FABRI-TEK Instruments, Inc

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New Products Number 1 in a series

Signal Averaging... Principles and Practices Input signal filtering or integrating? Signal averagers are fascinating instruments, and have contributed significantly to measurements in the physical sciences. They are easy to operate and almost any type will do a respectable job of improving noisy signals. But in both operation and design there are considerations which make very large differences in efficiency of noise reduction, accuracy of data, and convenience of use. This is a new kind of measuring device, which is not always understood. It is Fabri-Tek's intention in this series of notes to touch lightly on a few of the details involved in operation, and to describe a few of the features found in its new 1070 averagers, especially those features of significance in analytical chemistry applications. If you are fortunate enough to have used a good averager to reduce noise effects you are aware of the basic principles involved : Each of the recurrent signals is "sampled" at intervals called sampling times, and the digital values found for corresponding times are totaled or averaged. Typically 1024 samplings are made of each signal, and the 1024 totals or averages accumulated are the ordinate values of the 1024 coordinate points representing the average signal waveform. Because of the noise reducing effect of averaging, the spectrometer's sensitivity may be increased by a factor of a hundred or more. You may also be aware of the fact that instantaneous voltage measurements should not be used. The variations in digital values due to noise are lower if the input voltage is integrated (averaged) during each interval between sampling times, and this average used rather than the signal voltage at the instant of sampling. The efficacy of this technique increases in proportion to the bandwidth of the noise. In most applications a simple RC filter having a time constant equal to the interval between samplings does the job of minimizing noise variations almost as effectively as a true integrator. Furthermore the filter is linear, inexpensive, produces no calibration or drift errors, and is easily adjusted over an enormous range of time constants. For these reasons most averagers use some form of EC filtering. However, the high resolution spectroscopist must beware. Not because an RC filter will broaden a spectral line a little whereas an integrator will not, but because very narrow lines, for example one or two sampling intervals wide, will be measured differently with respect to the area under the peak. This difference can be several percent depending upon actual line width. Since an integrator preserves area information no matter how narrow the line, it enables use of wider sweep ranges for a fixed number of ordinate points than would be possible with RC filtering. For these reasons, we recommend to our customers involved with high resolution spectroscopy that they consider use of either the SD-71/I (12 bits) or SD-72/I (9 bits) Integrating Digitizer with our Series 1070 averagers. Write or phone to discuss your specific application. FABRI-TEK

n Isru tmens t, Inc.

5225 Verona Rd., Madison, Wis., 53711. Phone: 608/271-3333 Circle No. 96 on Readers' Service Card

92 A .

ANALYTICAL CHEMISTRY

Spectrophotometer cell housing accessory permits simultaneous observation and measurement of three variables: transmittance, light scattering at 90°, and fluorescence. The attachment is particularly valuable in studies of reaction kinetics in biological materials where only minute sample amounts are available. Although designed especially for the company's dual-wavelength scanning spectrophotometer, the accessory can be easily modified for use with other commercially available spectrophotometers. Phoenix Precision Instrument Co., 3803 N. Fifth St., Philadelphia, Pa. 19140. 421

Nondispersive Spectrometer for Electron Microscopes

The NEC spectrometer, designed for use on electron microscopes and scanning electron microscopes (all makes), adds microprobe capability to these instruments. X-ray detection efficiency is high ; as many as 80 elements can be detected. Nuclear Equipment Corp., 931 Terminal Way, San Carlos, Calif. 94070. 422

Photomultiplier Tubes

Two new vacuum ultraviolet photomultiplier tubes offer two orders of magnitude improved sensitivity over other standard detectors in the 2000 to 1150 A wavelength range. The tubes differ only in type of input window; the Model SBX 760 has a sapphire window (2000 to 1450 A) and the Model SBX 762 has a magnesium fluoride window for the range 2000 to 1150 A. Model SBX 760 has a peak photon detection efficiency of 5% at 1500 A, and the SBX 762 reaches 10% at 1250 A. Bendix Corp., Southfield, Mich. 48075. 423

Pump Systems

Reliable flow rates from 0.1 to 100000 ml per hour are available with new tubing pump systems. Designated the UltraMasterflex series, these pumps combine Masterflex pump head that accept a continuous length of tubing to