RF electronics
RF (radio frequency) electronics have two subsystems: the transmitter and the receiver.
Transmitter subsystem
This consists of the radio waveform synthesizer and power amplifier. The subsystem is responsible for generating waveforms of the required frequency, amplitude, phase and duration at specified times. Multiple RF synthesizers are required because many MR experiments require simultaneous application of RF pulses of different frequency. Earlier MR systems used waveform generators with subsequent phase modulation. However, more recent systems rely on DDS (direct digital synthesis).
Switching
The RF switch is responsible for coupling either the transmitter or the receiver subsystem to the probe. This ensures that the sensitive receiver subsystem is not overloaded with the high powered RF signal generated by the transmitter system. Also, the receiver is 'blanked' during the transmission and for a short duration afterward.
Receiver subsystem
This consists of the components:
- Pre-amplifier
- Amplifier
- Demodulator — responsible for subtracting a reference frequency of specified phase from the observed signal
- Analog-to-Digital converter (ADC)
Magnetic field gradients are controlled by an independent subsystem. Magnetic field gradients are generated by passing current through coils of appropriate geometry.