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KMAG4 – 260MHz Four-Sensor Magnetometer Counter Version 3.24.10

A magnetometer counter is a specific continuous to discrete converter in its nature. It converts the continuous signal from the magnetometer sensors, representing the value of the ambient magnetic field to discrete readings, representing the average magnetic field values for consecutive sampling intervals.

It is not unusual the magnetic field to contain noise components that need to be filtered out during data processing. This can be done properly only if the counter sample rate was at least twice higher than the highest frequency component of the ambient magnetic field. The nature of the noise, and hence its frequency composition can be different. Therefore, to reduce the likelihood of aliasing, would be better to use as higher, as possible sampling rate. KMAG4 allows 1,000 different sampling rates. Any sample interval between 1s (1 sample per second) and 1ms (1,000 samples per second) can be selected.

It seems, it would be the best always to use the highest possible sample rate. Unfortunately, higher sample rates have some drawbacks. One of them is the reduced resolution of the individual samples. The resolution of any individual sample depends on the length of the sample interval. Twice shorter sample interval would result in twice worse sample resolution for example. The solution of this problem is to increase the reference frequency of the counter. Twice higher reference frequency would result in twice better resolutrion of the individual magnetic field samples and compensate for the shorter sample intervals. KMAG4 uses 260MHz reference frequency, which provides more than 3 times better resolution than the counters on the market, using 80MHz reference frequency, for example.

Another drawback of the higher sample rate is the increased amount of data to be recorded and processed later. KMAG4 provides a soulution for this problem too. Its primary data, consisting of 1,000 samples per second, would properly represent the original magnetic field, even if it had very high frequency noise components. Then, the built-in low-pass filter can be used to filter out the noise and the resulting sequence of 1,000 samples per second will be a proper representation of the original magnetic field, not containing the noise already. Finally, the instrument can provide down sampling to the specified sample rate to reduce the amount of the output data.

KMAG4 can also be set to sample only during specified time intervals (for example, only during the transmitter OFF time of a time domain EM system) and can be easily integrated in virtually every data acquisition system, because it is not designed to be part of a particular one. Each KMAG4 sample has a precise time tag.

Features and Specifications

Physical Specifications

Box Width: ~ 17cm
Box Height: ~ 5.5cm
Box Depth: ~ 16.5cm1
Weight: ~ 850g
Notes:
1 - The specified depth is without cables connected.

Electrical Specifications

Power Supply: (12 ÷ 34)VDC1
Current Consumption: ~ 30mA from 28VDC2
Sample Time: (0.25 ÷ 1024)ms
Magnetic Field Range: (10,000 ÷ 100,000)nT
Counter Resolution: 3.85ns
Notes:
1 - This is the power supply range of the instrument itself. The actual restrictions on the power supply will come from the sensors, because they are powered from the same power supply as the instrument.
2 - This is the consumption of the instrument itself, without any sensors connected.

SyncIn and PPS Signals

Input Voltage Range: ±15V
Positive Going Threshold: ≥ +2.4V
Negative Going Threshold: ≤ +0.6V
Typical Hysteresis: = 0.5V

SyncOut Signal

Low Level: ≤ -5V at load resistance ≥3kOhm
High Level: ≥ +5V at load resistance ≥3kOhm

Resolution

The period of the signal, coming out from the magnetometer sensor (the Larmor signal) is inversely proportional to the ambient magnetic field. The magnetometer counter measures the period of the Larmor signal and then calculates the corresponding magnetic field value. The smallest difference between two periods, the counter can distinguish, defines its resolution, and the difference between the corresponding magnetic field values defines the magnetic sample resolution. The counter resolution is measured in time units (usually nanoseconds). The sample resolution is the smallest difference between two magnetic field values that can be reliably registered by the counter. It defines the survey sensitivity. The sample resolution can be calculated, using the formula:

Sres = Cres × (B / Ts), where

B is the magnetic field value,
Ts is the sample time,
Cres is the countrer resolution in same units as Ts.
Sres is the sample resolution in same units as B.

As can be seen from the above formula, the sample resolution (Sres) depends not only on the counter characteristics, but on the magnetic field value (B) and the sample time (Ts) also. Using the sample resolution as a counter characteistic can be misleading if the magnetic field value and the sample time are not mentioned explicitly. It is possible, a magnitometer counter manufacturer to claim, for example, its counter provides a resolution of 0.1pT without mentioning anything else. The number in such a case has most likely been calculated using the lowest magnetic field value (10,000nT) and the longest sample time (1s - corresponding to 1 sample per second), because this case provides the best number for the resolution. Most likely this counter has 10 nanoseconds resolution. The same counter would however provide only 10pT (100 times worse) sample resolution if the sample time was 50ms (20 samples per second) and the magnetic field was 50,000nT.

Better counter resolution gives the opportunity for faster sampling and still achieving the desired sample resolution. Faster sampling rate provides the opportunity for faster flying to reduce the cost of the survey or more detailed measurements at better resolution. Finally, faster sample rate provides opportunities for more sophisticated processing of collected data to get the most for the money, spent on the exploration.

