The signal generator provides the reference sinusoidal signals for the field generator emitter coils.
The waveform generator provides the reference sinusoidal signals for each emitter coil of the field generator. The generator comprises of a single Analog Devices AD9833 programmable integrated circuit which provides a reference sinewave at a desired frequency. The IC is programmed over the SPI bus using a microcontroller.
Programming the AD9833
The AD9833 synthesises the reference sine waves for each emitter coil. The integrated circuit is programmed over the serial peripheral interface (SPI) using a microcontroller. On powering up the IC needs to be configured to output the required frequency. The AD9833 contains a set of registers which can be written over the SPI interface.
The desired frequency register value (denoted 'FREQREG' in the datasheet) can be found using the following formula:
where is the desired output frequency of the AD9833 and is the frequency of the master clock signal input provided by a clock driver. For example, to program an output frequency of 20.5kHz using a 6MHz master clock signal:
The full programming sequence is detailed in the datasheet and application note. Typically a microcontroller platform (for example Arduino or MSP430 is used to program each waveform generator when the system powered-on.
Variable gain stage
The gain stage amplifies the output of the sinewave generator. The output of the AD9833 is capacitively coupled to the input of the TL081, a general purpose op-amp. The TL081 is configured with a potentiometer to allow the gain of the amplifier to be adjusted.
and form the feedback resistor network which controls the gain of the amplifier. The voltage gain is calculated using the following formula:
is potentiometer and can be varied to meet the gain requirements of the system. Detailed information regarding the circuit design and layout can be found in the schematics.