KMAG4 – 260MHz Four-Sensor Magnetometer Counter Version 3.27.11

Physical Specifications

Electrical Specifications

Resolution

The period of the signal, coming out of the magnetometer sensor (the Larmor signal) carries the information about the ambient magnetic field.The magnetometer counter measures the period of the Larmor signal and calculates the ambient magnetic field. To do so, the counter uses a generator of a reference frequency (a very stable high frequency signal). The period of the reference signal defines the counter resolution, which is the least difference between two time intervals, the counter can distinguish. It is measured in time units. Sometimes, the reference frequency can be specified instead. The counter resolution in such a case can be calculated, using the formula:

R = 1 / F_ref, where

R is the counter resolution,
F_ref is the reference frequency

Using the highest possible reference frequency is a key for achieving the best counter resolution. KMAG4 uses a reference frequency of 260MHz, which defines a counter resolution of 3.85ns. It provides 3.25 times better resolution than the counters on the market, using 80MHz reference frequency and 6.5 times better resolution than those, using 40MHz.

Survey Sensitivity

It is defined by the least change of the ambient magnetic field that can be reliably detected. The formula below shows how the counter resolution, ambient magnetic field and the length of the sample interval affect the survey sensitivity. As it is also affected by the system noise that can be different for every installation, the formula provides the best sensitivity that could possibly be achieved under the specified counter resolution, sample interval and ambient magnetic field.

S = R × (B / T_s), where

B is the magnetic field value,
T_s is the sample time,
R is the counter resolution in same units as T_s,
S is the survey sensitivity in same units as B.

As can be seen from the above formula, the higher counter resolution also provides opportunities for faster sampling while staying within the prescribed survey sensitivity.

Sample Rate

The magnetometer counter is a continuous to discrete converter. It samples the period of the continuous signal, coming out of the magnetometer sensors, at a specified sample rate and provides discrete readings, representing the average magnetic field values for the consecutive samples. Any sample interval between 1ms and 1s, multiple of 1ms can be selected for KMAG4. This corresponds to sample rates from 1 sample per second to 1,000 samples per second. Additionally, the external trigger mode allows the sampling interval to be synchronized to an external event like the OFF time of a time domain EM system transmitter. Faster sampling, combined with the higher counter resolution, provides opportunities for:

• improving the survey detail.
• reducing the costs by flying faster while keeping the specified survey detail.
• accurate representation of the original continuous magnetic field by the collected digital data, even if it contained higher frequency noise components. An example of such a noise is the one from the rotating helicopter blades. The Nyquist Theoreme requires the sample frequency to be at least twice higher than the highest frequency noise component and KMAG4 has no problems with that because of its capability of sampling at up to 1,000 samples per second. Having digital data, accurately representing the original continuous magnetic field, provides opportunities for more sophisticated data processing to get the most for the money, spent on the exploration.

Built-in Filter

A built in low-pass filter can be assigned to one of the four sensors. It attenuates more than 100,000 times (100dB) the magnetic field components, having a frequency, higher than 12Hz. This eliminates the noise from the rotating helicopter blades and of course, any higher frequency noise, including the 50 and 60Hz power line noise. KMAG4 can be configured to output both, the filtered and the raw data, or only the raw data, or only the filtered data. The transfer function of the filter is shown on the chart below. It is for frequency components up to 20Hz, but the attenuation for the higher frequency components is even higher. It could exceed 1,000,000 times (120dB). The filter uses an internal fixed sample rate of 1,000 samples per second. That means, magnetic fields, containing noise components below 500Hz will be represented properly by the digitized data, and the noise will be properly removed by the built-in filter.

Transfer Function [0 - 20]Hz

The built-in filter can provide the following advantages:

• Clean data during the flight.
• If too high sample rates were used to allow for subsequent filtration during data processing, the amount of data could be too big for some data acquisition systems to handle. Here the filter comes to help. Its data can be down-sampled to reduce the amount of information to a level, that can be handled easily by the data acquisition system.

Output Data

KMAG4 provides comma separated ASCII output records. Each record has a precise time stamp when a GPS receiver, providing a GGA string is connected to its GPS port and a PPS signal is connected to its PPS input.

Operating Modes

Two major modes of operations are available – Free Run Mode (FRM) and External Trigger Mode (ETM). When in FRM, KMAG4 starts sampling for the specified sample interval and sending data as soon, as it is powered-on. Any sample interval between 1ms and 1s, multiple of 1ms is allowed.

ETM allows KMAG4 sampling to be controlled by an external signal (SyncIn). Various External Trigger Modes are available.

SyncIn

SyncIn is used only in ETM. The sampling can be controlled by its level (high or low), or its transition from one level to the other. SyncIn is ignored in FRM.

SyncOut

KMAG4 can output a signal (SyncOut), to synchronize other instruments if necessary. SyncOut could be two types, depending on the current operating mode:

• A 500us pulse whose leading edge indicates the end of the current sample and the beginning of the next one. Any of the two edges (rising or falling) can be configured as a leading edge.
• A signal, whose level indicates if KMAG4 is sampling. Any of the two levels (high or low) can be configured for the purpose.

RS232 Ports

KMAG4 employs up to four RS232 ports - MAIN, GPS, AUX1 and AUX2. The GPS port is dedicated for connection a GPS receiver. The MAIN port is dedicated for the output data and for modifying the instrument settings. AUX1 and AUX2 can be used to connect two more instruments, having RS232 output. Their data could be included in the output string, available on the MAIN port. The GPS data can be included in the output string also. Thus, only one RS232 port would be necessary to record and display all data.

PPS

The PPS signal from the GPS receiver is used by KMAG4 to create a precise time stamp for each sample. It can be connected to the instrument's PPS input through pin 9 of any of the three connectors, labeled as Port 1, Port 2 and Port 3.