Quick and Automatic Solution of Electromagnetic Disturbance by Using the Intelligent Noise Filter

2.1 Noise measurement and filter specifications

The specifications of the intelligent noise filter and the newly developed EMI filters are listed in Table 1. The intelligent noise filter detects and suppresses noise. To detect noise, the intelligent noise filter measures the frequency range of an electromagnetic disturbance between 2 kHz to 12 MHz. This frequency range is also divided into three bands: 2 to 15 kHz (Low), 15 to 300 kHz (Mid), and 0.3 to 12 MHz (High). To suppress noise, the built-in EMI filters attenuate noise in accordance with three frequency bands.

Table 1. Specifications of the intelligent noise filter and newly developed EMI filters.

The newly developed EMI filters have the same specifications as the built-in EMI filters in terms of frequency range and attenuation level. Common mode attenuation, which indicates how much electromagnetic disturbance can be suppressed, is 40 dB or more in any frequency range. Differential mode attenuation, which indicates the attenuation of the telecommunication signal, is 1.5 dB or less when they are installed in an analog line and 3 dB or less when installed in a digital line.

Both the built-in EMI filters and the newly developed EMI filters effectively suppress electromagnetic disturbance while minimizing its impact on communication quality.

2.2 Noise detection

The intelligent noise filter automatically detects the frequency range of an electromagnetic disturbance and turns on an indicator corresponding to the detected frequency range (Low, Mid, or High). Electromagnetic disturbance voltage of –20 dBV (0.1 V) or higher can be detected, and it is possible to adjust the detection voltage level from –10 dBV (0.3 V) to –40 dBV (0.01 V) by adjusting the detection sensitivity volume.

2.3 EMI-filter-installation mode

The intelligent noise filter has two modes, i.e., manual and automatic, to install one of the built-in EMI filters according to the detected frequency range.

Manual mode

The appropriate built-in EMI filter can be applied by pressing the button corresponding to the detected disturbance-frequency range indicated by the illuminated indicator.

Automatic mode

When an electromagnetic disturbance is detected, the built-in EMI filter corresponding to that frequency range is automatically applied. The application flow of the corresponding filter when automatic mode is set is shown in Fig. 2.

Fig. 2. Flow of applying built-in EMI filter when automatic mode is set.

2.4 Using the intelligent noise filter

The intelligent noise filter is used according to the following procedure.

3.1 Countermeasure against telecommunication problems occurring on an ADSL

A customer who uses an ADSL reported that a link down of the ADSL frequently occurs. Although local service personnel took countermeasures such as replacing the ADSL modem, the link down could not be resolved. Therefore, TASC investigated the electromagnetic disturbance and implemented a countermeasure by using the intelligent noise filter (Fig. 3).

Fig. 3. Configuration of customer’s equipment and telecommunication network.

First, when the intelligent noise filter was installed between the ADSL modem and splitter, the Mid indicator was illuminated (Fig. 4(a)). Then, when the button of the Mid filter was pressed, i.e., the built-in EMI filter (Mid) was applied, the link down of the ADSL was resolved (Fig. 4(b)). Finally, TASC replaced the built-in EMI filter (Mid) with the newly developed EMI filter (Mid) for permanent measures.

Fig. 4. Operation screen of intelligent noise filter.

The transmission rate of the ADSL was improved by installing the appropriate newly developed EMI filter. The results of the measured bit maps before and after installing the filter are shown in Fig. 5. Transmission rate increased from 400 kbit/s to 1 Mbit/s on the uplink and from 130 kbit/s to 3 Mbit/s on the downlink.

Fig. 5. Bitmaps before and after installation of the newly developed EMI filter.

TASC also investigated the source of the electromagnetic disturbance and found that it was being generated by the power conditioner of a photovoltaic power system installed near the customer’s house. The noise was being transmitted through the power line cable and induced in the telecommunication cable, which was laid out parallel to it, thereby affecting the ADSL signal.

3.2 Countermeasure against audible noise occasionally heard on 3.4k leased line

A customer who uses a 3.4k leased line between two locations reported an audible noise (a “bee” sound) mixed into the telephone signal at an occurrence frequency of once a week. Local service personnel had difficulty in responding to the problem because the audible noise was not generated when they visited the customer’s house. Therefore, TASC investigated the noise and implemented a countermeasure using the intelligent noise filter (Fig. 6).

Fig. 6. Configuration of customer’s equipment and telecommunication network.

The intelligent noise filter was installed between the leased-line terminal and arrester in the customer’s house. Since the disturbance occurred sporadically, the automatic-mode button was pressed to detect and monitor the disturbance for one week (Fig. 7(a)). After that week, the customer informed us that the audible noise no longer occurred, so when the service personnel visited, the Low indicator was illuminated and the built-in EMI filter (Low) was applied automatically (Fig. 7(b)). Subsequently, that filter was replaced with the newly developed EMI filter (Low). It was found that low-frequency noise entered the power line cable. Thus, the source of the disturbance was determined as the power-supply unit or the power-supply source that starts up irregularly.

Fig. 7. Operation screen of intelligent noise filter (automatic mode).