A proton-resonance magnetic field stabilizer using a frequency stabilizer

NUCLEAR

INSTRUMENTS

AND

METHODS

387-393; ©

T[ 3 ( I 9 7 3 )

NORTH-HOLLAND

PUBLISHING

CO.

A PROTON-RESONANCE MAGNETIC FIELD STABILIZER USING A FREQUENCY STABILIZER I. K U M A B E , A. T A K I G U C H I

a n d H. N A K A M U R A

Department of Nuclear Engineering, Kyushu University, Fukuoka, Japan Received 13 April 1973 A p r o t o n - r e s o n a n c e m a g n e t i c field stabilizer using a frequency stabilizer was designed a n d constructed. T h e frequency o f an R F oscillator is stabilized by m e a n s o f a new frequency stabilizer which consists of a frequency counter, a t h u m b rotary switch, a B C D subtractor, a D A converter a n d a varicap located in the t a n k circuit o f the oscillator. T h e frequency stability o f < 1 x 10-6/h was obtained in this system. T h e magnetic field

stabilizer consists o f mainly the R F oscillator, a preamplifier, a selective b a n d amplifier, a phase sensitive detector, a D C voltage amplifier, a D C power amplifier, a stabilization coil a n d an L F oscillator. This system was simplified by the use o f m a n y integrated circuits a n d IC power packs. T h e stability o f this magnetic field stabilizer was m e a s u r e d to be < 2 x 10-6/h.

1. Introduction A proton-resonance magnetic field stabilizer using a frequency stabilizer was designed and constructed. In general, each of magnets used in nuclear physics research is controlled to 1 0 - 4 ~ 1 0 -5 by a current regulator. It is not meaningful with the magnet to increase current stabilization much better, because there are always variations in the magnetic susceptibility

of the magnet due to temperature variations and the other variations. Therefore some improvement should be made by the use of a feedback system coupled to the magnetic field. A possible method is obtained by the use of nuclear magnetic resonance. This method was applied to many magnets and is being used successfully1-4).

~ $elective Band Amp.

The stability of this magnetic field stabilizer is

Phose Sensitive Detector

DC Voltage Amp.

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Modulation

Coil

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Oscillator

Stabilization BCD Subt factor

1

DA Converter

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Fig. 1. Block d i a g r a m o f the magnetic field stabilizer.

387

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Coil

388

I. K U M A B E et al.

limited by the instability of the oscillator frequency. It is difficult to obtain the stability of <10 -5 for a non-regulated oscillator1). Br6once and Grennberg 2) have proposed a proton resonance magnetic field stabilizer using a quartz stabilized reference frequency. They have found the stability near probe as good as the quartz oscillator (+10-6). However, in this case only frequencies equal to the frequency of the quartz oscillator multiplied by an integer are available. Therefore the development of an oscillator with its frequency both continuously variable over a wide range and capable of being held with crystal accuracy is desirable. The use of a frequency synthesizer as a pilot oscillator is considered to be suitable to this requirement, but a frequency synthesizer is expensive. In this paper, we describe a new frequency stabilizer having a good frequency stability of < 1 0 - 6 and a proton-resonance magnetic field stabilizer using it. 2. General A block diagram of the present system is shown in fig. l. The N M R head consiting of an RF oscillator and a preamplifier is located at the magnet whose field is controlled by this system. The magnetic field at a probe is modulated by a small coil. The N M R signal is detected and amplified by the preamplifier. And the signal is fed into a selective band amplifier which gives an output signal being propor-

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3. Circuits These circuits except for the frequency stabilizer were mainly copied from those in refs. 1 and 2, so that they are described briefly.

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tional to the deviation of the magnetic field from its ideal value, and then it is compared with the modulating frequency in a phase sensitive detector. The error signal is amplified by a DC voltage amplifier and a DC power amplifier and then fed back into the system by means of a stabilization coil on the magnet. An LF oscillator supplies the modulation voltage, a compensating signal to the preamplifier, a phasesensitive-detectoi driver signal and a horizontal sweep for an oscilloscope. On the other hand, the frequency of the RF oscillator is stabilized by means of a new frequency stabilizer. The frequency stabilizer consists of a frequency counter, a thumb rotary switch, a BCD subtractor and a DA converter. The difference between the output frequency of the frequency counter and the required frequency set by the thumb rotary switch is calculated by the BCD subtractor, and the digital output signal is converted into an analog signal by the DA converter. Then this analog signal is fed back into the oscillator by means of a varicap which is located in the tank circuit of the oscillator, so as to cause the output frequency to change until the error is reduced to nearly zero.

