Frequency counters

To evaluate set-up costs we assume that we have to start from scratch. From our previous discussion about microwave frequencies it should be obvious that we want a cw X-band spectrometer as the central (frequently only) facility. What exactly is a complete spectrometer The answer depends a bit on the type of experiments planned, but for all cases the minimum requirements would be a basic spectrometer (bridge + resonator magnet control unit) and a frequency counter. 

Table 2.4 summarizes the above in terms of a shopping list for new items. It is once again emphasized that the numbers are to be understood as indicative (e.g., not for use in grant proposals). And recall that old spectrometers are not necessarily inferior at all to new ones, and they can be very cheap if standing in someone s way. Also, items such as frequency counters up to X-band appear regularly as second hand offers on the Internet for a fraction of their new price. And finally, the table does not show possible hidden costs, that is, of items that are taken for granted because they already happen to be around, but whose budgeting may be prohibitive when they have to be acquired, for example, square meters of lab space or dedicated operators. 

The Sauerbrey equation predicts a mass sensitivity per unit area of 0.226 Hz cm 2 ng . For a typical crystal the exposed area is c. 0.25 cm2 and the absolute mass sensitivity is 0.904 Hz ng 1. The resolution of modern frequency counters is easily +0,1 Hz in 10 MHz, giving a theoretical mass resolution of c. 9 x 10 10 g in practice this is usually found to be closer to 2 ng. 

AT-cut, 9 MHz quartz-crystal oscillators were purchased from Kyushu Dentsu, Co., Tokyo, in which Ag electrodes (0.238 cm2) had been deposited on each side of a quartz-plate (0.640 cm2). A homemade oscillator circuit was designed to drive the quartz at its resonant frequency both in air and water phases. The quartz crystal plates were usually treated with 1,1,1,3,3,3-hexamethyldisilazane to obtain a hydrophobic surface unless otherwise stated [28]. Frequencies of the QCM was followed continuously by a universal frequency counter (Iwatsu, Co., Tokyo, SC 7201 model) attached to a microcomputer system (NEC, PC 8801 model). The following equation has been obtained for the AT-cut shear mode QCM [10] ... 

Figure 3.24 — Typical system for piezoelectric crystal detector incorporating reference (C,) and test (CJ crystal sensors individually held in oscillating circuits (Or and 0 respectively) serviced by separate frequency counters (FC, and FCj, respectively) interfaced to a common microprocessor or other readout device. (Reproduced from [167] with permission of the American Chemical Society).

Ultraviolet and visible spectra were recorded with a Beckman DK-2 or Cary 14 spectrophotometer. ESR spectra were measured with a Varian model 4500 ESR spectrometer with 100-kHz field-modulation and detection. The klystron frequency was measured with a transfer oscillator and a frequency counter. The magnetic field was measured by a proton gauss meter monitored by the same frequency counter. 

The absolute frequency position of the two-photon transition is measured by comparing the infrared dye laser wavelength with an I - stabilized He-Ne reference laser at 633 nm (see Fig.2). The hey of the wavelength comparison is a nonconfocal etalon Fabry-Perot cavity (indicated as FPE in Fig.2) kept under a vacuum better than 10-6 mbar. This optical cavity is built with two silver-coated mirrors, one flat and the other spherical (R = 60 cm), in optical adhesion to a zerodur rod. Its finesse is 60 at 633 nm and 100 at 778 nm. An auxiliary He-Ne laser as well as the dye laser are mode-matched and locked to this Fabry-Perot cavity. Simultaneously the beat frequency between the auxiliary and etalon He-Ne lasers is measured by a frequency counter. 

The microwave frequency can measured with great precision using the variable-length cavity of a wavemeter Its resonant absorption is detected electronically by the extra "cavity dip," and its frequency is calculated from the mechanically calibrated dimensions of the cavity. The microwave frequency (say 10 GHz) can be read with a precision of even 1 Hz by digital counting and by using a built-in transfer oscillator in a microwave frequency counter. 

KH modulation amplitude of 2 Gauss and a microwave power of 30 mw at 9.5 GHz with proton Gauss meter and frequency counter for spectral marking. 

If the electrodes on both sides of the quartz disk are connected to an oscillator circuit, frequencies / can be measured with a frequency counter. The changes of superficial mass can be calculated from changes of the frequency A/ using the Sauerbrey-equation (-> quartz crystal microbalance). Changes at the sensitivity level of 0.1 Hz at a fundamental frequency of 10 MHz correspond to a mass of about 0.4 ng cm-2. 

The frequency can be measured by the help of a frequency counter with an accuracy of 0.1 Hz and sampling time 0.1-1 s. 

Facilities for a variety of multiple-resonance experiments, including broad-band, homo- or hetero-nuclear decoupling, are now generally built into the spectrometer, instead of being provided by auxiliary audio-oscillators. Oscilloscopes for optimization of magnetic-field homogeneity, precalibrated recorder-charts, and frequency counters for accurate calibration of charts and frequencies for multiple... 

