Nuclear Magnetic Resonance - The Instrument

This figure shows a simplified block diagram of the apparatus. The diagram does not show all the functions of each module, but it does represent the most important functions of each modular component of the spectrometer. The pulse programmer creates the pulse stream that gates the synthesized oscillator into radio frequency pulse bursts, as well as triggering the oscilloscope on the appropriate pulse. The rf pulse bursts are amplified and sent to the transmitter in the sample probe. The rf current bursts in these coil produce a homogeneous 12 gauss oscillating magnetic field at the sample. These are the time dependent B fields that produce the precession of the magnetization, referred to as the 90^o (or p/2) and 180^o (or p) pulses. The transmitter coils are wound in a Helmholz configuration to optimize rf magnetic field homogeneity.

Nuclear magnetization precessing in the direction transverse to the applied constant magnetic field induces an EMF in the receiver coil, which is then amplified by the receiver circuitry. This amplified radio frequency (15 MHz) signal can be detected (demodulated) by two separate and different detectors. The rf amplitude detector rectifies the signal and has an output proportional to the peak amplitude of the rf precessional signal.

The other detector is a mixer, which effectively multiplies the precession signal from the sample magnetization with the master oscillator. Its output frequency is proportional to the difference between the two frequencies. This mixer is essential for determining the proper frequency of the NMR signal. The precession frequency of the nuclei is well defined by the local magnetic field and the nuclear magnetic moment. The oscillator is tuned to this precession frequency when a zero-beat output signal of the mixer is obtained. A dual channel scope allows simultaneous observations of the signals from both detectors,

Magnet and Power Supply

The magnetic field is variable over a wide range. To detect the H and F signals, only lower currents (less than a third of the maximum) are needed.

The magnet and the power supply need water cooling. Before switching on the power supply, make sure that the water flows and the current setting is "0".

Fine tuning of the magnetic field is possible by adjusting the knob at the upper right corner. Notice that the fine tuning jumps at +50% to -50%. The field may exhibit a small drift over time. For studies when this may be an important issue, make sure that you run control tests from time to time, so that the drift is known and corrected for.

Two additional coils, connected to a separate (smaller) power supply, are installed to the magnet poles. The winding of these coils is opposite, and they can be used to create a magnetic field inhomogeneity.

Spectrometer

The case for the modules has a fused and switched power entry unit located on the back right side. The unit uses 2 A slow blow fuses. A spare set of fuses is stored inside the fuse case. The spectrometer case has a linear power supply enclosed. It has slots for five modules, which connect to the power supply through a back plane of electrical connectors. The modules should be located as follows:

15 MHz Receiver

This is a low noise, high gain, 15 MHz receiver designed to recover rapidly from an overload and to amplify the radio frequency induced EMF from the precessing magnetization. The input of the receiver is connected directly to a high Q coil wrapped around the sample vials inside the sample probe. The tiny induced voltage from the precessing spins is amplified and detected inside this module. The module provides both the amplified rf signal as well as detected signal.

The detector output rectifies the signal ("cuts" it in half) and passes only the envelope of the rf signal. It is also important to remember that the precession signal from spin system cannot be observed during the rf pulse from the oscillator/amplifier since these transmitter pulses induce voltages in the receiver coil on the order of 10 volts and the nuclear magnetization creates induced EMFs of about 10 mV; a factor of 10⁶ smaller!

• Gain: continuously variable, range 60 dB (typical)
• RF out: amplified radio frequency signal from the precessing nuclear magnetization
• Blanking: turns blanking pulse on or off
• Blanking In: input from blanking pulse, to reduce overload to the receiver during the power rf pulses.
• Time Constant: selection switch for RC time constant on the output of the amplitude detector. The longer the time constant, the less noise that appears with the signal. However, the time constant limits the response time of the detector and may distort the signal. The longest time constant should be compatible with the fastest part of the changing signal.
• Tuning: rotates a variable air capacitor which tunes the first stage of the amplifier. It should be adjusted for maximum signal amplitude of the precessing magnetization.
• Detector Out: the output of the amplitude detector to be connected to the vertical scope input.
• RF In: To be connected to the receiver coil inside the sample probe. This directs the small signals to the first stage of the amplifier.

