Loading Coils and Bridge Taps
Loading Coils and Bridge Taps
Loading Coils and Bridge Taps
This is a rarely documented but important item to be aware of, for any telecom Tech or Engineer. Voice circuits run
on long spans of twisted pair wires. Load coils counteract capacitance on the twisted pair lines by adding inductance.
To understand load coils, one must first understand capacitance and inductance (see the "Electronics" page for
details). Capacitors store electrical energy and inductors store magnetic energy.
Load coils are Inductors !!! They cancel out the capacitance in the lines !!!
Capacitance - Long lines of wire have capacitance, which is detrimental to voice. Capacitance is like a battery cell
and can store electrical energy. In fact, capacitors are made from two plates, that store a charge, just as batteries are.
The amount of energy that a capacitor can store is measured in "Farads" (usually uF which is "micro-farads"). A
capacitor's typical usage is to smooth our variation in voltage levels on a line. Capacitance acts as a filter, allowing
low frequencies to pass, but blocking high frequencies. This is fine for power supplies, where you want a constant
voltage. But with voice, you want the original signal to arrive intact, as close to the original transmitted signal as
possible. You need to retain the high frequencies.
There are no actual physical "capacitors" on the line - however, "extraneous", or "stray" capacitance does exist, and
just as it does on any line, it has the effect of reducing voltage variations - it "smooths out" the signal by blocking the
higher frequency range.
Inductance - the opposite of capacitance - it is simply a coil of wire. By adding inductance (load coils) periodically
into the cable facility, the capacitive effect can be cancelled, thus causing the attenuation across the voice band to be
equal.
Load Coils - inserted in series with the Twisted Pair Copper Lines
How Load Coils Work
All subscribers and trunk cable facilities consist of resistance and capacitance. The resistance is determined by the length and
gauge of the cable conductors. The capacity is determined by the length of the cable conductors and the spacing between the
conductors
The capacitive effect of the cable conductors has a direct relation on the voice band (300 Hz to 3000 Hz) from any given point. The
higher the frequency, the greater the loss or attenuation (3000 Hz would be attenuated more than 300 Hz). By adding inductance
(load coils) periodically into the cable facility, the capacitive effect can be cancelled, thus causing the attenuation across the voice
band to be equal. Non-loaded subscriber loops should not exceed 18,000 ft. of cable. It is recommended that loops longer than
18,000 ft. be conditioned with load coils.
Optimum loading can be achieved by selecting the desired loading coil, measured in millihenries (mh), and placing them in the
cable plant at prescribed intervals. For example, an 88mh coil will cancel 6000 ft. of capacity. Therefore, the recommended spacing
would be at intervals of 6000 ft., with the first coil place 3,000 ft. from the start of the cable run.
Examples
LC = load coil
1) Distance = 16k ft
CO------------------------------>cust
16Kft
2) Distance = 18,000 ft
load coils are optional, but since the distance = 18k ft, to be
3) Distance = 19k ft
6K ft in
between.
CO-------LC---------LC------LC------>cust
3Kft
9Kft
15Kft
19Kft
Note: NO 4th load coil added because it would be too close to customer (<3,000 feet)
4) Distance = 20k ft
6K ft in
between.
CO-------LC---------LC------LC-------->cust
3Kft
9Kft
15Kft
20Kft
Note: NO 4th load coil added because it would be too close to customer (<3,000 feet)
5) Distance = 21k ft
6K ft in
between.
CO-------LC---------LC------LC----LC------>cust
3Kft
9Kft
15Kft 18Kft
21Kft
you wonder this - "If the energy of the signal is only partially absorbed, what happens to the stuff that's not absorbed?"
All energy that is not absorbed by the termination load reflects back on to the copper pair and begins to interfere with
the original signal. Because the reflected signal is usually out of phase from the original signal, this starts to cause
"common mode rejection" or cancellation or loss of Amplitude at specific "standing wave" frequencies and their
mathematical derivatives. The net effect of that is, the original signal begins to get crippled by its own reflection.
The Evil Load Coils are placed in the circuit at specific intervals (varies depending on wire gauge and cable bundling
variables) to reduce the effective capacitance of the extended copper loop and therefore provide a higher level of
predictability across the copper segment. Remember, impedance matching, in theory, only works if you can predict the
signal characteristics of the waveform being impeded. Anything that drastically alters the characteristics of the
waveform will make the circuits termination less effective in absorbing the maximum signal.
