, measurement can be made. If the level of 
upstream ingress noise is substantially reduced upon the removal of that 
cable access unit from the infrastructure, the source of ingress has been 
isolated. One disadvantage of this approach is that the diplex filter must 
be installed in series with the subscriber signal path. The use of serial 
filters, however, degrades the upstream and downstream signals and adds to 
the return and insertion losses of the system. Also, because switches 
normally fail open, failure of the serial switches in the cable access 
unit normally causes a total loss of cable service in the respective line, 
contributing to the unreliability of the cable service. 
Another disadvantage of physically splitting the upstream and downstream 
bands by using a serial diplex filter is that reallocation of spectrum is 
precluded. For example, a bi-directional cable system may have an upstream 
band comprising frequencies between 5 and 42 MHz, a downstream band 
comprising frequencies between 50 and 750 MHz, and a guard band comprising 
frequencies between 42 and 50 MHz. In such a system, extension of the 
upper limit of the upstream band to a frequency of 45 MHz, would be 
impossible, unless the original diplex filter is replaced with a diplex 
filter with the appropriate attenuation characteristics. Therefore, 
spectrum reallocation is precluded when the upstream and downstream 
signals are physically split onto separate lines because upstream and 
downstream frequency allocation via a diplex filter is fixed. 
Another approach for isolating sources of ingress noise is to install a 
fixed attenuator in series with the subscriber path that is engaged to 
make ingress noise measurements. Because this approach avoids the use of a 
diplex filter, the reallocation problem discussed above is avoided. 
However, the placement of switches in series with the cable nevertheless: 
(a) decreases the reliability of the cable service due to occasional 
switch failure and (b) degrades the signal-to-noise ratio. Furthermore, 
upstream ingress noise measurements made by opening serial switches 
disrupt downstream transmission. Such disruptions are known to be a major 
cause of customer dissatisfaction. 
Therefore, it would desirable to isolate ingress on a cable communication 
system without increasing return and insertion losses, and without 
decreasing the reliability of the cable service.

DETAILED DESCRIPTION OF THE DRAWINGS 
Generally, the present invention provides methods and apparatus for 
evaluating ingress noise on a cable system. More specifically, the scope 
of the methods and apparatus of the present invention is particularly 
suited for isolating upstream ingress noise on a bi-directional cable 
system by attenuating upstream ingress noise originating from a subscriber 
premise via a cable access unit. 
A preferred embodiment of the present invention, as explained in greater 
detail below, comprises a DC switching element for changing the DC voltage 
on the subscriber path and a switchable notch filter responsive to the DC 
switching element for making an upstream ingress noise measurement. In 
accordance with the principles of the present invention, the DC switching 
element and the switchable notch filter are connected in parallel to the 
cable. 
Turning to the figures for a more detailed understanding of the principles 
of the present invention, FIG. 1 shows a schematic overview of a cable 
communication infrastructure configured for two way communications such 
as, for example, the placement of telephone calls or the bi-directional 
transmission of computer data. Referring to FIG. 1, telecommunication 
system 100 consists of a cable control unit 102 (CCU). The cable control 
unit 102 serves to receive and actively route signals (i.e., information) 
throughout the system 100, as well as to carry out other system 
administration functions. 
Extending from the cable control unit 102 (i.e., hub) are several spokes 
104, 106, 108 and 110, which are preferably fiber optic cables. The spokes 
104-110 may be of other suitable transmission medium, such as low-loss 
coaxial cable, depending upon the particular application, topography and 
system requirements. The spokes 104-110 serve as "trunks" for the 
telecommunication system 100, as is readily apparent to persons skilled in 
art. 
The fiber optic spokes 104-110 each lead to a particular one of the 
distribution portions 112, 114, 116 and 118, respectively. Each one of the 
distribution portions 112-118 is located at the end of the corresponding 
one of the spokes 104-110, each of which effectively terminates at one of 
the distribution points 120, 122, 124 and 126, respectively. Typically, 
the distribution portions 112-118 consist of coaxial cable, rather than 
fiber optic cable, that distributes signals to and from individual 
subscribers, as is shown in more detail in FIG. 2. 
The distribution portions 112-118 are shown in a variety of representative 
configurations in order to illustrate that such variety may be employed. 
For example, the distribution portion 112 is shown as a loop, whereas 
distribution portions 114 and 116 are shown as varying stubs. Thus, the 
specific configuration selected for each distribution portion is not 
considered to be a limiting factor of the present invention. 
In addition to distribution portions, cable control unit 102 may also be 
connected to other communication systems via gateways. For example, a 
spoke 128 extends from control unit 102 to a Public Switched Telephone 
Network (PSTN) 130. The spoke 128 can be a fiber optic cable similar to 
the spokes 104-110, or spoke 128 may be any other suitable transmission 
medium that is known to persons skilled in the art. Thus, the 
communication system 100 may be utilized to access all commonly available 
communications networks via cable 128 and various gateways, such as PSTN 
130. 
