SHORT NOTE – CONDENSED MATTER
NQR DETECTION SETUP*
O. S. STOICAN
INFLPR Bucharest, ostoican@k.ro
Received December 21, 2004
A setup aimed to detect nuclear quadrupole resonance (NQR) based on a
marginal oscillator is reported. Because NQR absorption signal is very weak and noisy,
a phase sensitive detection technique was employed. An experimental method to
optimize the setup parameters is described.
Key words: nqr, marginal oscillator, radiofrequency spectroscopy, phase sensitive detection.
An experimental setup meant to detect the weak RF energy absorption by
various solid substances, due to the nuclear quadrupole resonance (NQR) effect is
described. The main component of the setup consists of a marginal oscillator.
Generally, a marginal oscillator is characterized by a high sensitivity to the RF
energy loses and has the advantage that the operating frequency automatically
follows the resonator frequency.
The detailed circuit diagram of the marginal oscillator is shown in Fig. 1. The
circuit diagram is based on a configuration described in [1].
The core of the marginal oscillator consists of the two JFETs, Q1 and Q2,
respectively, embodying a differential stage. The operating frequency of the
marginal oscillator depends on the values of the tank circuit elements, namely
equivalent capacitance due to C1, C2, C3, D1 and inductance L1, respectively. To
perform the coarse adjustment of the operating frequency a variable capacitor C1
was used. The operating point of a marginal oscillator is chosen near to the
oscillation threshold by changing value of the resistor R5. The basic feature of a
marginal oscillator is that the oscillations level becomes very sensitive to the
quality factor of the tank circuit. It is necessary to keep the RF voltage across the
coil at low level to avoid saturation of the sample. The sample is contained in a
glass probe tube, around which is wrapped the coil of the tank circuit L1. Thus,
*
Paper presented at the 5th International Balkan Workshop on Applied Physics, 5–7 July
2004, Constanţa, Romania.
Rom. Journ. Phys., Vol. 51, Nos. 1–2, P. 311–315, Bucharest, 2006
312
O.S. Stoican
2
Fig. 1 – Detailed electrical diagram of the marginal oscillator.
sample is magnetically closely coupled to the RF field of the tank circuit. At the
frequency corresponding to NQR effect, the sample absorbs a very small amount
of RF energy and quality factor of the tank circuit decreases. By monitoring the
variation of the oscillations level, NQR absorption is detected. The amplitude
demodulation is performed using the nonlinear transfer characteristic of a JFET (Q4
in Fig. 1). The variations of the oscillations amplitude are amplified (∼X 670) by an
ac coupled low frequency amplifier. Due to the ac coupling, the circuit can detect
only variations of the oscillations amplitude. A buffer stage based on the dual gate
MOSFET Q3 is used to facilitate the oscillations frequency measurement. Due to
its high input impedance, the buffer stage avoids the perturbation of the marginal
oscillator by the frequency measurement circuit.
The whole setup is shown in Fig. 2.
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NQR detection setup
313
Fig. 2 – Block diagram of the NQR detection setup.
Circuit points A and B shown in Fig. 1 are corresponding to the points A and B
shown in Fig. 2. Because it may be detected only the variations of the oscillations
amplitude, the frequency must be swept around the NQR frequency and observed
the amplitude dip due to the RF energy absorption. In this purpose, a reverse biased
varactor diode D1 parallel connected to tank circuit is employed.
The RF energy absorption due to NQR effect is very weak and noisy so that
the absorption dip cannot be observed directly. In order to improve sensitivity a
phase sensitive detection technique was used. The varactor bias voltage represents
the superposition of three voltages V1+V2(t)+V3(t). The voltage V1 is a dc voltage
used to perform fine-tuning of the marginal oscillator center frequency. This
voltage is adjusted manually by mean of a potentiometer P so that the oscillation
frequency to be near of the expected NQR frequency of the studied sample. The
voltage V2(t) is a linear variable voltage provided by an external signal generator
G1. The voltage V2 varies slow (T=10s) allowing to sweep operating frequency of
the marginal oscillator around the NQR frequency of the sample. Supplementary, a
weak modulation voltage V3(t) at a frequency νm ≅70Hz provided by an external
signal generator G2 is applied to the varactor diode. This voltage modulates
marginal oscillator frequency. Consequently, when frequency of the marginal
oscillator passes through the absorption region by mean of variation of the voltage
V2(t), the oscillations appear to be amplitude modulated at a frequency 2νm ≅
140Hz. The amplitude demodulated signal is applied to the input of a band-pass
filter tuned at 2νm ≅ 140Hz. The output signal of the band pass filter is detected
with a lock-in amplifier (phase sensitive detector) and displayed on the
oscilloscope.
In order to obtain a maximum signal to noise ratio of the signal displayed on
the oscilloscope, the setup experimental parameters must be optimized.
Experimentally, it has been found that the amplitude and frequency of the voltage
V2(t) and the amplitude of the voltage V3(t) are critically. An inadequate choice of
these parameter leads to the failure of the experiment. Due to complexity of the
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O.S. Stoican
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setup an accurate prediction of the optimum values of above parameters are
difficult to be performed. We used an experimental method to find the setup
optimal parameters. In this purpose, an additionally test circuit has been built. The
block diagram of the experimental arrangement including the test circuit, used to
optimize the setup parameters is shown in Fig. 3. The test circuit simulates effect of
periodic RF losses connecting periodically an external resistive load (100kΩ to
10MΩ) to the tank circuit. The test circuit itself consists of a window comparator, a
monostable circuit and a RF switch. The operating frequency of the marginal
oscillator is maintained at a fixed value. The voltage V1+V2(t)+V3(t) is applied to
the input of the window comparator. When Vlow< V1+V2(t)+V3(t) <Vhigh then the
window comparator output state becomes logic "high". The positive edge of the
window comparator output pulse triggers a monostable circuit. Further, the
monostable
circuit
output
controls
a
RF
switch.
An
analog
multiplexer/demultiplexer circuit type 74HCT4051 is used as a RF switch [2]. This
connects to the ground one of a resistive load terminal while the other terminal is
connected to "hot" end of the tank circuit. In this way is simulated the periodic
dissipation of RF energy.
Fig. 3 – Block diagram of the experimental arrangement used to optimize the NQR detection setup
parameters. Dashed line box marks the additionally test circuit.
Using a monostable circuit, a better control of load connection time can be
obtained. Starting with lower value of the load resistance the NQR detection setup
parameters are adjusted progressively to obtain a maximum sensitivity.
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NQR detection setup
315
In Fig. 4 is shown absorption signal due to NQR effect in NaClO3 at room
temperature observed by mean of the setup described in this paper. The sample is
containing into a glass probe tube 16 mm in diameter. The tank circuit coil L1 has
the following characteristics: turns number 5, copper wire diameter 2 mm, inner
diameter 16.5 mm. The frequency was swept in the range from 29.942 MHz to
29.879 MHz.
The work was supported by IFA, contract number CERES 99/2001.
Fig. 4 – Absorption signal due to NQR effect in NaClO3 at room temperature. Bottom trace - phase
sensitive detector output voltage. Upper trace- bias varactor voltage V2(t) variation. The oscillation
frequency varies monotonically with the bias varactor voltage V2(t).
REFERENCES
1. P. A. Probst, B. Collet, W. M. MacInnes, Marginal oscillator optimized for RFSE measurements,
Rev. Sci. Instrum., 47, 1522–1526 (1976).
2. *** 74HCT4051 Datasheet, STMicroelectronics, 2001.