EM Field Head Tracking System specifications

Schematic of the EMF Head Tracking System

Introduction

The EM Field Head Tracking System is used for recording head movements of the subject during a trial. The system consists of the following parts:

Three oscillators at roughly 45, 55 and 65 kHz or 50, 60, 70 kHz for X, Y and Z direction.
Three signal amplifiers that amplify the oscillator signals.
six field coils that make a cube of roughly 3x3x3 meters.
A small pick-up coil mounted on a glasses frame that is worn by the subject during the experiment.
Three lock-in amplifiers that split the signal from the pick-up coil in the 3 frequency components and measures their envelope. They output analog signals between -10V and +10V.
An acquisition device converts the analog output signals from the lock-in amplifiers to digital.
A RZ6 multi I/O processor that records the acquired data.
Software that converts the recorded data to head orientations using calibration data.

The following table contains the channel convention with respect of the three directions:

Channel	Frequency	Orientation	Color	Polarity
Channel 1	45/50 kHz	Frontal	Red	Front = positive; Back = negative
Channel 2	55/60 kHz	Vertical	Blue	Right = positive; Left = negative
Channel 3	65/70 kHz	Horizontal	Yellow	Up = positive; Down = negative

Parts

Lock-in amplifiers

Three Femto LIA-BV-150-H Lock-in amplifiers are used in order to separate the Frontal, Horizontal and Vertical signals from the signal that is coming from the pick-up coil. Each Lock-in amplifier has a Sinus Oscillator Module (SOM-1) that is used both as Field Modulation Signal and as Reference Signal. The Lock-in amplifier settings are set at the front of the device (see picture), except for the modulation/reference frequency which is set inside on the SOM-board.

Kenmerk	LIA-BV-150-H
Model	Single Phase
Werkfrequentie	50 Hz - 120 kHz
Tijdconstanten	300 µs - 1 s (6 of 12 dB/oct.)
Verstelbaar Signaalfilter	Highpass 2 Hz - 10 kHz, Lowpass 100 Hz - 1 MHz
Datasheet	367 kB
Uitgangen	X = in fase, Y = kwadratuur, R = magnitude, ±10 V volledige schaal, kortsluitbeveiligd, Signaalmonitoruitgang
Gevoeligheid (Volledige Schaal)	Voltage: 3 µV - 1 V in 1-3-10 stappen
Spanningsingang	Echte differentiële instrumentatieversterker, ruis 12 nV/√Hz
Stroomingang	Transimpedantieversterker, versterking 100 kV/A, ruis 0.4 pA/√Hz
Referentie-ingang	±100 mV tot ±5 V, schakelbaar naar TTL
Fase	Aanpasbaar 0° - 360°; resolutie: 8-bit @ f ≤ 60 kHz, 7-bit @ f > 60 kHz, Temperatuur drift <0.01°/K
Max. Dyn. Reserve	80 dB
Digitale Controle	16 TTL/CMOS ingangen: 8-bit fase, 4-bit tijdconstante, 4-bit gevoeligheid
Voeding	±15 V, +120 mA / -60 mA
Afmetingen	160 x 100 x 20 mm (L x B x H), gewicht 100 g (0.22 lbs)

The front panel of each LIA has four dip-switches:

Mode: 1f means that the fundamental harmonic is measured (2f = second harmonic signal)
Dyn.Res.: For dynamic reserve the settings Low (L) is preferred because the signal is normally large in comparison with the noise.
PLL: For the Phase Lock Loop (PLL) the settings Slow (S) is preferred because the frequency is at a fix value. The phase is more stable in this setting.
Ref.Thr.: Reference Threshold is set to 0V for analog signals (2V is for digital signals).

Oscillators

Amplifiers

The EM Field amplifiers are mounted in a 3U 19" rack. Each channel (F, V and H) has a standard amplifier board based on the TDA7294 audio amplifier chip with a maximum output of 100 Watt. Each board is powered by its own toroidal transformer. Each amplifier chip is connected to a 10 x 10 cm heat sink . At the front there are three BNC connectors for the input signal. At the back there are 3 pairs of banana jacks for the output.

Resonance circuits

Field coil resonator box

Each pair of field coils in opposite walls of a lab have box with a transformer and a resonance circuit (except for the Patient Lab). The boxes are mounted next to the door inside the lab.

The transformer and resonator box has three inputs signals (H,V and F) coming from the field amplier fed into the primary coils of three transformer that are wound 3:1. The secondary coils are connected to the field coils and have a capacitor added into the circuit in order to create a resonance condition. The capacitor are chosen in order to match the resonance frequency of the LC circuit to the frequency of the channel. These frequencies are determined by the SOM-1 sinewave generator in the Femto LIA for the corresponding channel.

The transformers consist of a ferrite ring of 21 mm diameter with 15 primary winding's and 5 secondary winding's. The winding's are made of 1.0 mm (18 AWG) coated copper wire with a current rating of 7.0 A.

Wall coils

The wall coils make up a cube of about 3 x 3 x 3 m around the subject and create low frequency magnetic fields in three orthogonal directions (Frontal, Horizontal, Vertical). The opposite wall coils are paired into one loop that create one field together.

Auditory Sphere Lab:

two winding's per coil.
resonator circuit.
3 m x 3 m coils

Auditory Motion Lab:

two winding's per coil.
resonator circuit.
3 m x 3 m coils

Auditory Perception Lab:

five winding's per coil
no resonator circuit.
2.3 m x 2.8 (2.3) m coils

Pick-up coil

There are several head mounted pick-up coils. For the Auditory Sphere Lab and the Auditory Motion Lab the coils have a flux area of 152 cm2. For the Auditory Perception Lab the flux area is 298 cm2.

ADC's

The signal coming from the output of each Lock-in amplifiers is fed into an ADC. The ADC device is optically linked to the recording device (RA16 or RZ6). The range of the ADC's is -10 V to +10 V

Auditory Sphere Lab     : RA16 with RA8GA channels 1, 2 and 3 (system 3 version)
Auditory Perception Lab : RZ6  with RA8GA channels 1, 2 and 3 (earlier version)
Auditory Motion lab     : RZ6  with RA8GA channels 1, 2 and 3 (system 3 version)

Recording device

The recording of the output of the ADC's is done by either a RA16 Madusa Base Station or a RZ6 Multi I/O processor. The ADC devices are optically linked to the recording device.

Auditory sphere lab     : RA16 on 6103.5 Hz
Auditory perception lab : RZ6 on 48828.25 Hz (divide by 10 ??)
Auditory motion lab     : RZ6 on 48828.25 Hz (divide by 10 ??)

The raw data is saved. For display a 150 Hz low pass filter is used.