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Measuring MEMS in Motion by Laser Doppler Vibrometry
Published in Wolfgang Osten, Optical Inspection of Microsystems, 2019
Christian Rembe, Georg Siegmund, Heinrich Steger, Michael Wörtge
An example of a digital decoding scheme on an intermediate-frequency (IF) level is given in Figure 11.8. It utilizes the arctangent decoding method, but in combination with a numerical down-conversion process equivalent to Figure 11.7. Prior to quantization, the original heterodyne signal is mixed down to an IF level using analog RF circuitry. A numerical oscillator (NCO) generates two data streams representing the reference frequency in I&Q format, which are then multiplied with the input data coming from the ADC. The reference frequency is matched to the IF center frequency, thus providing offset-free down-conversion. Obviously, numerical down-conversion does not suffer from errors introduced by analog components. Thus, the I&Q signals are inherently ideal and do not require any further correction as indispensable when using the analog method. Low-pass finite impulse response (FIR) filters define the operating bandwidth and remove high-frequency components from the I&Q signals. The arctangent calculation, based on a fast algorithm, generates the primary phase data. Instead of phase unwrapping and outputting displacement data, velocity data v(tn) are calculated by numerical differentiation. This results in a significant saving of DSP power and processing time. The remaining capacity of the DSP chip is exploited to realize digital low-pass filtering of the velocity signal with selectable cutoff frequency. With an extended approach, φ(tn) raw phase data are unwrapped and scaled to provide an additional displacement output. Digital output data can be converted to voltage as shown, but can also be fed to subsequent processing blocks in an adequate digital format.
FPGA Realisation of n-QAM Digital Modulators
Published in IETE Technical Review, 2019
J. A. Galaviz-Aguilar, J. C. Nuñez-Perez, F. J. Perez-Pinal, E. Tlelo-Cuautle
Each NCO achieves a maximum resolvable frequency , and a frequency resolution , at DDS. For a clock at 125 MHz, the minimum output frequency resolution is 0.0291 Hz in NCO. The design of the signal generator allows obtaining up to four complex tones at the output; these signals are decomposed in real and imaginary form equivalent with the and in-phase and quadrature modulation components expressed as, where is the resulting modulated signal, is the RF carrier signal, whereas amplitude, and phase are both the shifts applied to sine and cosine waves.