Datenblatt-Suchmaschine für elektronische Bauteile |
|
74VHC4046 Datenblatt(PDF) 10 Page - Fairchild Semiconductor |
|
74VHC4046 Datenblatt(HTML) 10 Page - Fairchild Semiconductor |
10 / 16 page www.fairchildsemi.com 10 Detailed Circuit Description (Continued) FIGURE 2. Logic Diagram for Phase Comparator I and the Common Input Circuit for All Three Comparators FIGURE 3. Typical Phase Comparator I. Waveforms Thus in normal operation VCC and ground voltage levels are fed to the loop filter. This differs from some phase detectors which supply a current output to the loop filter and this should be considered in the design. (The CD4046 also provides a voltage.) Figure 4 shows the state tables for all three comparators. PHASE COMPARATOR I This comparator is a simple XOR gate similar to the 74HC86, and its operation is similar to an overdriven bal- anced modulator. To maximize lock range the input fre- quencies must have a 50% duty cycle. Typical input and output waveforms are shown in Figure 3. The output of the phase detector feeds the loop filter which averages the out- put voltage. The frequency range upon which the PLL will lock onto if initially out of lock is defined as the capture range. The capture range for phase detector I is dependent on the loop filter employed. The capture range can be as large as the lock range which is equal to the VCO fre- quency range. To see how the detector operates refer to Figure 3. When two square wave inputs are applied to this comparator, an output waveform whose duty cycle is dependent on the phase difference between the two signals results. As the phase difference increases the output duty cycle increases and the voltage after the loop filter increases. Thus in order to achieve lock, when the PLL input frequency increases the VCO input voltage must increase and the phase differ- ence between comparator in and signal in will increase. At an input frequency equal fMIN, the VCO input is at 0V and this requires the phase detector output to be ground hence the two input signals must be in phase. When the input fre- quency is fMAX then the VCO input must be VCC and the phase detector inputs must be 180 ° out of phase. The XOR is more susceptible to locking onto harmonics of the signal input than the digital phase detector II. This can be seen by noticing that a signal 2 times the VCO fre- quency results in the same output duty cycle as a signal equal the VCO frequency. The difference is that the output frequency of the 2f example is twice that of the other exam- ple. The loop filter and the VCO range should be designed to prevent locking on to harmonics. PHASE COMPARATOR II This detector is a digital memory network. It consists of four flip-flops and some gating logic, a three state output and a phase pulse output as shown in Figure 5. This comparator acts only on the positive edges of the input signals and is thus independent of signal duty cycle. Phase comparator II operates in such a way as to force the PLL into lock with 0 phase difference between the VCO output and the signal input positive waveform edges. Fig- ure 6 shows some typical loop waveforms. First assume that the signal input phase is leading the comparator input. This means that the VCO’s frequency must be increased to bring its leading edge into proper phase alignment. Thus the phase detector II output is set HIGH. This will cause the loop filter to charge up the VCO input increasing the VCO frequency. Once the leading edge of the comparator input is detected the output goes 3-STATE holding the VCO input at the loop filter voltage. If the VCO still lags the sig- nal then the phase detector will again charge up to VCO input for the time between the leading edges of both wave- forms. |
Ähnliche Teilenummer - 74VHC4046_03 |
|
Ähnliche Beschreibung - 74VHC4046_03 |
|
|
Link URL |
Privatsphäre und Datenschutz |
ALLDATASHEETDE.COM |
War ALLDATASHEET hilfreich? [ DONATE ] |
Über Alldatasheet | Werbung | Kontakt | Privatsphäre und Datenschutz | Linktausch | Hersteller All Rights Reserved©Alldatasheet.com |
Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
Family Site : ic2ic.com |
icmetro.com |