Pulse Programmer PP-101

The pulse programmer is a complete, self contained pulse generator which creates the pulse sequences used in all the experiments. The pulses can be varied in width (pulse duration), spacing, number, and repetition time. Pulses are about 4 V positive pulses with a rise time of about 15 ns. The controls and connectors are described below and pictured here

• A-width: width of A pulse 1-30ms continuously variable
• B-width: width of B pulse 1-30ms continuously variable
• Delay Time:
1. with number of B pulses set at 1, this is the time delay between the A and B pulses.
2. with number of B pulses set at 2 or greater, this is the time between A and the first B pulse and one half of the time between the first B pulse and the second B pulse.
3. delay range can be varied from: 10 ms (which appears as 000x10⁰ on the front panel) to 9.99 s (which appears as 9.99 x 10³)

Accuracy: 1 part in 10⁶ on all delay times.

• Mode: This switch selects the signal that starts the pulse sequence. There are three options
• Int (Internal): The pulse stream is repeated with a repetition time selected by the two controls at the right of the mode switch.
• Ext (External): The pulse stream is repeated at the rising edge of a TTL pulse.
• Man (Manual): The pulse stream is repeated every time the manual start button is pushed. This allows the experimenter to choose arbitrarily long repetition times for the experiment.
• Repetition Time: four position, 10 ms, 100 ms, 1 s 10 s, variable 10 - 100% on any of the four positions. Thus for 100 ms and 50%, the repetition time is 50 ms. The range of repetition times is 1 ms (10 ms 10%) to 10 s (10 s 100%).
• Number of B Pulses: This sets the number of B pulses from 0 to 99
• Ext-Start: Rising edge of a TTI pulses will start a single pulse stream.
• Man-Start: Manual start button which starts pulse stream on manual mode.
• Sync Switch: This switch allows the experimenter to choose which pulse, A or B, will be in time coincidence with the output sync pulse.
• A-switch: turns on or off the A pulse output
• B-switch: turns on or off the B pulse output
• Blanking out: A blanking pulse used to block the receiver during the rf pulse and thus to improve the receiver recovery time.
• M-G out: Meiboom-Gill phase shift pulse, connected to the oscillator, to provide a 90^o phase shift after the A pulse.
• Sync out: A fast rising positive 4 V pulse of 200 ns duration used to trigger an oscilloscope or other data recording instrument.

15 MHz OSCILLATOR / AMPLIFIER / MIXER

There are three separate functioning units inside this module. A tunable 15 MHz oscillator, an rf power amplifier, and a mixer. The oscillator is digitally synthesized and locked to a crystal oscillator so that its stability is better then 1 part in 10⁶ over 30 minutes. The frequency in MHz is displayed on a seven digit LED readout at the top center of the instrument. This radio frequency signal can be extracted as a continuous signal (CW-RF out, switch on) or as rf pulse burst in to the transmitter coil inside the sample probe.

The second unit is the power amplifier. It amplifies the pulse bursts to produce 12 Gauss radio frequency magnetic fields incident on the sample. It has a peak power output of about 150 watts.

The third unit is the mixer. It is a nonlinear device that effectively multiplies the cw rf signal from the oscillator (w₀) with the rf signals from the precessing nuclear magnetization (w  =  g B₀). If the two rf signals have different frequencies a beat structure will be superimposed on the signal. The beat structure is clearly evident on the lower trace of the signals from a free induction signal, shown here. The frequency of the beats equals to the absolute value of w - w₀ . When the frequency of the precession signal is equal to the oscillator frequency, then the beat frequency is zero, and the mixer output and the detector output from the receiver module have identical shapes. At this setting the magnetic field is tuned to frequency of the spectrometer.

Here is a list of the controls:

• Frequency in MHz: The LED displays the synthesized oscillator frequency in megahertz (10⁶ cycles/second ).
• Frequency Adjust: this knob changes the frequency of the oscillator. When the switch (at its left ) is on course control, each "click" changes the frequency by 1,000 Hz, when it is switched to fine, each click changes the frequency by 10 Hz. The smallest change in this digitally synthesized frequency is 10 Hz.
• The Mixer (inside black outline)
• Mixer In - rf input signal from receiver, 50 mV rms (max.)
• Mixer Out - detected output, proportional to the difference between cw-rf and rf from precessing magnetization. Level, 2 V rms (max.) bandwidth 500 kHz.
• CW-RF switch: on-off switch for cw-rf output.
• CW-RF OUT: continuous rf output from oscillator - 13 dbm into 50 W load.
• M-G Switch: turns on phase shift of 90^o between A and B pulse for multipulsed Meiboom-Gill pulse sequence.
• A & B in: input for A & B pulses from pulse programmer.
• RF out: TNC connector output of amplifier to the transmitter coil inside sample probe. Radio frequency power bursts that rotate magnetization of the sample.
• Coupling: This adjustment should only be made by the instructor using a small screwdriver. Adjusting the screw inside the module optimizes the power transfer to the transmitter coils in the sample probe. This adjustment has been made at the factory and should not need adjusting under ordinary operating conditions.

