| 1.Sound Level Meter | a) a box-style case made of leather (occasionally aluminium), stretched over a hinged frame that opens into two compartments |
| 2.Acoustic Calibrator | b) a device used to prevent electrostatic discharge by safely grounding a person working on electronic equipment. It consists of a stretchy band of fabric with fine conductive fibers woven into it |
| 3.Hard Attache Case | c) two or more wires running side by side and bonded, twisted or braided together to form a single assembly |
| 4.Wrist Strap | d) a combination of one or more electrochemical cells, used to convert stored chemical energy into electrical energy |
| 5.Manual | e) measures sound pressure level and are commonly used in noise pollution studies for the quantification of almost any noise, but especially for industrial, environmental and aircraft noise |
| 6.Download Cable | f) a booklet that instructs on the usage of a particular machine |
| 7.Battery | g) an instrument that provides a reference noise source that is used to calibrate and check the performance of a Sound Level Meter |
Look at the scheme and fill in the gaps in the text with appropriate words from the box.
| fluctuations phase detector devices reference source quadrature phase difference voltage output |
Phase-noise measurements are an important part of many microwave designs, especially when developing oscillators, phase-locked loops (PLLs), prescalers, frequency converters, and frequency synthesizers.
The technique of using a PLL with a is very useful in research laboratories because it can be configured for numerous measurements and many types of . In this setup, the phase difference of the two input signals to the are converted to a voltage at the detector's . When the phase difference is set to quadrature or 90 deg., the output is 0 V. Phase fluctuations from quadrature result in voltage at the output of the detector. When is not maintained, an error can be introduced into the results based on the amount of the from quadrature.
Using the scheme from ex.13 speak about phase-noise measurements.
Read and translate text B.
TEXT B
The noise voltage over a band can be measured with either a spectrum analyzer or with a filter having a known noise bandwidth and a voltmeter. The noise can be referred to the input of the circuit under test by dividing by the total gain between its input and the measuring device. The measuring voltmeter should have a bandwidth that is at least 10 times the noise bandwidth of the filter. The voltmeter crest factor is the ratio of the peak input voltage to the full-scale rms meter reading at which the internal meter circuits overload. For a sine-wave signal, the minimum voltmeter crest factor is . For noise measurements, a higher crest factor is required. For Gaussian noise, a crest factor of 3 gives an error less than 1.5%. A crest factor of 4 gives an error less than 0.5%. To avoid overload on noise peaks caused by an inadequate crest factor, measurements should be made on the lower one-third to one-half of the voltmeter scale.
A true rms voltmeter is preferred over one that responds to the average rectified value of the input voltage but is calibrated to read rms. When the latter type of voltmeter is used to measure noise, the reading will be low. For Gaussian noise, the reading can be corrected by multiplying the measured voltage by 1.13. Noise voltages measured with a spectrum analyzer must also be corrected by the same factor if the spectrum analyzer responds to the average rectified value of the input voltage but is calibrated to read rms.
Noise measurements with a spectrum analyzer require a knowledge of the noise bandwidth of the instrument. For a conventional analyzer, the bandwidth is proportional to frequency. When white noise is analyzed, the display exhibits a slope of +10 dB per decade. However, the measured voltage level at any frequency divided by the square root of the noise bandwidth of the analyzer is a constant equal to the spot-noise value of the input voltage at that frequency. Band pass filters that have a bandwidth proportional to the center frequency are called constant-Q filters.
A second type of spectrum analyzer is called a signal analyzer. Such an instrument uses digital signal processing techniques to calculate the spectrum of the input signal as a discrete Fourier transform. The noise bandwidth of these instruments is a constant so that the display exhibits a slope of zero when white noise is the input signal.
Fairly accurate rms noise measurements can be made with an oscilloscope. A filter should be used to limit the noise bandwidth at its input. Although the procedure is subjective, the rms voltage can be estimated by dividing the observed peak-to-peak voltage by 6. One of the advantages of using the oscilloscope is that non-random noise that can affect the measurements can be identified, e.g., a 60-Hz hum signal.
Another oscilloscope method is to display the noise simultaneously on both inputs of a dual-channel oscilloscope that is set in the dual-sweep mode. The two channels must be identically calibrated and the sweep rate must be set low enough so that the displayed traces appear as bands. The vertical offset between the two bands is adjusted until the dark area between them just disappears. The rms noise voltage is then measured by grounding the two inputs and reading the vertical offset between the traces
Дата: 2016-10-02, просмотров: 182.
