A transmitter modification which provides increased sensitivity for homonuclear decoupling experiments

JOURNAL

OF MAGNETIC

RESONANCE

100,

57 l-574 ( 1992)

A Transmitter Modification Which Provides IncreasedSensitivity for Homonuclear DecouplingExperiments ROBERT Department

of Physical

Methodology,

W. DYKSTRA

Searle,

4901

Searle Parkway,

Skokie,

Illinois

60077

Received November 26, 199 1; revised April 17, 1992

Homonuclear decoupling experiments in Fourier transform NMR spectroscopy frequently employ the time-sharing procedure introduced by Jesson et al. ( 1) . In this method, the acquisition period dwell time is shared between B2 irradiation and signal acquisition. To minimize loss of the receiver’s signal-to-noise ratio (S:N), the B2 irradiation time is limited to approximately 20% of the dwell period. To avoid experimental difficulties it is essential to maintain a high level of isolation between the irradiating transmitter and the receiver’s preamplifier during the signal sampling period of the dwell time (2-5). Homonuclear decoupling experiments on our AMX-500 NMR spectrometer have produced spectra with a 17-fold loss in S: N when compared to experiments without homonuclear decoupling. We have determined that this S: N loss is due to inadequate isolation of the proton transmitter from the preamplifier. In this report we describe a modification to the AMX-500 which has restored the spectrometer sensitivity for these experiments. Our AMX-500 NMR spectrometer has recently been upgraded to incorporate a multichannel interface that is capable of rapidly varying the amplitude, phase, and frequency of three observation and irradiating radiofrequency fields. The proton transmitter/decoupler comprises transmitters of different power levels that provide a dynamic range from 0.1 PW to 50 W while utilizing this rapid switching capability. Solid-state switches are used to select the input frequency source, and to combine the transmitters to a single output port. Rapid commutation between power levels and RF frequencies should increase spectrometer performance for a variety of experiments. For example, the AMX-500 transmitter can provide a water presaturation pulse (50 PW ), the r/2 excitation pulse ( 50 W), and a time-shared selective irradiation field (4 mW) at a different frequency on a common RF output. This feature should provide a reduction of transmission-line losses since the need for a directional coupler to combine separate transmitters is now eliminated. A reduction in transmission-line losses in the signal path from the probe to the preamplifier yields greater sensitivity (6). A simplified schematic drawing of a portion of the AMX-500 proton transmitter is shown in Fig. 1A. Switches Sl-S5 are silicon semiconductor multithrow RF switch modules which provide rapid commutation between RF sources and RF power levels. Power level changes involve selection of either the 50 W amplifier, for output levels of 125 mW to 50 W, or the 5 W amplifier for output levels between 0.1 PW and 200 571

0022-2364192 $5.00 Copyright 0 1992 by Academic Press, Inc. All rights of reproduction in any form reserved.

572

NOTES

TO Probe

B

IlF Y

TO SwitchSS

FIG. 1. (A) A simplified schematic drawing of the proton transmitter /decoupler circuit from an AMX500 NMR spectrometer. Switch Sl selects the frequency source to be amplified in the transmitter. The gate control signal controls the on/off status of the RF gate. This signal is also used to control the on/off status of the PIN diodes comprising our modification (see text). Switches S2 and S5 are used to select either the 5 or the 50 W amplifier. The location of the circuit modification within the transmitter is indicated by the block in dashed lines. (B) A schematic drawing of the circuit developed to modify the proton transmitter/ decoupler. Inductors Ll-L4 are epoxy-molded devices with an inductance of 8.2 PH and a Q of 60. All resistors are l/4 W deposited carbon film fixed resistors. The circuit input is from the output of the second 20 dB attenuator (S4), and its output is connected to switch S5. The circuit is fabricated on a double-sided copper-clad epoxy-fiberglass circuit board that is mounted in a shielded enclosure. The RF input and output connections are facilitated with BNC coaxial connectors.

mW. To extend the dynamic range of output power at levels of 2 mW or less, at least one of the 20 dB attenuators is in series with the output of the 5 W amplifier. At power levels between 2.5 and 200 mW, switches S3 and 54 are closed to bypass the 20 dB attenuators. It is under these latter conditions that the experimental sensitivity decreased the most. Using the 2.5 to 200 mW power range for homonuclear decoupling experiments resulted in a 1‘I-fold loss of S: N. The use of decoupling irradiation at power levels of 2 mW or less resulted in a 3-fold loss of S: N, but this amount of power was inadequate for homonuclear decoupling experiments. To regain the original S: N would require a 9-fold increase in experimental time to compensate for the 3-fold S: N loss, and a nearly 300-fold increase in experimental time is required to compensate for the 17fold loss of S: N. This problem has been explored by the manufacturer, but the problem remained even after an exchange of the proton transmitter module. Spectra showing