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A PROTON-RESONANCE

3.1. THE PROTON SAMPLE AND THE PROBE A proton sample consists of about 0.5 c m 3 of a ferric nitrate solution, containing one part by weight of F e ( N O a ) a ' 9 H 2 0 to 100 parts of water, kept in a sealed glass envelope. A coil wound on the proton sample consists of 15 turns of l m m copper wire, 8 m m in diameter and pitched five turns to the centimeter. The frequency range of 14-28 M H z is achieved by using this coil. 3.2.

THE R F OSCILLATOR AND THE PREAMPLIFIER

A circuit diagram of the RF oscillator is shown in fig. 2. These circuits are similar to fig. 3 in ref. 1. Some changes have been imposed. The varicap IS48 which provides the frequency control is located in tank circuit. This is a voltage-variable capacitor with a static capacitance of 18 pF at 10 V bias. The oscillator is fed from a constant current source Q2, whose current is determined by the voltage drop across the resistor in the emitter circuit of Q2. The proton resonance signal is detected by a 1S953 silicon diode and is fed to the ring-of-three preamplifier. The noise at the output of the preamplifier is about 5 mV peak to peak. This low value was achieved by using the low noise transistor (2SC350) as a first transistor. 3.3.

THE SELECTIVE BAND AMPLIFIER AND THE

LF

389

MAGNETIC FIELD STABILIZER

OSCILLATOR

These circuits are similar to those in ref. 2. The IC operational amplifiers (/LA741) were used in these circuits. The frequency of the LF oscillator is about

160 Hz which is chosen to have no factor in common with the 60 Hz mains. 3.4.

THE PHASE-SENSITIVE DETECTOR DRIVER, THE PHASE-SENSITIVE DETECTOR~ AND THE COMPENSATION CIRCUIT

These circuits are similar to those in ref. 1. The phase sensitive detector was simplified from a full wave rectifier-amplifier circuit to a half wave one. The phasesensitive-detector driver is a kind of the phase shifting network which provides an output of completely variable phase. The compensation circuit supplies a 160 Hz singal of completely variable phase to the preamplifier. 3.5. THE

DC

VOLTAGE AMPLIFIER, THE FILTER

NETWORK AND THE D C POWER AMPLIFIER

These circuit diagrams are shown in fig. 3. An IC power pack TH9013P (Toshiba) was used as a D C power amplifier. This pack having a low zero drift is very convenient for this purpose and saves a lot of labor. Another IC power pack was also used as a power amplifier for the modulation coil. 3.6.

THE FREQUENCY STABILIZER

The frequency of the oscillator is measured by the frequency counter. The frequency counter used is TR-5501 (Takeda) and its stability is 1 x 10-V/day. Its gate time and repetition time were chosen to be 0.1 s and 0.15 s, respectively, therefore the lowest decade of the measured frequency corresponds to l0 Hz. Its 1.5K

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A PROTON-RESONANCE

MAGNETIC FIELD STABILIZER

data output provides a BCD output signal which is transfered in parallel. The difference between the output frequency of the frequency counter and the required frequency is assumed to be less than 1 kHz. In the first place, the lowest three decades of the required frequency are set manually on the thumb rotary switch, which is of three decades and provides the BCD output signal. Secondly, by adjusting the tuning condenser of the controlled oscillator, the higher decades without the lowest two decades of the oscillator frequency are coincided with the required frequency in absence of stabilization, and then the stabilization is carried out by switching on a switch as schown in fig. 5. A circuit diagram of the BCD subtractor is shown in fig. 4. Since the data output impedance of the frequency counter is so high (60 k~2) that an input buffer circuit to the subtractor shown in the left side of fig. 4 was used for each bit Ai. To subtract Bi from Ai, nine's complement arithmetic is used. The first stage of the subtractor is a true/complement converter which transfers its

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complement to the output. The second stage is the BCD adder stage which consists of four SN7483 and one SN7480. The SN7483 four-bit binary full adder performs the addition of two 4-bit binary number. In order to perform the correct subtraction the information of the lowest bit of the third decade is needed even under the condition that the difference is smaller than 100. Therefore the SN7480 single-bit full adder was used. The third stage is a complement/ true converter. A circuit diagram of the DA converter is shown in fig. 5. The input requirements for the DA converter are that logical " 0 " voltage is exactly 0 volt and logical " 1 " is exactly same as each other. The following emitter followers whose collector voltages are supplied in common by a regulated power supply are suitable to these requirements. The DA converter consists of a weighted resistor network and the HA 741 integrated circuits which are used as current adders and a current subtractor. The output of the DA converter is fed back into the varicap after passing through an attenuator and a time constant circuit.

