Volume 80, Issue 12, December 2009
Index of content:
- ELECTRONICS; ELECTROMAGNETIC TECHNOLOGY; MICROWAVES
Attofarad resolution potentiostat for electrochemical measurements on nanoscale biomolecular interfacial systems80(2009); http://dx.doi.org/10.1063/1.3245343View Description Hide Description
We present an instrument that enables electrochemicalmeasurements (cyclic voltammetry, impedance tracking, and impedance spectroscopy) on submicrometric samples. The system features a frequency range from dc to 1 MHz and a current resolution of 10 fA for a measurement time of 1 s, giving a sensitivity of few attofarads in terms of measurablecapacitance with an applied voltage of only 100 mV. These performances are obtained using a low-noise wide-bandwidth integrator/differentiator stage to sense the input current and a modular approach to minimize the effect of input stray capacitances. A digitally implemented lock-in filter optimally extracts the impedance of the sample, providing time tracking and spectroscopy operating modes. This computer-based and flexible instrument is well suited for characterizing and tracking the electrical properties of biomolecules kept in the physiological solution down to the nanoscale.
80(2009); http://dx.doi.org/10.1063/1.3263908View Description Hide Description
To fully appreciate sensitivity enhancement achievable by reducing thermal noise at the detection coil for nuclear magnetic resonance(NMR), a duplexer operational at low temperature is required. In this work, we developed a cryogenic duplexer that can work below 50 K using GaAsdiodes and rf-microelectrical mechanical systems switches. As the observed leakage of a rf pulse (100 W and ) through the duplexer is below 1 mW, it can be used not only for NMR in liquids but also for NMR in solids. We reported recently the cryocoil magic-angle spinning (MAS) probe [T. Mizuno et al., Rev. Sci. Instrum.79, 044706 (2008)] into which was incorporated with this duplexer and a commercial rf preamplifier, we show that signal-to-noise ratio gain of 4.0 times can be achieved for MAS-NMR by lowering the detection-coil temperature to 12 K and the temperatures for the rf preamplifier and the duplexer to 43 K.
A radio-frequency source using direct digital synthesis and field programmable gate array for nuclear magnetic resonance80(2009); http://dx.doi.org/10.1063/1.3271379View Description Hide Description
A radio-frequency (rf) source for nuclear magnetic resonance(NMR) is described. With the application of direct digital synthesis (DDS), the rf source has the ability to yield rf pulses with short switching time and high resolution in frequency and phase. To facilitate the generation of a soft pulse, a field programmable gate array (FPGA) cooperating with a pulse programmer is used as the auxiliary controller of the DDS chip. Triggered by the pulse programmer, the FPGA automatically controls the DDS to generate soft pulse according to predefined parameters, and the operation mode of the pulse programmer is optimized. The rf source is suitable for being used as transmitter in low-field NMR applications, for example, magnetic resonance imaging and relaxation measurement. As a compact and low-cost module, the rf source is of general use for constructing low-field NMRspectrometer.