The argument is that, three discrete segments of 6,000 feet copper has less negative capacitive effects than a single
copper segment that is perhaps 18,000 feet long. Therefore, in a long copper loop, Load coils are placed at strategic
points to reduce the negative effects that capacitance will have on the signals characteristics. Capacitance is the enemy
of high BAUD rate applications. Amplitude can be overcome with amplifiers, Capacitance is much harder to control
cheaply.
This load coil works wonderfully for voice, however; load coils, with their purpose to limit excess capacitance, also
greatly limits the frequency spectrum available to the end devices. By the way, load coils also help restrict impulse
noise or interference from one copper segment to adjacent copper segment. So in effect may reduce the additional
noise characteristics from loop interference. Now back to the limit of frequencies.
There are a few variables that affects this discovery either positive or negative. But in general, placing a load coil in a
Telco voice line reduces the effective bandwidth by chopping off the top 25 percent of the available frequencies. If you
are to take a tone generator to the lines you would see the highest quality signal at approximately 1,000 to 2,000 Hz.
When the tone generator reached 2900 Hz there would be a significant "role off" of over 12dB per octave. This
reduction explains why you can't use load coils in digital circuits. As a matter of reference if you examined a T1 signal
with a scope it would look like a very phase distorted 772kHz analog signal. Certainly any facility that limited the
bandwidth to 2900 Hz would seriously choke a signal running at 772kHz. The same holds true for ISDN BRI which
has an effective signal rate of approximately 40kHz.
So, it becomes obvious that load coils can cripple the high speed signal. Please note, that removing the load coils will
only make the signal worse. Because of the nature and characteristics of how modems manipulate an analog signal,
removing the load coils will just cause exponentially more distortion across the copper segment. If you want to remove
the load coils you must completely re engineer the way the data signal is presented to the copper pair. Thats what they
did with ISDN.
Bridge Taps
These are perhaps the most annoying and offensive of all the anomalies found in a Telco copper segment. They are by
far the number one problem you all have getting modems to connect and stay connected at high speeds. Unfortunately
they are riddled throughout most residential neighborhoods and corporate business parks.
W hen the phone company runs a cable down the street the cable may extend a mile or so passed your house.
Although no other house or device is using your specific copper pair, the pair runs out the length of the cable. All an
installer does is take the wires that come from your demarc, drag them out to a junction box or splice box on a poll or
pedestal and "Tap" the wires coming from your house on to a spare copper pair that runs out the length of the cable.
They do not cut the cable pair at the junction box just incase they have to use the same pair for one of your neighbors
down the road when you move out. Also, in order not to drastically reduce the amplitude and of the signal coming
from your telephone they do not to terminate the extended cable end either.
That means there may be a one mile cable running from the central office out past your house and your telephone line
is simply tapped into the middle of it. It's sort of like having an additional half-mile antenna picking up all the garbage
in the air and feeding it to your telephone equipment.
The fix, is to have a technician come out and cut the wires off where they extend down the block.
But you, as a consumer, won't know if you have this problem, since there is no way you can see
that part of the Telco infrastructure.
Assuming for a moment we can deal with the additional idle channel noise on the copper facility, which even the most
unqualified lineman can test for, (but if you ask him what he's testing for he probably can't tell you), and tell you some
story how your line is the quietest line on the street, the next hurdle beyond the noise is the reflected signal coming
back off of the half-mile unterminated antenna they built just for you at no additional charge.
When an electrical signal hits the end of a wire it has to go somewhere. If there is no impedance load to absorb the
signal, then the signal in its entirety gets reflected back over the entire copper segment. The signal that comes from
your modem headed for the central office arrives at a specific time interval and the reflected signal coming back off
the unterminated copper extension comes then just behind yours causing your signal to appear phase distorted.
When two out of phase signals are received at a certain impedance load of a cause a rejection affects and begin to
cancel each other out. If the signals arrives 180 degrees out of phase your signal can be canceled out completely. The
more the second reflected signal it is closer to 180 degrees the more the signal will be attenuated and phase distorted.
At lower frequencies problem is not terribly dramatic as these reflections are only fractions of a waveform out of
phase. But when you make the waveform's smaller as is the case with higher frequencies the problem becomes
exponentially more apparent. Phase is much more an issue with smaller, shorter or higher frequency wave forms. In a
nutshell, we are screwed.
Even when you can get them to, a customers residence and have a line tested, they DO NOT test the frequency