
Pathfinder DVL Guide
April
2018
EAR-Controlled Technology Subject to Restrictions Contained on the Cover Page.
Page 55
Identifying EMI
Purpose
Observation of the plots will provide information about interference to the DVL. This will probably NOT
help in determining if the DVL emits interfering signals to other devices. The NGSPFFT software pro-
vides the user with a frequency domain plot (~100% bandwidth) of the Pathfinder’s four receiver chan-
nels during a sampling interval.
NGSPFFT.
Adjustments
The NGSPFFT software has several easy to use adjustments. For example, the “Filter” is a moving av-
erage of the last 30 samples; and the “FFT N” is the FFT sample size, with a default of 512. The FFT sam-
ple size is directly proportional to the bandwidth of the FFT resolution. Additionally the, “Hardware
Bandwidth” and “Hardware Gain” and “System Frequency” can be adjusted. In addition, the commu-
nication port number and communication baud rate can be adjusted to match the communication pa-
rameters of the Pathfinder.
Operation
The NGSPFFT software connects to the Pathfinder DVL using a computer’s serial port and sends the
Pathfinder DVL a special set of commands. The Pathfinder responds with a stream of digitally processed
values, from the receive portion of the circuit and firmware. The sonar transmitter is not used for the FFT,
i.e. the FFT routine uses the sonar receive transducer, circuitry and firmware only, not the sonar transmit.
The NGSPFFT computer software processes the values it receives from the Pathfinder, and then plots the
resulting frequency response in a graphical form for each sonar channel, in terms of RSSI versus Fre-
quency. RSSI is a Teledyne RD Instruments nomenclature for Received Signal Strength Intensity in
A/D counts. The frequency span is centered at the carrier frequency (for Pathfinder, this is 614,400 Hz)
and spans ±12.5% bandwidth in NB mode, from (for Pathfinder or other 614400 kHz systems this is
308,400 Hz to 921,600 Hz.)
Once the user learns the basics of the FFT software, you can use the Pathfinder cables and transducer as
probes by holding them or locating them next to equipment you would like to include in your vehicle
(ROV, AUV). The user will see the effect of “in air” interference. If interference is noted, different strate-
gies can be incorporated to mitigate the interference.
For example, separating devices or shielding may be help. Note that the four transducer faces
(where the urethane is on the transducer) will always be submerged in water during real operation. You
can simulate this in water performance by placing the transducer at the bottom of a small bucket, with
just enough water to wet the urethane faces of the transducers. Often what appears to be interference
when the transducer is in air may be completely gone when the transducer is submerged in water. The in-
verse may be true as well. If there are other sonars or mechanical devices such as hydraulic power units
that create acoustic signals, these may interfere with the Pathfinder system and the interference may only
be detected when both our transducer and the other device are submerged in a larger bucket or tank of
water or somehow acoustically coupled through a hull or structural framework.