Spectrum Analyzer
xDSL achieves higher data rates over the existing copper by utilizing the frequencies well above the voice spectrum. POTS service uses the 300 Hz to 3200 Hz bandwidth; adsl DMT uses up to 1.1 MHz and adsl CAP extends up to 1.5 MHz. The frequency range is divided into 256 frequency bands of 4.3125 kHz each. The upstream signal uses the lower frequency tones (20 to 138 kHz) while the downstream signal uses the higher frequency tones (140 to 1100 kHz).
The actual assignment of channels is flexible. And, the number of bits within each channel is flexible as the modems try to optimize the bit distribution based on the signal-to-noise ratio for each tone. Since DSL uses such a wide frequency spectrum, it is particularly susceptible to interference from outside sources. Interference can be very harmful to DSL’s data rate and signal to noise margin. It may lead to a lower data rate, or may prevent the modems from synchronizing at all.
The most likely culprit for interference is other digital signals transmitted in the same or adjacent binder groups. These signals can crosstalk at common frequencies. AM radio transmission could be another potential noise source as many AM stations broadcast their frequencies in the same range as DSL. DSL circuits carried over aerial cables are more at risk to AM interference than those carried underground. At the TestGearStore we have been using a Power Spectral Density (PSD) measurement anaylzer to effectivly identify potential interfering services. It checks for both intrinsic and extrinsic noise, including RFI from AM transmission, crosstalk from adjacent digital services in cable binders, as well as thermal and impulse noise sources. A field spectrum analyzer such as the Sunset xDSL tester with a resolution bandwidth of 4.3125 kHz and a range from 13 to 1600 kHz can be used for this. The SunSet xDSL’s background noise measurement can identify potential interfering noise sources. This test can be valuable for troubleshooting marginal circuits or for prequalifying a cable pair for DSL. Interferer masks can be superimposed on your measurement to help you determine what type of service is creating the interference. For example, you can compare your results to a sample template of noise from 10 HDSL signals in the same binder group. Other background noise tests place filters to check for noise at the frequencies used for ISDN BRI or HDSL.
Calibrating
The frequency range is divided into 256 frequency bands of 4.3125 kHz each. The upstream signal uses the lower frequency tones (20 to 138 kHz) while the downstream signal uses the higher frequency tones (140 to 1100 kHz). The actual assignment of channels is flexible. And, the number of bits within each channel is flexible as the modems try to optimize the bit distribution based on the signal-to-noise ratio for each tone. Since DSL uses such a wide frequency spectrum, it is particularly susceptible to interference from outside sources. Interference can be very harmful to DSL’s data rate and signal to noise margin. It may lead to a lower data rate, or may prevent the modems from synchronizing at all. The most likely culprit for interference is other digital signals transmitted in the same or adjacent binder groups. These signals can crosstalk at common frequencies. AM radio transmission could be another potential noise source as many AM stations broadcast their frequencies in the same range as DSL. DSL circuits carried over aerial cables are more at risk to AM interference than those carried underground. A Power Spectral Density (PSD) measurement is an effective tool to identify potential interfering services. It checks for both intrinsic and extrinsic noise, including RFI from AM transmission, crosstalk from adjacent digital services in cable binders, as well as thermal and impulse noise sources.
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