Digital filters conversion

Figure 10.8.3 Procedure for generating a complex excitation waveform, b) Shows the chosen amplitudes for the various frequencies and a) shows randomized phase angles. In (c) there is a complex plane representation of the arrays in (a) and (b), (d) The time domain representation, which is subjected to digital/analog conversion to produce (e) and in turn, low-pass filtering yields (/). Only a small part of the waveform period is shown in (e) and (/). [From S. C. Creason et al., 7. Electroanal. Chem., 47, 9 (1973), with permission.]...

As a rule analog filters are connected on line with the signal source. Digital filtering takes place after AD conversion and storage of the data it is seldom possible to work up the data source immediately. They should thus be stored temporarily on the PC-integrated RAM or permanently on a discette or on tape. 

Scattered light collected by the FI.5 lens (f = 30 cm) is relayed to the photomultiplier tube via 1 mm slits, a 1 nm bandwidth interference filter and a polarization filter, to reduce background from flame luminescence. The PDP-11/34 computer instructs the A/D convertor to make a conversion every 100 vsec. The resulting digital data are stored sequentially in core memory. The memory is saturated at 16,000 temperature measurements, at which time the data are transferred to a hard disk memory. The data in this transfer constitute one time... 

Figure 4 shows the frequency domain spectra obtained with our pulsed Fourier transform spectrometer for c(13)h20 in natural abundance displayed on an oscilloscope. The formaldehyde pressure was approximately 1 mTorr. The spectra cover 25 MHz and each frequency point corresponds to 100 KHz. The displayed line corresponds to the 111 llO t otational transition of c(13)h20 at 4593.3 MHz. The carrier frequency, ff/ o, was kept at 4 MHz off-resonance in the upper spectrum and 21 MHz off-resonance in the lower spectrum. Both spectra were obtained after an averaging time of 15 sec and an optimum exponential filter was used in the digital conversion. The line has an absorption coefficient of 6 x 10 8 cm"l. The obtained signal-to-noise ratio (peak signal amplitude to rms noise amplitude) is approximately 50 1. 

Another kind of analyzer has been developed which offers the best features of parallel- and swept-filter spectrum analyzers. So-called dynamic signal analyzers use analog-to-digital conversion followed by frequency-to-time-domain transformation, usually using hard-wired computational machines, to mimic the function of a parallel-filter analyzer with hundreds of filters, and yet are cost-competitive with swept-filter analyzers. In addition, dynamic spectrum analyzers are capable of measuring amplitude and phase accurately these are basically time domain instruments, and their function will be discussed in Section 3.1.4. 

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