仪表与量测PPT课件-Chapter 5 Time and Frequency Measurement.ppt

chapter 5 time and frequency measurement,p. pataki,introduction accuaracy of frequency standards frequency sources frequency and period time measurement time-interval measurement phase measurement,introduction,frequency is by definition the number of occurences of a given periodic phenomenon in a given time. time has two meanings in practice: the position of an event on a conventional time axis. astronomy, space research, navigation, etc, a redio signal emitted from a very accurate frequency standard (the atomatic clock) synchronized to a well defined starting point (1958.1.1 00:00:00) the time elapsed between two events (time-length or time-interval) topics: time, frequency, and phase.,theoretical limit,frequency standards - accuracy,frequency accuracy of 10-13 can be taken as a theoretical limit in the earth-fixed coordinate system. the relativistic effects and the change of the gravitational potential cause a time shift in this range. in space, it is 10-10 (reinhard 1974, ramsey 1987, .),frequency standards - accuracy,in early 60, different mathematical descriptions were usedto characterize the accuracy of high-stability. 1964, nasa and ieee has a conference on the standardization - (1) fractional frequency offset (2) phase-time,frequency offset:,instantaneous frequency:,voltage of a sine wave:,fractional frequency offset:,phase-time:,y(t) and x(t) are unchanged by frequency multiplication and division if the devices are noiseless. they are usually for direct comparisons of the stability of sources with very different nominal frequencies (barnes 1971),frequency sources - atomic frequency standard,the most precise way to produce a stable frequency generator is the atomic clock. it is based on the principle of quantum mechanics where frequenc is determined by an atomic resonator. the frequency of the microwave radiation is proportional to the energy difference between the discrete p and q energy level. h=6.63x10-34 j is plancks constant. the atomic clock elements are used from the first column of mendelejevs periodical system (alkali metals) the material have only one valence electron on their outer energy level. the rediation frequency depends on the energy difference between two outer levels. (hayatt 1973; kartaschoff 1978, ramsey 1991).,frequency sources - quartz crystal oscillator,the most generally used frequency standard of practical measurement techniques. the piezoelectric effect of the quatz crystal makes it easy to excite mechanical vibrations by means of an oscillating electric field. the resonant frequency of the crystal oscillator is very stable kartaschoff 1978.,frequency sources - frequncy synthesizer,to produce a quatz-accurate but changeable frequency signal. the frequency of the crystal oscillator is transformed into another frequency signal. direct frequency conversion is done with a freuquency multiplier and a divider. frequency multipliers or harmonic generators are based on nonlinear components for the harmonics. frequency dividers are counters. frequency synthesizer performs four basic arithmetic operations (+ - x /) on standard frequencies.,frequency sources - frequncy synthesizer,the synthesizers are based on phase-locked loops (pll). the basic elements of a pll are a phase detectro, a loop filter, and a voltage controlled oscillator (vco). plls are to generate a clean waveform a appropriate shape that is locked-in to the average input frequency. (kroupa 1973),lf,vco,lpf,frequency summing phase-locked loop,frequency sources - frequncy synthesizer,the feed-back arm divides the output frequency of the voltage controlled oscillator (vco). in steady state, the divided frequency equals fin.,frequency multiplier phase-locked loop,lf,vco,freq divider,phase detector,frequency sources - frequncy synthesizer,a frequency synthesizer is composed of decadic frequency dividers, filters, frequency multipliers, and addition loops. the circuit can produce an arbitrary frqeuency from a constant frequency signal of the quatz oscillator. the accuracy and stability are high (kroupa 1973; mcnamee 1989).,frequency and period time measurements,two major frequency measurement principles. compare to the known frequency. resonant method which are determined by frequency dependent elements.,frequency measure - counter,1,2,9,5,8,4,3,7,6,0,frequency measure - counter - frequency measure,the unknown frequency is connected to input1. the known frequency signal is connected to input2. depending on tm(1s, 1ms, 1ms), the displayed number can be scaled in frequency (hz, khz, mhz). the uncertainty is limited by two factors: +/- 1 count ambiguity, and accuracy of the standard frequency. frequency range: 50-100mhz.,frequency measure - counter - period measure,unknown frequency is connected to input2. standard frequency is connected to input1. it is the inverse of frequency measurement. the relative error of the unknown frequency is: consider the snr of the triggering error. n- measuring periods, up- peak signal/noise. if n=1, snr=40db, error = 0.3%. the total relative error:,frequency measure - counter - direct freq. meas mode,n=1,n=10,n=100,n=1000,tm=0.01s,tm=0.1s,tm=1s,tm=10s,tm=100s,frequency measure - counter - direct freq. meas mode,t,correct,incorrect,incorrect,incorrect,incorrect period length,frequency measure - counter - freq. range extension,frequency meter with counter,slave-dividers,pre-dividers,use frequency division frequency range : up to 1.5ghz.,frequency measure - counter - freq. range extension,use frequency converter frequency range : 10-15ghz,if amplifier 1 53hz,tunable bp filter,harmonic generator,v,frequency meter with counter,dial,mixer,indicator,frequency measure - counter - freq. range extension,use transfer oscillator. frequency range : 20-25ghz. oscilator is tuned until the frequency of one of the upper harmonics will be equal to the measurement frequency. the counter measures the fundamental harmonic of the oscillator signal.,error: 10-7 to 10-10,frequency measure - counter - freq. range extension,harmonic heterodyne converter. frequency range : up to 40ghz. frequency synthesizer increases the frequency until the if fd1 appears. need 2nd step to determine the upper or lower bands (df).,harmonic generator,frequency synthesizer,if amplifier,if detector,controller,frequency meter with counter,mixer,frequency measure - resonators,the unknown frequency is determined by the resonance of a resonator. the frequency is calculated from parameters of resonators.,type,measure range,accuracy,freq. span,rc bridges/ rc selective,cavity resonators,rc bridges/ rc selective,frequency measure - frequency discriminators,t,t,discriminator,low-pass filter and amplifier,frequency,pulse train,voltage,(a frequency discriminator can be used as a fm demodulator),frequency measure - frequency comparison,lissajous method,h,v,screen,t,t,frequency measure - frequency comparison,synchronization mehod two signals are connected to the vertical and trigger inputs. if the figure stops, the ratio of the two frequency is a rational number. if the figure moves to right, the frequency of the vertical input is higher than the frequency of the trigger input signal. if the figure moves to left, the frequency of the vertical input signal is slightly smaller.,frequency measure - accuracy,+ + + + +,tm=1000,tm=100,tm=10,tm=1,direct freq. meas,ratemeter. rc circuit,lc circuit,xxxxxxxxxxxxxxxxxx,cavity resonators,harm, heterodyne converters,transfer oscillators (man.),freq. dividers,transfer oscillators (pll),hz,khz,mhz,ghz,time-interval measurement - basic architecture,input circuit a,input circuit b,flip-flop,time base,counter,1 3 4 7 8 5 6 2 9 0,n:,display,stop,start,start,stop,t,t,t,t,measuring the average time-interval, the uncertainty due to noise will decrease by a factor of 1/n0.5. ex. for f=100mhz, the uncertainty is 10ns. if n=106, the uncertainty will be 10ps. this holds only if the measurement processes are independent.,time-interval measurement - average,time-interval measurement - interpolation,0 1 2 3 4,0 1 2 3,start,stop,time base,sync. start,sync. stop,gate,n=(7),n1=(4) slave,n2=(3) slave,t,t,t,t,t,t,t,t,t,coincidence 1,coincidence 2,phase measurement,phase and time difference between the zero crossing of two sinusoidal signal.,in