Kāhu Spectrometer

The Kāhu spectrometer is a multi-channel MR console with integrated gradient and shim controllers. The base model includes 2 transmit channels and 4 receive channels, while the upgraded model provides 16 receive channels. The Kāhu QuickStart Guide explains the basics of how to set up the console.

../_images/kahu_console.jpg — Kāhu spectrometer, 16 channel model

Specifications

Unless stated otherwise, these specifications apply to both the base and 16 channel models of Kāhu.

General

Power Supply	24 V, 90 W
Dimensions	300 x 200 x 60 mm (Width x Depth x Height)
Weight	1.8 Kg

RF Transmit

Channels	2
Frequency Range	100 kHz - 140 MHz
DAC Sample Rate	400 MSPS
Resolution	16-bit
Output Power	0 dBm (0.63 Vpp)
Output Impedance	50 Ω
Output Coupling	AC coupled
Connector	SMA

RF Receive

	Base Model	16 Channel Model
Channels	4	16
Receive Processor	FPGA-based 32-bit quadrature	FPGA-based 32-bit quadrature
Frequency Range	100 kHz - 140 MHz [1]	100 kHz - 140 MHz [1]
Crosstalk, worst neighbor	-80 dB	-70 dB [2]
Sample Rate	100 MSPS	50 MSPS
Resolution	16-bit	14-bit
Input Power Range	10 dBm (2 Vpp)	10 dBm (2 Vpp)
Input Impedance	50 Ω	50 Ω
Input Coupling	AC	AC
Connector	SMA	MMCX

Gradient Controller

Channels	4
Resolution	16-bit
Sampling Rate	200 kSPS
Voltage Range	±10 V
Max Current Output	10 mA
Output Type	Single ended
Connector	BNC

Shim Controller

Channels	16
Resolution	16-bit
Sampling Rate	50 kSPS
Voltage Range	±5 V
Max Current Output	1 mA
Output Type	Single ended
Connector	High Speed I/O (Cable provided)

Digital I/O

General Purpose/Pulse Progammable Outputs	28
Pulse Progam Trigger Inputs	4
General Purpose Inputs	4

Temperature Control

Temperature sensor inputs	2x PT100 inputs, with 24-bit ADC
PID control outputs	2x 5 V PWM outputs

Embedded Platform

Main Processor	Quad-core ARM Cortex-A53
Pulse Sequencer	FPGA-based real-time streaming engine
Operating System	Linux
Memory	4 GB 64-bit DDR4
Ethernet	1 Gbps
USB	4x USB 3 ports
Other Features	Real Time Clock

Detailed Description

GPIO

The Kāhu console has 4 GPIO ports (see Connections). Each port interfaces with 3.3 V TTL logic and has 7 output pins, 2 input pins and 1 ground pin. Following the pin numbering on the mating connector, the pin configuration for all ports is:

Part No.	Molex 545991016
Pin No.	Function
1, 2, 3, 4, 5, 6, 8	Output
7, 9	Input
10	GND

Note

The metal shielding on the mating connector will need to be removed in order to access the solderable pins and see the printed pin numbering.

The image below shows the mating connector in GPIO port 3.

../_images/GPIO_mating_conn.JPG — GPIO Connector

For information on controlling the GPIO pins see the General Purpose Outputs page.

Interlock

Kāhu has 5 interlock output pins available on the i-lock connector, 1 for the shims and 1 for each gradient channel. These can be used to enable amplifier modules when a pulse sequence is active, and disable them when in standby. The interlock pins are not directly controllable, they will automatically be brought high for the entire duration of a pulse sequence. They will be brought low when the pulse sequence finishes or is aborted.

../_images/i_lock_connection.png — Kāhu interlock connector pinout, viewed from the back

Cable Part No.	SFSDT-07-28-G-20.00-SR
Pin No.	Function
1, 3, 5, 7, 9	GND
2	5 V TTL Shim interlock Output
4, 6, 8, 10	5 V TTL Gradient interlock Outputs
11-14	NC (reserved)

Shim Controller

The pinout of the shim connector is as follows:

../_images/shim_connection.png — Kāhu shim connector pinout, viewed from the back

Cable Part No.	FCF8-20-01-L-12.00-S
Pin No.	Shim Channel Index
1	0
2	1
3	2
4	3
5	4
6	5
7	6
8	15
9	14
10	13
11	12
12	11
13	10
14	9
15	8
16	7
17 - 20	NC

Caution

When plugging in the shim cable to Kāhu ensure the gold contacts are facing upwards.

Temperature control

Kāhu provides capability to monitor and control temperature through the PT100 connector. Two sensor inputs can be connected and then used to automatically control the duty cycle of their respective PWM outputs to achieve a stable temperature. The two channels maybe be operated independently or in conjuction. Each PWM signal is 5 V and can be used to control a heater, fan, or peltier module.

In the JupyterLab environment, the temperature can be monitored and the PID settings adjusted using the System dashboard app, see Temperature Monitor.

The pin configuration for the PT100 connector is:

Cable Part No.	SFSDT-05-28-G-08.00-SR
Pin No.	Function
1	PT100 2 Reference
2	PT100 1 Reference
3	PT100 2 +
4	PT100 1 +
5	PT100 2 -
6	PT100 1 -
7, 8	GND
9	PWM 1, 5 V output
10	PWM 2, 5 V output

The diagram below illustrates the connector pinout and an example of how to connect a PT100 sensor to Kāhu. Note that the reference wire should connect at the sensor itself so that it is the same length/resistance as the ‘-’ connection (3-wire PT100 configuration).

../_images/pt100_connections.png — PT100 Sensor 1 Wiring Diagram

Typical Performance Characteristics

Crosstalk Measurement Setup

The crosstalk has been characterised by connecting Transmit channel 0 to each Rx channel in turn with a coaxial cable and transmitting a frequency sweep while acquiring on all Rx channels. Unconnected Rx channels were left unterminated for this test.

../_images/kahu_crosstalk_connection.png — Crosstalk Measurement Setup

A frequency sweep from 0 - 200 MHz was then transmitted, while acquiring signal on all Rx channels.

Crosstalk 16-Channel

The following plots show test results measured on the 16-channel Kāhu. Note that the sampling rate of the 16-channel Kāhu is 50 MSPS, which causes spikes at mutliples of 50 MHz due to the aliasing of the DC offset.

Crosstalk 4-Channel

The 4-channel Kāhu crosstalk measurements are summarised below:

Rx loopback channel	Rx 0 dB	Rx 1 dB	Rx 2 dB	Rx 3 dB
0	0	-85	-90	-100
1	-85	0	-100	-105
2	-105	-95	0	-83
3	-100	-100	-80	0