Sample Rate

In its nature, mag counter is a specific continuous to discrete signal converter. It takes the continuous signal from the sensors and provides discrete readings, representing the average magnetic field values for consecutive sample intervals. Those discrete readings however, would not represent properly the original magnetic field, if the sample rate wasn't at least twice higher than its highest frequency component. They would be distorted by the phenomenon, known as aliasing. To reduce the probability of aliasing, as higher, as possible sample rate is recommended. The higher sample rate provides better opportunities for making the best use of the data during the data processing and interpretation phase. KMAG4, with its highest sample rate of 1000 samples per second, can provide data, allowing for best use of the money, spent on geophysical exploration. Any sample interval, between 1ms and 1024ms, multiple of 1ms, can be selected for KMAG4. The longer sample intervals are used when higher frequency magnetic field components are not expected, like in a magnetic base station application for example.

It is true, the higher sample rates decrease the resolution of the individual samples, but it is not a problem, during the data processing, to convert 10 consecutive samples for example, into one, ten times longer and having 10 times better resolution. That means, data, collected at higher sample rate can always be converted to data, collected at lower sample rate, but the opposite is not possible. And only data, collected at high sample rates provide the opportunity for sophisticated data processing.

Output Data

KMAG4 has built in decouplers for up to 4 sensors. For each sample, it provides a precisely time stamped ASCII output string with comma separated fields, allowing for easy integration with virtually any data acquisition system. The fields, corresponding to the unused sensor inputs, can be disabled if necessary to reduce the amount of the output data. KMAG4 can also output a fixed-length output record if the data acquisition system requires it.

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 allows eliminating 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 exceeds even 1,000,000 times (120dB). The filter uses an internal fixed sample rate of 1000 samples per second, but any longer sample interval up to 1024ms, multiple of 1ms can be selected for the output data.

Transfer Function [0 - 20]Hz Filter Transfer Function

Operating Modes

Two major modes of operations are available – Free Run Mode (FRM) and External Trigger Mode (ETM). When in FRM, KMAG4 starts sampling at the specified sample interval and sending data as soon, as it is powered. Any sample interval between 1ms and 1024ms, multiple of 1ms is allowed.

ETM allows KMAG4 sampling to be controlled by an external signal (SyncIn). Various External Trigger Modes are available. They can be divided into two categories – Continuous Sampling Mode (CSM) and Partial Sampling Mode (PSM). In CSM, the instrument samples during the entire sample interval. The end of the current sample is also the start of the next sample in CSM. The same edge of SyncIn signal is used to end the current sample and to start the next one.

In PSM, the instrument samples only during a part of the sample interval. The end of the current sample is followed by idle time and then, the next sample starts. PSM can be divided in two categories – Simple Partial Sampling Mode (SPSM) and Parametric Partial Sampling Mode (PPSM). In SPSM, the instrument samples only when SyncIn is in a specified state (high or low). In PPSM, only one edge of SyncIn is used. The parameters Sample Delay and Sample Length should be specified in advance. Detection of the specified SyncIn edge causes KMAG4 to start a sample after the specified Sample Delay and to sample for the specified Sample Length. Any number between 0ms and 1024ms, multiple of 0.25ms can be specified for Sample Delay. Any number between 0.25ms and 1024ms, multiple of 0.25ms can be specified for Sample Length.

SyncIn

SyncIn is used only in ETM. Its level, high or low (programmable), controls the instrument whether to sample the magnetic field or not in SPSM. Its transition, from low to high or from high to low (programmable), controls both, the start and the stop of a sample in CSM or only the start in PPSM. SyncIn is ignored in FRM.

SyncOut

KMAG4 can output a signal (SyncOut), to synchronize other instruments. SyncOut could be two types, depending on the current sampling mode. In Free Run Mode and Continuous Sampling Mode it is a constant width (500us) pulse. Its leading edge marks 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.

In Partial Sampling Mode, SyncOut, when active, indicates the instrument is sampling. Its leading edge indicates beginning of the sample and the trailing one – the sample end. Any of the two states (high or low) of the signal can be configured as active.

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 can be used to ensure a precise timestamp for each sample. KMAG4 samples it every 20ns to detect its active edge. The PPS signal from the GPS 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.

Front View

ON MAG1 MAG2 MAG3 MAG4 [ KMAG4 Front View ]

ON

Power Supply LED

MAG1, MAG2, MAG3, MAG4

Two rows of BNC connectors. The bottom one is for the four sensors. They are inputs to the 4 decouplers and provide power to the sensors. Each sensor is powered through an individual fuse to protect against short in the sensor cable. The table below provides the specifications for the four fuses.

Current Rating Manufacturer Part #
3A Littelfuse 0452003.MRL

The top row is to connect an oscilloscope, if necessary to observe the signal from the associated sensor.