IMPORTANT: DO NOT OPERATE THE POWER AMPLIFIER WITHOUT ATTACHING TNC CABLE FROM SAMPLE PROBE. DO NOT OPERATE THIS UNIT WITH PULSE DUTY CYCLES LARGER THAN 1%. DUTY CYCLES OVER 1% WILL CAUSE OVERHEATING OF THE OUTPUT POWER TRANSISTORS. SUCH OVERHEATING WILL AUTOMATICALLY SHUT DOWN THE AMPLIFIER AND SET OFF A BUZZER ALARM. IT IS NECESSARY TO TURN OFF THE ENTIRE UNIT TO RESET THE INSTRUMENT. POWER WILL AUTOMATICALLY BE SHUT OF TO THE AMPLIFIER IN CASE OF OVERHEATING AND RESET ONLY AFTER THE INSTRUMENT HAS BEEN COMPLETELY SHUT OFF AT THE AC POWER ENTRY.

Sample Probe

The figure shows an artist sketch of the sample probe. The transmitter coil is wound in a Helmholz coil configuration so that the axis is perpendicular to the constant magnetic field. The receiver pickup coil is wound in a solenoid configuration tightly around the sample vial. The coil's axis is also perpendicular to the magnetic field. The precessing magnetization induces an EMF in this coil which is subsequently amplified by the circuitry in the receiver. Both coaxial cables for the transmitter and receiver coils are permanently mounted in the sample probe and should not be removed. Caution should be exercised if the sample probe is opened since the wires inside are delicate and easily damaged. Care should be exercised that no foreign objects, especially magnetic objects are dropped inside the sample probe. They can seriously degrade or damage the performance of the spectrometer.

The glass sample vial should be positioned so that the sample is at the center of the pickup coil. This can be achieved by adjusting the position of an O-ring at the top of the vial.

Auxiliary Components

• PICKUP PROBE: A single loop of # 32 wire with a diameter of 6mm is used to measure the B, of the rotating rf field. This loop is encapsulated with epoxy inside a sample vial and attached to a short coaxial cable. The coaxial cable has a female BNC connector at the other end. To effectively eliminate the effects of the coaxial cable on the pickup signal from the transmitter pulse, a 50 ohm termination is attached at the pickup loop end as shown in the diagram. Since the single loop has a very low impedance, the signal at the oscilloscope is essentially the same as the signal into an open circuit. Note: the orientation of the pickup loop inside the sample holder is important, since the plane of the loop must be perpendicular the the rf field. (Faraday's Law!)
• DUMMY SIGNAL COIL: A second single loop of # 32 wire in series with a 22k resistor is used to create a "dummy signal". This probe is also placed in the sample holder and located at the proper depth to produce the maximum signal. The loop is also connected to the terminating resistor to eliminate cable effects. This probe is attached to the cw output of the oscillator to create a signal which can be used to tune and calibrate the spectrometer.

Computer and software

The oscilloscope has powerful and convenient numerical integration and fast Fourier transform (FFT) features. A short and sketchy introduction to the principles of the FFT is given in the "HP Product Note 54600-3: FFT Lab Experiments Notebook"

The oscilloscope is interfaced to a PC, and data from the scope can be transfered to the computer in two different formats: "screen shot" or numerical file. The corresponding menus in the HP 34810A BenchLink/Scope software are called IMAGE and WAVEFORM.

The screen shot is great for purposes of illustration. (This is how the spectra in this manual were made.) However, the data transferred this way ends up in a file foramt (*.PCX) that can not be processed numerically. The "wave" format allows exporting the data to an ASCII (or EXCEL) file, and it is recommended if further evaluation is planned.

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