FIG. 2 is a schematic diagram of a portion of telecommunication system 100 
that provides a detailed illustration of a cable communication 
infrastructure set up for bi-directional communications. In particular, 
the principles of the present invention may be applied to enable a cable 
operator to isolate ingress noise to a single cable access unit within a 
given distribution portion. For purposes of illustration, distribution 
portion 116 is shown in FIG. 2, although the principles are equally 
applicable to all distribution portions. 
Cable access units 210 and 212, used in connection with communication 
system 100, are single transceiver devices that act as interfaces between 
cable control unit 102 and subscriber appliances (such as telephone 214, 
television 216, and computer 218). Cable access units, which code and 
decode between analog and digital signals, are normally mounted on the 
side of subscriber homes 206 and 208, or in some other unobtrusive place, 
such as a basement or attic. Cable access units 210 and 212 may also be 
directly incorporated into subscriber appliances, such as televisions, 
computers, and telephones for use with the principles of the present 
invention. 
Bi-directional cable systems using cable access units 210 and 212 usually 
comprise two bands of operation--an upstream band and a downstream band 
for carrying upstream signals and downstream signals, respectively. The 
upstream band provides uplink communication for transmitting information 
from cable access units 210 and 212 (i.e., the tailends of the system) to 
cable control unit 102 (i.e., the headend of the system). The upstream 
band may include frequencies, for example, between about 5 MHz and about 
42 MHz. Similarly, the downstream band provides downlink communication for 
transmitting information from the headend to the tailends of the cable 
system. The downstream band may include frequencies, for example, between 
about 50 MHz and about 750 MHz. Finally, a non-operative band, referred to 
as a guard band, is typically located between the upstream and downstream 
bands to prevent interference between the operative bands. 
In typical cable environments, the downstream consists of approximately 100 
6 MHz channels, each containing video programming. These downstream 
channels are usually located in the 50 MHz-750 MHz band. When telephony 
and data communications capability is added to the system, as is 
consistent with the present invention, a portion of one or more of the 6 
MHz video channels may be dedicated to telephony/data channels which have 
associated carriers, typically spaced 600 KHz apart. 
The typical upstream spectrum in a two-way cable system is 5 MHz-42 MHz. In 
the past, this upstream has been used for simple, low data rate traffic of 
set-top box status, pay-per-view requests, etc. When telephony and data 
communications capability is added to the system, as is consistent with 
the present invention, telephony/data channels having associated carriers 
are provided in this spectrum, typically spaced 600 KHz apart. 
The cable communication system, used in accordance with this invention, 
should have frequency agility--namely, the ability to move carriers to 
different channels in either the uplink and/or downlink direction. 
Frequency agility can be used to move a carrier to a new RF channel that 
is not affected by ingress noise. 
As shown in FIG. 3 and in accordance with the principles of the present 
invention, upstream ingress filter 300 may be used to isolate upstream 
ingress noise. Upstream ingress filter 300 includes (1) DC switching 
element 310 for changing the DC voltage on subscriber signal path 301 and 
(2) switchable notch filter 340 which is responsive to DC switching 
element 310 for isolating upstream ingress noise. Both DC switching 
element 310 and switchable notch filter 340 are connected in parallel with 
subscriber signal path 301. Preferably, DC switching element 310 comprises 
two serial elements: upstream choke 320 and switch assembly 330. Upstream 
choke 320 may be activated by switch assembly 330. When switch assembly 
330 is activated, upstream choke provides: (1) a low-independence path to 
ground 370 for a first predetermined range of frequencies and (2) a 
high-impedance path for a second predetermined range of frequencies. The 
first predetermined range of frequencies preferably includes frequencies 
substantially less than the lower frequency limit of the upstream 
frequency band, and most preferably 0 Hz (e.g., DC voltages). The second 
predetermined range of frequencies preferably includes frequencies which 
are about equal to or greater than the lower limit of the upstream 
frequency band. Most preferably, the second predetermined range of 
frequencies includes the frequencies between about the lower limit of the 
upstream frequency band and about the upper limit of the downstream 
frequency band. 
As shown in FIG. 3, switch assembly 330 preferably includes transistor 334 
and DC source 338 for biasing transistor 334. Transistor switch assembly 
330, such as the one shown in FIG. 3, (a) enables very rapid switching 
(i.e., typically within a fraction of a microsecond), (b) allows many 
different circuits to be switched with a single control signal, and (c) 
permits "cold switching" in which only dc control voltages are transmitted 
to a switch assembly rather than information containing signals that are 
susceptible to capacitive pickup and signal degradation. Preferably, 
transistor 334 is a field effect transistor (FET) having collector 335, 
base 336, and emitter 337. In the embodiment shown in FIG. 3, the 
transistor is "inactive" when the base voltage is low. When inactive, 
there is no base current and therefore no collector current in transistor 
334. Therefore, when the base voltage is low, upstream choke 320 is off. 
On the other hand, when the base voltage is high, transistor 334 is turned 
"on," permitting current to flow between collector 335 and emitter 337 and 
thereby providing a low-impedance path to ground 370 for upstream choke 
320 and signal path 301. 