Rear View

28 VDC Port 1 Port 2 Port 3 [ KMAG4 Rear View ]

Port 1

Pin 1 - Four are the possible configurations:

  • NC
  • SyncIn input in ETM
  • NU
  • RxD for the AUX2 port, when enabled in FRM. Baud rates of 4800, 9600, 19200, 38400, 57600, 115200, 128000, 230400, 256000, 460800, 576000 or 768000 bits/sec are available in this case.

Pin 2 - Two are the possible configurations:

  • NU
  • RxD for the GPS port. Baud rates of 4800, 9600, 19200, 38400, 57600, 115200, 128000, 230400, 256000, 460800, 576000 or 768000 bits/sec are available in this case.

Pin 3 - Two are the possible configurations:

  • NU
  • GPSD Replica (The instrument outputs through this pin everything, it receives through the RxD pin of the GPS port)

Pin 4 - Two are the possible configurations:

Pin 9 - Two are the possible configurations:

Pin # Signal Description Signal Type
1 NC/SyncIn/NU/AUX2-RxD -/In/In/In
2 NU/GPS-RxD In/In
3 NU/GPSD Replica Out/Out
4 NC/SyncOut -/Out
5 GND -
6 NC -
7 NC -
8 NC -
9 NC/PPS -/Input
Notes:
NC - Not Connected (There is no connection between this pin and the instrument circuitry).
NU - Not Used (The pin is connected to the instrument circuitry, but the instrument ignores the applied signal if it is an input, or does not modifies it if it is an output).

Port 2

Pin 1 - Four are the possible configurations:

  • NC
  • SyncIn input in ETM
  • NU
  • RxD for the AUX2 port, when enabled in FRM. Baud rates of 4800, 9600, 19200, 38400, 57600, 115200, 128000, 230400, 256000, 460800, 576000 or 768000 bits/sec are available in this case.

Pin 2 - Three are the possible configurations:

  • NU
  • RxD for the AUX1 port. Baud rates of 4800, 9600, 19200, 38400, 57600, 115200, 128000, 230400, 256000, 460800, 576000 or 768000 bits/sec are available in this case.
  • RxD for the GPS port if AUX1 is not enabled. Baud rates of 4800, 9600, 19200, 38400, 57600, 115200, 128000, 230400, 256000, 460800, 576000 or 768000 bits/sec are available in this case.

Pin 4 - Two are the possible configurations:

Pin 9 - Two are the possible configurations:

Pin # Signal Description Signal Type
1 NC/SyncIn/NU/AUX2-RxD -/In/In/In
2 NU/AUX1-RxD/GPS-RxD In/In/In
3 NC -
4 NC/SyncOut -/Out
5 GND -
6 NC -
7 NC -
8 NC -
9 NC/PPS -/Input
Notes:
NC - Not Connected (There is no connection between this pin and the instrument circuitry).
NU - Not Used (The pin is connected to the instrument circuitry, but the instrument ignores the applied signal if it is an input, or does not modifies it if it is an output).

Port 3

The MAIN port is connected to Port 3 connector. No flow control, 8 data bits, No parity, 1 stop bit setup is used. Only 3 RS232 lines (RxD, TxD and GND) are necessary. A data logger can be connected here and the instrument settings can be modified through this port also. The instrument settings are saved in a non-volatile memory and stay the same until they are modified again. A communication program like Hyper Terminal can be used to modify the instrument settings. Baud rates of 9600, 19200, 38400, 57600, 115200, 128000, 230400, 256000, 460800, 576000 or 768000 bits/sec are available.

Pin 1 - Four are the possible configurations:

  • NC
  • SyncIn input in ETM
  • NU
  • RxD for the AUX2 port, when enabled in FRM. Baud rates of 4800, 9600, 19200, 38400, 57600, 115200, 128000, 230400, 256000, 460800, 576000 or 768000 bits/sec are available in this case.

Pin 4 - Two are the possible configurations:

Pin 9 - Two are the possible configurations:

Pin # Signal Description Signal Type
1 NC/SyncIn/NU/AUX2-RxD -/In/In/In
2 MAIN-RxD In
3 MAIN-TxD Out
4 NC/SyncOut -/Out
5 GND -
6 NC -
7 NC -
8 NC -
9 NC/PPS -/Input
Notes:
NC - Not Connected (There is no connection between this pin and the instrument circuitry).
NU - Not Used (The pin is connected to the instrument circuitry, but the instrument ignores the applied signal if it is an input, or does not modifies it if it is an output).

28VDC

This connector provides power to the instrument and to the sensors. Pin 1 is connected to pin 3 and pin 2 - to pin 4 inside the instrument. Four-wire 16 AWG cable is recommended. Pins 1 and 3 connect to the positive wire and pins 2 and 4 – to the negative one. Special attention should be paid when making the power cable. The instrument will be damaged if connected to reversed power supply. The table below lists the parts, necessary for the power cable.

Item Description AMP Part # Quantity
1 Plug Standard Sex 206060-1 1
2 Socket Contacts 18AWG-16AWG 66181-1 4
3 Cable Clamp 1-206062-6 1