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STAB. FACTOR

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4. Performance

4.t. T H E FREQUENCY STABILIZER There exists an optimum value of the stabilization factor. The smaller stabilization factor leads to the larger deviation of the oscillator frequency from the required frequency set by the switch, because the deviation from the required set-frequency is inversely proportional to the stabilization factor under stabilization. Therefore the stabilization factor should be as large as possible, but the larger stabilization factor leads to the larger fluctuation due to hunting of the feedback system. The frequency deviation and fluctuation were measured as follows. The output of the DA converter was connected in series with a mercury cell in a mercury pulser so as to produce pulses having the pulse height corresponding to the frequency deviation, and the pulses were analyzed with a pulse height analyzer. The results of the frequency fluctuation are shown in fig. 6. In the case of C = 20/~F, the frequency fluctuation is nearly constant up to the stabilization factor of 100, but it becomes gradually large for the stabilization factor larger than 100. Thus, the optimum value of the stabilization factors was found to be about 100 and about 50 in the cases of the time constants of 18.8 s ( C = 20/~F) and 9.4 s ( C = l0/iF), respectively. Fig. 7 shows an example of the frequency variation of the R F oscillator for 1 h running under stabilization. It is seen from this figure that the frequency variation is less than +1(_+10 Hz), which corresponds to the frequency stability of < 1 0 - 6 / h for 20 MHz. Thus, this

frequency stabilizer described here achieves the design aims. 4.2. THE MAGNETICFIELD STABILIZER The magnetic field stabilizer using the frequency stabilizer described above was applied to a beam analyzing magnet in the Cockcroft-Walton laboratory in Kyushu University. This magnet has already been controlled to < 4 x 10-5/h by a current regulator. After several trials, in the case of the time constant of 22 s a stabilization factor of 20 ~ 30 was found to be suitable. The larger stabilization factor leads to the

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Fig. 7. A n example o f the frequency variation o f the R F oscillator for 1 h r u n n i n g under stabilization.

A PROTON-RESONANCE

MAGNETIC

large fluctuation due to hunting of the feed back system. The stability of < 2 x 10-6/h is expected in the case of the stabilization factor of 20, because the magnet has been controlled to < 4 x 10-6/h by the current regulator. The stability was measured by recording the output voltage of the power amplifier in a recorder and was proved to be < 2 x 10-6/h. The output of the power amplifier is neary proportional to the magnetic field variation except for short time variations ( < 1 s) and the long time zero drift of the system, because the effective time constant of this system is about 1 s. The amplitude of short time variations is not so large because of the large inductance of the magnet. The long time zero drift was measured to be < 1.2 x 10-6/h in absence of stabilization. This drift corresponds to < 6 x 1 0 - 8 / h under stabilization. Therefore the measurement of the stability mentioned above would be justified. The stability of the magnetic field using the present system is strongly dependent on the stability of the main coil current, therefore better stability of the

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/i Frequency

Frequency

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Fig. 8. Block diagram of the measuring system of the magnetic field variation.

FIELD

STABILIZER

393

current regulator is desirable for better stability of the present system. 5. Application to the measurement of the magnetic field variation A method of the measurement of the magnetic field variation is proposed. The magnetic field variation can be accurately measured by changing inner connections in the present system. A block diagram of a measuring system of the magnetic field variation is shown in fig. 8. The output of the power amplifier is fed back into the varicap in the tank circuit of the RF oscillator. In this way the oscillator frequency is controlled so as to hold always the N M R signal at the center of the field modulation. That is, the oscillator frequency follows automatically the proton resonance frequency for the existing magnetic field. The difference between the oscillator frequency and the set-frequency is recorded in a recorder after passing through the frequency counter, the BCD subtractor and the DA converter. Since the frequency variation is exactly proportional to the magnetic field variation, the magnetic field variation can be measured. The experimental test of this system was carried out and was eminently successful.

The authors would like to thank Drs S. Matsuki and H. Tawara for their valuable discussions. References 1) w. c. Olsen, Nucl. Instr. and Meth. 31 (1964) 237. 2) p. Br6once and B. Grennberg, Nucl. Instr. and Meth. 84 (1970) 83. z) j. W. M. DuMond, Ann. Phys. 2 (1957) 283. 4) j. A. Jungerman, M. E. Gardner, C. G. Patten and N. F. Peek, Nucl. Instr. and Meth. 1S (1962) 1.