In order to prevent damage to DC switching element 330 when the ingress 
filter 300 is "on," switch assembly 330 also preferably includes resistors 
331 and 332. Resistor 332 limits electrical current flowing between DC 
source 333 and ground 370 when transistor 334 is providing a low-impedance 
path to ground 370. Resistor 332 should have a resistance between about 1 
kilo-Ohms and about 10 Mega-Ohms, and preferably should have a resistance 
of about 10 kilo-Ohms. Additionally, DC switching element 310 preferably 
includes resistor 331 connected in series with choke 320 and transistor 
334 for limiting electrical current when transistor 334 provides a 
low-impedance path to ground 370. Resistor 331 should have a resistance 
between about 100 Ohms and about 10 kilo-Ohms, and preferably has a 
resistance of about 470 Ohms. DC source 333 is preferably a 12 volt DC 
power supply, but need only be sufficient to drive transistor 334. 
Switchable notch filter 340, shown in FIG. 3, preferably comprises switch 
350 connected in series with broad-band notch filter 360. Switch 350 is 
preferably a voltage-variable resistor, such as PIN diode 351. Preferably, 
PIN diode 351 is Model #MA4P274, available from MA-COM, Inc., of Lowell, 
Mass. Broad-band notch filter 360 may be any circuit which effectively 
attenuates at least a portion of the upstream band. More particularly, 
broad-band notch filter 360 preferably has a relatively low Q-factor 
(i.e., Q-factor scales inversely with the band width of broad-band notch 
filter 360). Preferably, broad-band notch filter 360 includes inductor 361 
connected in series with capacitor 365, as well as capacitor 362 and 
resistor 363 that are connected in parallel with each other and in series 
with inductor 361 and capacitor 365. 
Capacitor 362 should have a capacitance between about 500 pico-Farads and 
about 2,000 picoFarads, and preferably has a capacitance of about 1000 
pico-Farads. Capacitor 365 should have a capacitance between about 1,000 
pico-Farads and about 1 micro-Farad, and preferably has a capacitance of 
about 0.1 micro-Farads. Inductor 361 should have an inductance between 
about 220 nano-Henrys and about 50 nano-Henrys, and preferably has an 
inductance of about 100 nano-Henrys. Resistor 363 should have a resistance 
between about 2 Ohms and about 1 kilo-Ohms, and preferably has a 
resistance of about 12 Ohms. Switchable notch filter 340 also preferably 
includes a DC source 364 which reverse biases PIN diode 351 when 
transistor 334 is off. DC source 364 is preferably a 5 volt DC power 
supply, but need only have a voltage sufficient to forward bias PIN diode 
351 when filter 300 is active. It should be readily apparent to persons of 
ordinary skill in the art that broad-band notch filter 360 may be any 
band-reject filter that attenuates upstream frequencies without strongly 
attenuating downstream frequencies. 
When transistor 334 is "on," the dc voltage on subscriber signal path 301 
is lowered by choke 320. Preferably, choke 320 is any broad band choke 
capable of providing a low-impedance path to ground for a first 
predetermined range of frequencies when the choke is active. Moreover, 
choke 320 preferably provides a high-impedance path to ground for 
substantially all frequencies in the upstream and downstream bands. 
Preferably, choke 320 is the shunt leg of a directional coupler 321, such 
as Model #EMDC-10-1-75, available from MA-COM, Inc., of Lowell, Mass. When 
the dc voltage on subscriber signal path 301 is sufficiently low, PIN 
diode 351 is forward biased, providing a low-impedance path to ground 370 
through broad-band notch filter 360 for signals and/or ingress in the 
upstream band. The low-impedance path to ground 370 prevents upstream 
signals and/or ingress that originate at or near cable access units 210 
and 212 from affecting cable control unit 102. Thus, local sources of 
upstream ingress noise can be isolated from cable system 100 by 
activating, or turning on, one or more upstream ingress filters. 
