Possible HF Interference caused by ADSL service?


I installed the Telkom ADSL (HomeDSL 192) offering on 11 September 2005. Soon after, I discovered I am getting some consistent interference on HF ...  Was not sure if it was caused by the ADSL or not.  Here is what I found:

The original problem "appeared" at about the same time my ADSL line was installed, so I was concerned if this was the cause. However, the particular interference I found, approx. every 100 kHz, was traced to a (cheap) faulty DVD player in the lounge. Of interest is that the interference occurs whether it is switched off or on: The reason is that the power supply is always active, and when you switch it "off" it just puts the equipment into standby mode, but the power supply is still running ... And generating interference! When I pull the power plug out of the wall the interference disappears as if by magic.

While checking everything else, I found that the ADSL modem does cause some interference as well. When powered off there is no interference from the ADSL service. Perhaps this is enhanced where, in the absence of a signal, the "exchange" end stops transmitting and just listens for a signal from the modem. When the modem is powered on, it first goes through a self test. During this phase I cannot pick up any interference, but once this is complete it appears to turn on its output (analogue) signal, and from this point on I can pick it up on HF bands. There is a very definite carrier at 14152, 14213, 14272, 14335 etc.... IE: with a spacing of 61kHz. This is annoying if I want to listen to a faint signal on these frequencies, but the level is fairly low and the signal is "clean" so it can be notched out. The signal strength does not lift the S meter at all.

I would be interested in hearing about other experiences, particularly those using ADSL services.


I found some interesting information on the web, included below.
 
From Wikipedia:
(Note: In South Africa, Telkom uses ITU G 992.1)

On the wire

ADSL uses two separate frequency bands. With standard ADSL, the band from 25.875 kHz to 138 kHz is used for upstream communication, while 138 kHz - 1104 kHz is used for downstream communication. It is, however, possible to alter this frequency division, but it will cause issues with crosstalk.

 
Frequency plan for ADSL

Each of these is further divided into smaller chunks of 4.3125 kHz. During initial training, the ADSL modem tests which of the available chunks have an acceptable signal-to-noise ratio. The distance from the telephone exchange, or noise on the copper wire, may introduce errors on some frequencies. By keeping the chunks small, an error on one frequency thus need not render the line unusable: the chunk will not be used, resulting in reduced throughput on the ADSL connection.

There is a direct relationship between the number of chunks available and the throughput capacity of the ADSL connection. The exact data capacity per chunk depends on the modulation method used.

Modulation

ADSL can use any of a variety of modulation techniques, CAP was the de facto standard for xDSL deployments up until 1996, deployed in 90 percent of xDSL installs. Now it is deprecated in favour of DMT/OFDM modulation schemes.

OFDM is used in ADSL connections that follow the G.DMT (ITU G.922.1) standard. (Annex A refers to ADSL-over-POTS).

It is worth noting that in contrast to the modulation schemes that baseband technologies like Gigabit Ethernet use, ADSL uses primarily analog modulation schemes, so the 'D' in ADSL is a misnomer -- ADSL is simply a very fast analog connection (using PPPoE or PPPoA) with much higher symbol rates and much faster handshaking between modems.

ADSL standards

Standard name Standard type Downstream rate Upstream rate
ANSI T1.413-1998 Issue 2 ADSL 8 Mbit/s 1.0 Mbit/s
ITU G.992.1 ADSL (G.DMT) 8 Mbit/s 1.0 Mbit/s
ITU G.992.2 ADSL Lite (G.Lite) 1.5 Mbit/s 0.5 Mbit/s
ITU G.992.3/4 ADSL2 12 Mbit/s 1.0 Mbit/s
ITU G.992.3/4 Annex J ADSL2 12 Mbit/s 3.5 Mbit/s
ITU G.992.5 ADSL2+ 24 Mbit/s 1.0 Mbit/s
ITU G.992.5 Annex L ADSL2+ 24 Mbit/s 3.5 Mbit/s

Additionally, the non-Annex ADSL2 and ADSL2+ support an extra 256 kbit/s of upstream if the bandwidth normally used for POTS voice calls is allocated for ADSL usage.

The downstream and upstream rates displayed are theoretical maximums. Note also that because DSLAM and ADSL modems may have been implemented based on differing or incomplete standards some manufacturers may advertise different speeds. For example, Ericsson has several devices that support non-standard upstream speeds of up to 2 Mbit/s in ADSL2 and ADSL2+.

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Peter Hers 
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