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AD9901 Folha de dados(PDF) 6 Page - Analog Devices |
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AD9901 Folha de dados(HTML) 6 Page - Analog Devices |
6 / 8 page AD9901 REV. B –6– THEORY OF OPERATION A phase detector is one of three basic components of a phase- locked loop (PLL); the other two are a filter and a tunable oscil- lator. A basic PLL control system is shown in Figure 3. LOW- PASS FILTER 1/N VCO AD9901 REFERENCE INPUT OSCILLATOR OUTPUT OPTIONAL 1/N PRESCALER TYPICAL OF DIGITAL PLLs Figure 3. Phase-Locked Loop Control System The function of the phase detector is to generate an error signal that is used to retune the oscillator frequency whenever its out- put deviates from a reference input signal. The two most com- mon methods of implementing phase detectors are (1) an analog mixer and (2) a family of sequential logic circuits known as digital phase detectors. The AD9901 is a digital phase detector. As illustrated in the block diagram of the unit, straightforward sequential logic de- sign is used. The main components include four “D” flip-flops, an exclusive-OR gate (XOR) and some combinational output logic. The circuit operates in two distinct modes: as a linear phase detector and as a frequency discriminator. When the reference and oscillator are very close in frequency, only the phase detection circuit is active. If the two inputs are substantially different in frequency, the frequency discrimina- tion circuit overrides the phase detector portion to drive the oscillator frequency toward the reference frequency and put it within range of the phase detector. Input signals to the AD9901 are pulse trains, and its output duty cycle is proportional to the phase difference of the oscilla- tor and reference inputs. Figures 4, 5 and 6 illustrate, respec- tively, the input/output relationships at lock; with the DC MEAN VALUE REFERENCE INPUT OSCILLATOR INPUT REFERENCE FLIP-FLOP OUTPUT OSCILLATOR FLIP-FLOP OUTPUT XORGATE OUTPUT Figure 4. AD9901 Timing Waveforms at “Lock” DC MEAN VALUE REFERENCE INPUT OSCILLATOR INPUT REFERENCE FLIP-FLOP OUTPUT OSCILLATOR FLIP-FLOP OUTPUT XORGATE OUTPUT Figure 5. Timing Waveforms ( φ OUT Leads φIN) DC MEAN VALUE REFERENCE INPUT OSCILLATOR INPUT REFERENCE FLIP-FLOP OUTPUT OSCILLATOR FLIP-FLOP OUTPUT XORGATE OUTPUT Figure 6. Timing Waveforms ( φ OUT Lags φIN) oscillator leading the reference frequency; and with the oscillator lagging. This output pulse train is low-pass filtered to extract the dc mean value [Kφ (φI – φO)] where Kφ is a proportionality con- stant (phase gain). At or near lock (Figures 4, 5 and 6), only the two input flip- flops and the exclusive-OR gate (the phase detection circuit) are active. The input flip-flops divide both the reference and oscilla- tor frequencies by a factor of two. This insures that inputs to the exclusive-OR are square waves, regardless of the input duty cycles of the frequencies being compared. This division-by-two also moves the nonlinear detection range to the ends of the range rather than near lock, which is the case with conventional digital phase detectors. Figure 7 illustrates the constant gain near lock. PHASE DIFFERENCE AT INPUTS 2 1 0 –2 0 – FO = 50MHz FO = 70MHz FO = 200MHz TYPICAL PHASE DETECTOR GAIN IS 0.2865V/RAD VOUT = 1.8V Figure 7. Phase Gain Plot When the two square waves are combined by the XOR, the output has a 50% duty cycle if the reference and oscillator in- puts are exactly 180 ° out of phase; under these conditions, the AD9901 is operating in a locked mode. Any shift in the phase relationship between these input signals causes a change in the output duty cycle. Near lock, the frequency discriminator flip- flops provide constant HIGH levels to gate the XOR output to the final output. The duty cycle of the AD9901 is a direct measure of the phase difference between the two input signals when the unit is near lock. The transfer function can be stated as [Kφ(φI – φO](V/RAD), where Kφ is the allowable output voltage range of the AD9901 divided by 2 π. For a typical output swing of 1.8 V, the transfer function can be stated as (1.8 V/2 π = 0.285 V/RAD). Figure 7 shows the rela- tionship of the dc mean value of the AD9901 output as a func- tion of the phase difference of the two inputs. |
Nº de peça semelhante - AD9901 |
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Descrição semelhante - AD9901 |
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