There are several methods of isolating sources of ingress noise on a 
bi-directional cable system in accordance with the principles of the 
present invention. Generally, sources of upstream ingress noise on a cable 
infrastructure 100 may be isolated by activating an upstream ingress 
filter. Activating upstream ingress filter 300 prevents ingress noise 
associated with cable access unit 210 or 212 from being transmitted to 
cable control unit 102. Once upstream ingress filter 300 is active, a 
first upstream ingress measurement can be made. An upstream ingress noise 
measurement is the determination of the ingress level at a particular 
frequency or range of frequencies. By comparing the first ingress level 
measurement made when filter 300 is on to the upstream ingress level 
measurement made when filter 300 is off, it can be determined whether or 
not the subscriber premise associated with that particular cable access 
unit is a substantial source of ingress noise. If the upstream ingress 
level measurement decreases upon activation of upstream ingress filter 
300, it can be determined that the particular subscriber premise is a 
source of ingress noise. Alternatively, if the ingress level measurement 
does not decrease when upstream ingress filter 300 is activated, it can be 
determined that the particular subscriber premise is not a source of 
ingress noise. 
In accordance with the principles of the present invention, ingress noise 
measurements are made by activating upstream ingress filter 300 remotely 
or locally. Preferably, ingress measurements are made remotely. Remote 
activation may be in response to a downstream activation signal 
transmitted by cable control unit 102 and received by one or more cable 
access units on cable infrastructure 100. Preferably, cable control unit 
102 periodically transmits downstream activation signals to periodically 
activate one or more upstream ingress filters. By periodically activating 
one or more upstream ingress filters and simultaneously monitoring the 
ingress noise level at cable control unit 102 for corresponding drops in 
ingress noise, sources of ingress noise may be determined. 
Periodic activation is especially useful for performing general 
maintenance-like functions on cable system 100, such as the automatic 
detection of unacceptable sources of ingress noise. For example, to 
automatically detect unacceptable sources of ingress noise, previous 
ingress noise measurements of known quality can be made and the ingress 
noise levels stored for subsequent analysis. Previous ingress measurements 
of known quality may be analyzed to determine a threshold level, above 
which ingress is considered to have an unacceptable level. Once a 
threshold level is determined, ingress measurements are periodically made 
and compared to the threshold. Alternatively, an upstream ingress source 
may be determined to be present if the first upstream ingress level is 
found to be sufficiently similar to a previous ingress measurement known 
to be indicative of an upstream ingress noise source. 
It should be understood that the predetermined threshold level may vary, 
for example, with carrier frequency and time of day or year. During 
operation, if the upstream ingress level is determined to be unacceptable, 
the cable control unit may notify a cable operator that an unacceptable 
source of ingress noise has been detected. Alternatively, automatic 
upstream attenuation may be desirable to remove major or even catastrophic 
sources of ingress. In fact, it should be clear to persons skilled in the 
art that a number of actions may be taken in response to the isolation of 
one or more ingress sources on cable system 100. 
Moreover, upstream ingress filter 300 may be activated locally by a 
subscriber or cable maintenance personnel. For example, upstream ingress 
filter 300 may be engaged by a toggle switch located at the subscriber 
home or office. Local activation could be used, for example, when remote 
activation becomes inoperable or undesirable. 
A further advantage of the present invention is that upstream ingress 
filter 300 allows for spectrum reallocation. Spectrum reallocation is the 
reassignment of portions of the spectrum from one band of operation to 
another band of operation. For example, consider a bi-directional cable 
system having an upstream band between 5 MHz and 42 MHz, a guard band 
between 42 MHz and 50 MHz, and a downstream band between 50 MHz and 750 
MHz. In such a system, the cable operator may desire to expand the upper 
limit of upstream band from 42 MHz to 45 MHz, thereby reducing the 
bandwidth of the guardband. Because ingress noise generally scales 
inversely with frequency, most serious sources of ingress noise have a low 
frequency (i.e., from about 5 MHz to about 22 MHz). Therefore, because 
upstream ingress filter 300 is designed to strongly attenuate the lower 
portion of the upstream band, reallocation of a lower portion of the guard 
band for use as an upstream band does not substantially effect upstream 
ingress filter utility. 
It will be understood that the foregoing is only illustrative of the 
principles of the invention, and that various modifications can be made by 
those skilled in the art without departing from the scope and spirit of 
the invention. For example, the broad-band notch filter may be replaced 
with a plurality of narrow band notch filters, each of which may be 
activated by same or different downstream activation signals. In this 
configuration, different portions of the upstream band could be 
selectively attenuated. Also, the upstream ingress filter need not be 
installed exclusively in cable access units. Upstream ingress filters 
could also be installed regularly along the length of communication 
infrastructure to isolate ingress noise from points other than cable 
access units, such as gateways. 
Furthermore, the broad-band notch filter, in accordance with the principles 
of the present invention, may include, for example, a high pass filter and 
a low pass filter in parallel with one another, wherein the high pass 
filter provides a low-impedance path for frequencies below a first 
threshold frequency and the low pass filter provides a low-impedance path 
for frequencies above a second threshold frequency. When the first 
threshold frequency is higher than the second threshold frequency, the 
notch filter provides a low-impedance path between the cable and ground 
for frequencies between the first threshold frequency and the second 
threshold frequency.