Radio-Frequency Amplifiers

In contrast to the high power required by the light modulator, relative little power is needed to modulate the gain of the FMT. This amplifier is typesUy near 1W. and can be less. In fact, we often direct use the output of the fiequency synthesizer without amplification for gain mediation of the PMT. 

The approximate upper fiequem limits of commonly used mTs are listed in Ihble 4.1. These values ate estimated based on our experience and product literature. The upper fiequency limh of the side window R928 is near 200 MHz. Much higher fiequency measurements ste posdbie with MCP PMTs (Section 5.7), but special circuits ate needed for cross coiidation outside of the mr. 

As described in Section 4.6A I fTs can displi a wavelength-dependent time response. These effects can also be present in the PD measurements. In FD measurements the effects are somewhat more difficult to understand. There can be systematic errors in the phase or modulation values, and the direction of the errors is not always intuitively obvious. Fortunately, the color edicts are minor with presently used side-window dynode PMTs. and they ap- 

The decay time of the reference results in a phase dday (4a) and demodulation (mg) of the reference emission compared to that which would have been observed using a scatteier with a zero lifetime. Of course, 4g and mg depend on the modulation fiequency. The measured values 

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A2. Alderman, D. W., Improvement of signal-to-noise ratio in continuous-wave nuclear magnetic resonance at liquid-helium temperature by using a metal oxide semiconductor field-effect-transistor radio-frequency amplifier. Rev. Set. Instrum. 41, 192-197 (1970). 

Fourier Transform Carbon-13 Solid State MMR. The C-13 spectra were recorded on a Nlcolet Technology MT-150 spectrometer operating at 37.7 MHz and equipped with a cross-polarlzatlon accessory. Radio-frequency amplifiers delivered 450 watts at 150 MHz and a 800 watts at 37.7 MHz were adjusted to satisfy the Hartmann-Hahn condition at roughly 72 KHz (9). All spectra obtained by the CP-MASS and CP-PCS used spin-lock cross-polarlzatlon. The contact time was 1 msec and the delay between pulse sequence repetitions was 2 sec. unless otherwise noted. The spectrometer used quadrature detection. 

The microprocessor handles all of the housekeeping chores for the keyboard and display, deals with conunand and control signaling with the base station, and iso coordinates the rest of the functions on the board. The ROM and flash memory chips provide storage for the phone s operating system and customizable features, such as the phone direaory. The RF and power section handles power management and recharging, and also deals with the hundreds of FM channels. Finally, the RF (Radio Frequency) amplifiers handle signals in and out of the antenna. 

A direct-current SQUID constitutes the core of both the world s most sensitive magnetometer and its quietest radio-frequency amplifier. 

Power supply, x-ray tube, constant-potential, 245, 246 full-wave, 244, 245 half-wave, 244, 245 radio frequency, 149 three-phase, 246 Pre-amplifier, use, 60, 243 Precision, 266... 

Optical parametric oscillator (OPO, see 20) is the real equivalent to the radio frequency shifter however OPO can be replaced by a simple addition of a local oscillator (e.g. laser) through a beam splitter. Multiplication takes place at the level of detectors. For sake of S5mimetry, detectors can be placed at both output of the beam splitter, the intermediate frequency is then the output of the differential amplifier. 

In the conventional NMR experiment, a radio-frequency field is applied continuously to a sample in a magnetic field. The radio-frequency power must be kept low to avoid saturation. An NMR spectrum is obtained by sweeping the rf field through the range of Larmor frequencies of the observed nucleus. The nuclear induction current (Section 1.8.1) is amplified and recorded as a function of frequency. This method, which yields the frequency domain spectrum f(ai), is known as the steady-state absorption or continuous wave (CW) NMR spectroscopy [1-3]. 

In 1993, the first ultrafast vibrational echo experiments on condensed matter systems were performed using a free electron laser as the source of temporally short, tunable infrared pulses (11). Recently, the development of Ti sapphire laser-based optical parametric amplifier (OPA) systems has made it possible to produce the necessary pulses to perform vibrational echoes using a tabletop experimental system (12,13). The development and application of ultrafast, IR vibrational echoes and other IR coherent pulse sequences are providing a new approach to the study of the mechanical states of molecules in complex molecular systems such as liquids, glasses, and proteins (14-20). While the spin echo, the photon echo, and the vibrational echo are, in many respects, the same type of experiment, the term vibrational echo is used to distinguish IR experiments on vibrations from radio frequency experiments on spins or vis/UV experiments on electronic states. In this chapter, recent vibrational echo experiments on liquids, glasses, and proteins will be described. 

Figure 18-5 shows a block diagram of the ENDOR apparatus. This equipment was first used in conjunction with a Varian E-12 EPR spectrometer, but is meant to show the general features of apparatus that could be used with any EPR spectrometer. The EPR cavity (Figure 18-1) was modified for the ENDOR experiment with the inclusion of a hair-pin loop around the Cryo-Tip. The loop is fed with a 50 0 transmission line from a 50-100 Watt broadband radio-frequency (rf) amplifier and is terminated with a non-inductive, water cooled 50 O resistance. 

The field is modulated at some audio- or radio-frequency (usually -100 kHz), and this produces signals at the crystal detector whose output can be amplified at the modulation frequency. In general, sensitivity is higher at high-modulation frequencies because of the linear variation of excess crystal noise with the inverse of frequency. However, the recently developed Schottky diode makes it possible to obtain good sensitivity at relatively low frequency. 

It would be necessary to decouple the radio frequency to prevent it interfering with the function of the potentiometric recorder (or signal processing amplifier). The resistance capacity decoupling shown in their circuit appears hardly sufficient to achieve this in a satisfactory manner and consequently, the circuit shown in figure 11 may be only schematic. The column was connected directly to the sensor and the eluent passed through the annular channel between the central electrode and the sensor wall. 

Two Radio-Corporation amplifiers (CX 371 A) with a filament temperature of about 800° C. are used as a source of current. Variations of temperature have very little effect on the frequency. The gas-filled condenser Co is compared with a condenser Cl (filled with nitrogen) of exactly the same construction. Each condenser consists of nine heavily gilt precision cylinders with a total capacity of 2070 cm. The gas and air condensers are fitted into a metal boiler in a bath of about 50 litres of paraffin oil. The range of temperature of the bath is from room temperature to about 300° C. A quartz compensation manometer connected with the boiler by an invar-quartz joint enables the pressure of the gas to be measured correct to i/io mm. One side of... 

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