Online

Electrical Noise

Deploying an acoustic instrument as an online system, which is supplying real-time data over a subsea cable, a rigorous assessment of potential electromagnetic interference is required. Interference from electrical noise can significantly degrade both recorded and transmitted data. It can manifest in the data as an overall elevated noise floor or periodically raised return signal strength.

Electrical noise may originate from:

• the  power supply (For power supplies other than what is provided by Nortek)
• DC/DC converters
• other operational sensors or electronic equipment installed in the vicinity of the instrument

These sources can introduce noise that overlaps with the acoustic instrument’s operational frequency range, especially when power supplies use switching regulators or are poorly filtered.

To minimize interference and ensure high-quality data collection:

• Power Supply Compatibility: Always verify that any external power supply has a switching frequency that lies outside the acoustic sensor’s operational bandwidth.
• Measurement Timing: If multiple acoustic instruments or sensors are deployed within a close range, their acoustic signals can interfere with one another. Staggering their measurement intervals—so they do not ping simultaneously—can reduce cross-talk and improve overall data quality.  (See also:  Acoustic interference between multiple instruments)

• Shielding and Grounding: Proper cable shielding and grounding practices can help reduce transmission paths and protect signal integrity.

   

Using long cables

Cables arguably provide the most reliable means of communication. When working over large distances, it is important to be aware of the following challenges as well as the precautions that can be taken to minimize them:

Maximum cable length
RS232 data communication at 9600 baud can be used for cable lengths up to 50 meters, depending on environmental conditions. If a longer cable is required, we recommend switching to the RS422 communication protocol, as RS232 may be too slow and could result in data loss. For cable lengths exceeding 100 meters, the use of an interface box is strongly advised.

Multiple power supplies
When multiple power sources are present, the instrument will draw current from the source with the highest voltage until the voltages equalize, after which it will draw from both. It is also possible to add backup batteries and memory to online systems. The battery ensures continued data collection if the main cable connection is interrupted, while additional memory allows for data to be stored locally without loss.

Voltage drop

When supplying power to your instrument over a long cable, the voltage drop due to the cable's internal resistance must be taken into account.  It is essential to consider both the minimum operational voltage and the maximum voltage rating of the instrument—these values are available in the technical documentation on our website. A stable power supply is just as critical as correct voltage. An unstable supply can result in corrupted data or intermittent faults. In extreme cases, severe power surges in systems lacking an interface box or DC/DC converter can damage the electronics irreparably.

Voltage drop occurs due to the resistance in a cable, causing energy to be lost as heat. The thickness and the length of the cable are two variables affecting the amount of resistance, and are also two factors you can affect when planning your deployment. The voltage drop \(V_{\text{drop}\) along a cable can be calculated using (1):

When calculating voltage drop in a circuit, we have to account for the power travelling to the instrument and back. To do so, we time the cable length by two.

Example calculation

As an example, we want to supply power to a Signature 1000 located 100 m away using Nortek’s 20 AWG Ethernet cable. The resistance of this cable is 34.3 Ω/km. Assuming the Signature has a current draw of 1 A we can input these numbers into the equation as follows:

Calculating this, we find \(V_{\text{drop}\) equals 6.86 V, leaving approximately 17 V to power the instrument. As Signature 1000's DC input is 12-48 V, we can the assume a 24 V power supply would be sufficient for this mission.

Resistance for Nortek’s cables

The thicker the cable, the lower the resistance. The longer the cable, the higher the resistance.

Solutions when working with long cables

At Nortek we provide the following solutions to combat the downsides of longer cables and to ensure high quality data during the whole deployment:

• For transmission over 50m we recommend the use of the RS422 communication protocol, as the 232 protocol is the simplest form of serial data transmission and will be too slow and might cause loss of data.
• Online cables. These cables are tougher and are designed to be deployed under water and survive the harsh conditions over a long period of time.
• DC/DC converter. This converter is placed inside the instrument, scales down the incoming voltage to 15V, and has an input range of 24-48V giving a lot more freedom and peace of mind when designing a power supply.

• Interface Box. We recommend this for all applications over 100m and for customers that are less familiar with electronics and want a simple and reliable solution. The interface box is powered by 220 AC voltage and communicates with the instrument only via RS422 and uses a specific round connector at the dry end. Its output is 48 DC voltage.

   

Note: Having a stable supply is just as important as having the correct voltage. An unstable supply of power can lead to bad data and intermittent issues. In case of bad power surges without interface box or DC/DC it can be fatal to the electronics.