+View of ESR Equipment – Overview

+

+A) The Control Unit

+The control unit has a variety of functions:

• +Reports the frequency, f , of the RF field in the B)plug-inRF coil that is being driven by the C)amplifier unit.
• +Reports the amplitude of RF field in the B)Plug in sample coil, which will decrease whenever the sample absorbs energy. A processed version of the field amplitude, the “ESR Signal”, can be displayed on the E)Oscilloscope.
• +Applies a swept magnetic field to the sample by driving a slowly varying current through the Helmholtz coils. The magnetic field is continuously varied at a frequency of 50 Hz, but the experimenter can set the average magnitude (A=) and modulation amplitude (A∼) of the current. The values of the average current (A=) and current modulation (A∼) can both be displayed on the numeric output, while the instantaneous current, the “B Signal” can be displayed on the E)Oscilloscope.

+When first trying to find resonance at a new RF frequency, f, it can be helpful to start with a large current modulation, A∼, to quickly scan a wide range of magnetic field strengths

+

+B) The Plug-in RF Coil

+You are given three different plug-in RF coils. Each has a space for the DPPH sample in the middle. The different coils support oscillations over a different ranges of frequency. To cover the full range of available RF frequencies you will need to take several measurements with each coil. The A) Control unit will report the frequency, f , whenever the amplifier is producing an RF field. If the control unit reports a frequency of 0 MHz, the amplifier is not currently producing an RF field – consult the student guide or your tutor for what to do in this situation.

+

+C) The Amplifier Unit

+The amplifier unit holds the B) Plug-in RF coil and the DPPH sample, and generates the oscillating RF magnetic field. The frequency of the RF field can be changed using the dial on the top of the unit. The amplitude adjustment knob and power switch are located on the back and do not normally need to be adjusted.

+

+D) Helmholtz Coils

+The Helmholtz coils produce the magnetic field the sample is subject to. It is important to position them to produce the most uniform field, which in theory occurs when the current flows in the same direction around each coil, the average distance between the coils, d, is equal to the coil radius, and the sample sits at their centre.

+

+E) Oscilloscope

+The oscilloscope can visualize the magnetic resonance by graphing the “ESR Signal” versus the “B Signal” from the A) Control Unit. This requires the oscilloscope to be running in XY mode, with the current strength “B Signal”) as the X variable (Chan 1) and the “ESR Signal” as the Y variable (Chan 2). It can also be helpful to set the “B Signal” channel (Chan 1) to AC coupling, so the the oscilloscope subtracts the average value the current, A=, leaving the sweep centred even as the average current, A=, is adjusted to locate the resonance.