Frequency square-wave modulation

In connection with the square-wave technique, mention can be made of high-frequency polarography, also called radiofrequency polarography and developed by Barker53, in which a sinusoidal radio-frequency cu, (100kHz to 6.4 MHz) square-wave modulated at co2 (225 Hz) is superimposed on to the dc potential ramp as the wave form includes (apart from additional higher... 

Fig. 9. Polarization curve of an Fe-disc Pt-split-ring electrode with hydrodynamic square wave modulation. In 1 M NaOH with anodic and cathodic scan including capacity of the Fe disc (dashed curve), modulation frequency of rotation co = 0.05 Hz (insert), simultaneous detection of Fe(II) and Fe(III) ions at Pt half rings [12].

Another example for the HMRRD electrode is given in Fig. 9 for Fe in alkaline solutions [12, 27]. The square wave modulation of the rotation frequency co causes the simultaneous oscillation of the analytical ring currents. They are caused by species of the bulk solution. Additional spikes refer to corrosion products dissolved at the Fe disc. This is a consequence of the change of the Nemst diffusion layer due to the changes of co. This pumping effect leads to transient analytical ring currents. Besides qualitative information, also quantitative information on soluble corrosion products may be obtained. The size of the spikes is proportional to the dissolution rate at the disc, as has been shown by a close relation of experimental results and calculations [28-30]. As seen in Fig. 7, soluble Fe(II) species are formed in the po-... 

Figure 10 Measurement of the lifetime of NO molecules adsorbed on the Pd particles, (a) Stationary desorbed NO pulse in response to a square wave modulated NO beam at a frequency of 12.5 Hz. The continuous curve represents an exponential decay with a time constant t (from Ref. [91]). (b) Arrhenius plot of the life time t for two different Pd particles sizes (from Ref. [89]).

Simultaneous with the publication of Hocker et al., there appeared the results of Yardley and Moore [142] on laser-excited vibrational fluorescence in CH4. A mechanically chopped He-Ne 3.39-micron laser [143, 144] was used to excite the asymmetric stretching [/ = 2948 cm-1 (36.55 X 10-2 eV)] vibration, i>3 (see Figure 3.17). The optical arrangement is shown in Figure 3.18. The He-Ne laser tube, 220 cm in length, is shown on the left. Mx, M2, and Ms are mirrors Bx and B2 are baffles to eliminate stray light Lx and L2 are lenses which focus the laser output into a collimated beam having a diameter of 2 mm, and thence, into a Pyrex fluorescence cell. At the focal point between Li and L2 is a chopper wheel, to produce a nearly perfect square wave modulated at frequencies between 600 and 10,000 Hz. An audio oscillator and a 60-W amplifier are used to drive the synchronous chopper motor. An InSb infrared detector (response time of about 4 nsec) is used to... 

The pressure response to the voliune perturbation was recorded with a high-accuracy differential Baratron pressure transducer (MKS 698A11TRC) (Eq. 10) at each frequency over three to five square-wave cycles (256 pressure readings per cycle) after the periodic steady-state had been established. The isotherm describing the equilibrium sorption conditions can be linear or curved. However, the horizontal region of a rectangular isotherm cannot be used as there is no sorption/desorption following the square-wave modulation of the equilibrium volume. 

Figure 3.10 Square wave modulation yields sidebands at intervals of the modulation frequency...

In addition to sinusoidal modulation, square wave modulation [3, 33] was also sometimes used. Generally, the applied modulation frequency lies between 50 and... 

Dissolved oxygen is removed by purging (5 min) with nitrogen. A new mercury drop is made and adsorption is carried out (1-3 min) at a deposition potential of -1.0 V. Then a quiescence period of 8 s is used, followed by a negative going potential scan. The square-wave modulation is used at an optimal frequency of 50 Hz the differential-pulse mode can also be used. The chromium peak appears at -1.3 V. The scan is repeated to check for peak stability a sig-... 

Polarization curve of the preactivated iron disk of a HMRRD electrode in IM NaOH (dE/df = lOmV S ) with hydrogen oxidation, Fe(ll) and Fe(lll) formation at peak A1 (Epi), Fe(ll) oxidation at All (Epj), and oxide reduction at peak C. Simultaneous capacity measurements (dashed curves) (a,b) Fe(IlI) and (b,c) Fe(ll) dissolution as detected at the Pt half-ring electrodes. Frequency, W = 450min with square wave modulation amplitude, Aiv = 250min- and modulation frequency,/= 0.1 s. (From Haupt, S. and Strehblow, Langmuir, 3,873,1987)... 

A considerable decrease both in the deposition rate and in powder formation was found when the 13.56-MHz excitation was modulated with a square wave of 2 Hz [510, 511]. In the y -regime, Biebericher et al. [512] have observed a decrease in deposition rate from 1.0 nm/s in a continuous-wave (cw) 50-MHz SiH4-H2 plasma, to 0.2 nm/s in a similar (i.e. with the same average power of 10 W) plasma, modulated by a frequency of 100 kHz. 

Here, results are shown from experiments performed in ASTER, reported by Biebericher et al. [512. 519], A SiH4-H2 (50 50 flow ratio, total flow 60 seem) plasma was generated at an RF excitation frequency of 50 MHz. The substrate temperature was 250°C. The RF signal was ampitude modulated (AM) by a square wave. The modulation frequency has been varied in a range of 1-400 kHz. The modulation depth was always 90%. The duty cycle was fixed at 50%. The pressure amounted to 0.2 mbar, and the average power was kept at 10 W. With a duty cycle of 50%, this leads to a power of 20 W during the plasma-on period. 

For example, the light emitted by a faint source may be chopped by means of a chopper producing a square wave-shaped light signal on an optical detector (see Fig. 10.8). Other spurious optical signals which are not modulated at the frequency of the chopper, as we... 

Sinusoidal excitation provides only one harmonic at the modulation frequency. In contrast, pulsed light provides a large number of harmonics of the excitation repetition frequency. The harmonic content, the number of harmonics and their amplitude, is determined by the pulse width and shape.(25) For example, a train of infinitely short pulses provides an infinite number of harmonics all with equal amplitude. A square wave provides only three modulation frequencies with sufficient amplitude to be usable. Equation (9.74) gives the harmonic content of a train of rectangular pulses R(t) of D duty cycle (pulse width divided by period) and RP peak value ... 

Note that the product /A yields the scan rate of the square-wave potential modulation. If the delay period is sufficiently long, the additional adsorption during the potential scan is negligible. Otherwise, the additional adsorption complicates the theoretically expected dependencies, in particular the relationships between the net peak currents and potentials on the frequency [114]. 

The electrochemical TNT sensor was mounted onto the UUV as a separate module, and it serially sent data values to the REMUS UUV. The electrode assembly was mounted on the cone nose of the vehicle and connected to the internal microanalyzer (Fig. 7). Major attention was given to the optimization of variables of the square-wave waveform (including the frequency, amplitude, and potential step) essential for attaining high speed... 

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