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NOTES

the change in S: N at different power levels can be seen in Fig. 2. The spectra in Figs. 2C and 2D were acquired at & power levels of 2 and 2.5 mW, respectively, prior to our modification. These same power levels were selected to produce the spectra in Figs. 2A (2 mW) and 2B (2.5 mW) following the installation of our modification to the proton transmitter. All spectra were acquired in the fast switching mode using identical parameters except for the 1 dB power level change.

pm

4.1

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3:9

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317

3j6

FOG.2. Proton NMR spectra (500 MHz) from an AMX-500 NMR spectrometer showing a portion of the spectrum of a 1 mM sucrose sample in DrO. All spectra are phase-corrected Fourier transforms of FIDs from eight accumulations using a 45” flipangle pulse. The homonuclear decoupling irradiation was applied to a resonance at 4.3 ppm (not shown). The signal-to-noise ratios of the spectra in A and B are 187 and 183, respectively. These spectra were taken following our modifications. Decoupling powers of 2 mW (A) and 2.5 mW (B) were used. The spectra shown in (C) and (D) were acquired prior to our modification using decoupling power levels of 2.0 and 2.5 mW, respectively. The signal-to-noise ratio is 61 in (C) and 10.5 in (D). The signal shown at 3.77 ppm was used for the signal-to-noise calculations. The spectra are drawn to the same absolute scale. Therefore, spectral intensities (signals and noise) are directly comparable.

574

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Our investigation of the problem indicated that the transmitter was functioning normally (i.e., as designed), and that the core of the S: N loss was due to insufficient isolation of the residual noise output of the 5 W amplifier from the receiver preamplifier. Our modification provides an increase of 40 dB in the isolation between these elements. This increased isolation during the signal acquisition period has resulted in a 17-fold increase in S:N for these experiments. Figure 1B shows schematically the circuit developed as a modification to the AMX-500 proton transmitter. The key elements in this circuit are the two PIN diodes (Hewlett-Packard Type 5082-3043) that are controlled by the proton transmitter gate signal. The computer-controlled gate signal synchronizes the turn-on time of the RF signal at the amplifier input and the switching of the PIN diodes. A PIN diode is a semiconductor device that provides a variable RF resistance when controlled by a direct current signal. The range of RF resistance of the PIN diode varies from about 1000 ohms with zero bias to less than one ohm with a direct current of 10 mA. For these diodes, the carrier lifetime is 15 ns and the reverse recovery time is 10 ns. The circuit was evaluated using a signal generator (HewlettPackard 8657A) and a spectrum analyzer (Hewlett-Packard 8590A). At 500 MHz the insertion loss is less than 0.4 dB, and the isolation is 40 dB. Test results indicate that the circuit is useful over a frequency range from 20 to 1040 MHz. The insertion loss remains less than 0.5 dB over that frequency range. The circuit isolation is >50 dB from 20 to 130 MHz, >40 dB from 13 1 to 500 MHz, and >35 dB from 501 to 1040 MHz. The circuit rise time is 50 ns and the fall time is 150 ns. The total switching time, including propagation and device storage time, is 0.2 ps for turn-on and 0.6 ps for turn-off. The resistors in the RF path (R 1-R3 in Fig. 1B) establish a 3 dB attenuator circuit. This resistive termination reduces transmission-line standing-wave reflections that are present when the probe is not impedance matched to the transmission-line impedance of 50 ohms. When the AMX-500 transmitter 20 dB attenuators are bypassed, a severe impedance mismatch can result in damage to the output stage of the 5 W amplifier ( 7). This 3 dB attenuator is not essential to solving the current problem, but it may preclude premature failures in the transmitter circuit. ACKNOWLEDGMENTS The author thanks Dr. Roy H. Bible, Jr., for support and encouragement, Judy Balazs for preparing the drawings, and Andrea Matsuda for typing the manuscript. REFERENCES 1. J. P. JESSON,P. MEAKIN, AND G. KNEISSEL, J. Am. Chem. Sot. 95,618 ( 1973). 2. T. C. FARRAR AND E. D. BECKER, “Pulse and Fourier Transform NMR,” Academic Press, New York, 1971. 3. J. M. K. SANDERS AND B. K. HUNTER, “Modem NMR Spectroscopy,” Oxford Univ. Press, New York, 1987. 4. N. R. KRISHNA, J. Magn. Reson. 22,255 (1976). 5. R. W. DYKSTRA, J. Magn. Reson. 88,388 (1990). 6. R. W. DYKSTRA, J. Magn. Reson. 86,391 ( 1990). 7. R. S. CARSON, “High Frequency Amplifiers,” Wiley, New York, 1982.