DMM
There are 5 different DMM tests used to prequalify and troubleshoot DSL: ACV measurement, DCV measurement, capacitance measurement, loop resistance measurement, and isolation resistance measurement. Each one is described in this application note. Thisapplication note divides the tests into 4 different testing categories: verify metallic criteria, check loop length, verify POTS voltage, and check for power.
During prequalification, DMM measurements can verify that the copper pair meets the requirements for the DSL service. For example, ANSI T1.413 (ADSL) specifies that the tip-ground/ring-ground isolation resistance measurement should be a minimum of 5 Mohm for ADSL service. These tests allow you to verify propermetallic criteria prior to installing DSL equipment.
Measure resistance
A resistance meter verifies isolation resistance of tip-ground and ring-ground with an open at the far end. Both should be a minimum of 5 Mohm per ANSI 1.413
(ADSL standard).
Check for balanced pair
You can run a capacitance test to check for a balanced pair. A balanced pair means that both the tip and ring leads are of equal length.
CHECK LOOP LENGTH
Much attention has been focused on the distance between the customer and the central office. The distance can influence the maximum data rates. It may determine if DSL service can be supported at all. For this reason, it is critical to know the exact loop length during prequalification or installation testing. In addition, a distance measurement can be a good indication of a fault on the circuit. If your reading is significantly shorter than expected, there’s a good chance you have a cut, open, or even short on the line. If you see a longer reading, you may have a bridge tap or split pair.
There are two basic methods for determining loop distance: capacitance and loop resistance. Each has its advantages and disadvantages. Capacitance can be less reliable, but does not require a short at the far end. Loop resistance is more reliable, but requires a short at the far end. Each method is discussed below.
Measuring distance with capacitance
The capacitance measurement can provide a rough estimate of distance to the far end. It is only a “rough estimate,” since any resistive faults like bridge taps or wet cable will be included in the calculation. Be wary; if your cable has any faults, this measurement will give a longer distance than reality. For example, if your cable pair is actually 8,000 feet long, but also has a bridge tap with a lateral length of 2,000 feet, the capacitance reading will give 10,000 feet (not 8,000). Capacitance does have its advantage since it should be performed with an open at the far end, as opposed to the loop resistance test that requires a short at the central office or far end.
Distance is calculated using the following conversion ratio for tip-ring measurement:
• 83 nF= 1 mile (5,280 feet)
• 51 nF= 1 km
You can identify possible bridge taps on the circuit by measuring distance using the two methods discussed above: loop resistance and capacitance. If the length given by the CAP measurement is longer than the length from Loop Resistance, this could indicate a bridge tap. This is because the capacitance measurement includes any resistive faults, like bridge taps, in its measurement. The difference between the two measurements could also indicate the length of the lateral.
If the capacitance measurement is larger, you should try a TDR measurement or Insertion Loss test (”Detaptor”) to confirm the presence of a bridge tap.
VERIFY POTS VOLTAGE
The POTS loop is powered by 48-VDC from the central office. The voltage is placed on the ring lead, with the tip as an insulated ground. You can run a DCV measurement to verify that the cable pair is connected to the POTS switch and is properly powered.
TROUBLESHOOT AC INTERFERENCE
An AC voltage measurement can check for unwanted power influence. This power influence is caused by induction between the tip-ground and ring-ground from power lines running near the cable pair.
Leave a Reply