# Difference between revisions of "Signal generator"

The signal generator provides the reference sinusoidal signals for the field generator emitter coils.

# Waveform generator

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.

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:

${\displaystyle {\text{FREQREG}}={\frac {f_{OUT}\times 2^{28}}{f_{MCLK}}}}$

where ${\displaystyle f_{OUT}}$ is the desired output frequency of the AD9833 and ${\displaystyle f_{MCLK}}$ 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:

${\displaystyle {\text{FREQREG}}={\frac {20500\times 2^{28}}{6\times 10^{6}}}=DFEA2_{16}=11011111111010100010_{2}}$

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.

Non-inverting amplifier configuration

${\displaystyle R1}$ and ${\displaystyle R2}$ form the feedback resistor network which controls the gain of the amplifier. The voltage gain is calculated using the following formula:

${\displaystyle A=1+{\frac {R_{2}}{R_{1}}}}$

${\displaystyle R2}$ 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.