With the advent of inexpensive, fast frequency counters, which count the individual cycles over a precisely fixed period (usually 1 s) and display the frequency digitally, it is more convenient to connect the radio-frequency output of the variable-frequency oscillator directly to the frequency counter and determine the total capacitance with the aid of Eq. (27). This technique is highly suitable for the present experiment if a WTW Dipol-meter or another LC oscillator is available or can be constructed. (With the Dipolmeter only the variable-frequency oscillator is used.) A simple LC oscillator circuit that can be constructed from inexpensive components has been described by Bonilla and Vassos this circuit, with a small modification to provide for one side of the tank to be grounded, is shown in Fig. 2. In this circuit, as in the WTW Dipolmeter circuit, all tank capacitances are in parallel. [This is not true of the circuit described in Ref. 4 of Exp. 30, as that circuit incorporates some series capacitance. If that circuit is employed, Eqs. (28) to (30) are not valid and Eqs. (30-3) to (30-5) must be used instead, unless the null mode is employed.]... 

Although several methods for making the capacitance measurements have been suggested, we will limit our discussion to the use of the LC oscillator and frequency counter shown in Fig. 2. 

C, 300 pF Cj, 0 to 100 pF. The circuit should be housed in a metal box for shielding, and connections with the capacitance cell and the frequency counter should be made with shielded coaxial cable with BNC or similar connectors. 

With the cell capacitor and cell beaker clean and dry, assemble the cell and mount it in a constant-temperature bath set at 25°C. Set the cell capacitor at the open position (a). Turn on the oscillator and the frequency counter, and adjust the tank capacitance Cj to yield a frequency in the range of 1.3 to 1.5 MHz. Wait a while to make sure that the apparatus is operating stably and not drifting in frequency. Determine the frequencies and 4 in alternation at intervals of 30 s by alternating the position of the pointer knob between (a) and (b) until four to six frequency values have been recorded at each position. Make certain that you do not alter Cj or move the leads so as to affect Cs during either set of measurements. 

Capacitance cell as shown in Fig. 1 oscillator as described in the text and Fig. 2 frequency counter (range at least 0.5 to 5 MHz) shielded coaxial cables with connectors five 50- or 100-mL volumetric flasks a 5-mL Mohr pipette acetone wash bottle rubber pipette bulb. 

The rapid development of solid-state electronic devices in the last two decades has had a profound effect on measurement capabilities in chemistry and other scientific fields. In this chapter we consider some of the physical aspects of the construction and function of electronic components such as resistors, capacitors, inductors, diodes, and transistors. The integration of these into small operational amplifier circuits is discussed, and various measurement applications are described. The use of these circuit elements in analog-to-digital converters and digital multimeters is emphasized in this chapter, but modern integrated circuits (ICs) have also greatly improved the capabilities of oscilloscopes, frequency counters, and other electronic instruments discussed in Chapter XIX. Finally, the use of potentiometers and bridge circuits, employed in a number of experiments in this text, is covered in the present chapter. 

Active/passive device Active devices require input of power, most often low-level (5-24 V) DC, to achieve their specified function, with the ctmse-quence that their output RF power level can exceed RF input powo-. Passive devices, on the other hand, effect some transformation of the input signal without use of any external power source, so that the output power is always less than or equal to the input power. In the following, simple components are specified as active or passive (note that the addition of an electronic control system, e.g., a motor drive to set the value of a variable attenuator, is not considered grounds for calling a component active). The active/passive distinction is made only for circuit components having an input and output, not for measurement instrumentation (e.g., a frequency counter), the output of which is a visual indication or computer bus-compatible signal. 

Directional cottier (passive) A device having three or more ports that passes the majority of an input signal straight through to its ouqnit while splitting off a small, specified fraction of the signal to send to another device (e.g., a frequency counter). The device is directional because any power returning to the split-off port from the external circuit is diverted either to a fourth port or to an internal load where it is dissipated. 

Overall, the prospects for development of practical electronic noses based on AW sensor arrays and computerized pattern recognition are very good. A current design for such an instrument consists of an array of four 250-MHz SAW resonators along with RF electronics, frequency counters, interface circuitry, and neural-network pattern-recognition computer. The complete instrument occupies a volume of 500 cm (i.e., 11.4 X 11.4 X 3.8 cm) and uses less than 2 W of power. Improvements in the near future should allow the volume to shrink to less than 160 cm with a power consumption of about 0.7 W. While this is still a rather large nose, further improvements in size and performance are quite likely. Thus, the quest for a small instrument having a volume of a few cubic centimeters and the ability to detect and identify vapors at the part-per-million concentration level (i.e., a truly versatile electronic nose) appears to be achievable. 

Fig. 7.18. Low-pressure interfaces to detectors based on flow injection. (A) Interface to a photometric detector across a membrane. (Reproduced with permission of the American Chemical Society.) (B) Interface to a flow-through photometric sensor with prior derivatization by the modified Griess reaction. (Reproduced with permission of the American Chemical Society.) (C) Interface to a piezoelectric detector. P peristaltic pump, C collector, CUC clean-up column, DB debubbler, SA sulfamic acid, NEDD /V-( 1-naphthyl)ethylenediamine dihydrochloride, SV switching valve, W waste, DF displacement flask, IV injection valve, FC-PZ flow-cell-piezoelectric crystal, OC oscillator circuitry, F frequency counter, PC personal computer. (Reproduced with permission of Elsevier.)...

In the use of a QCM as a sensor, oscillator circuits are the most common electrical interface. A frequency counter can then measure the frequency output from the oscillator, which is identical to the resonant frequency of the quartz crystal. There are two general classifications for the oscillator circuits operation in series resonance and operation in parallel resonance. The performance of these circuits and the choice of the proper circuit are discussed extensively elsewhere. 

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