RF electronics

As in radar, nuclear magnetic resonance has to manage strong transmitted and weak received signals. It does so by switching between transmit and receive modes.

Transmitter subsystem

It consists of the radio waveform synthesizer and power amplifier. This 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 subsystem

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:

• Preamplifier
• Amplifier
• Demodulator: The demodulator is 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.