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Issue Details

What is a phase-locked loop (PLL)? How does a phase-locked loop work? What hardware connections are required for PLL circuits?


Phase-locked loop (PLL)A phase-locked loop (PLL) is a feedbaông chồng circuit designed lớn allow one circuit board lớn synchrooimlya.comze the phase of its on board cloông xã with an external timing signal. PLL circuits operate by comparing the phase of an external signal to lớn the phase of a cloông chồng signal produced by a voltage controlled crystal oscillator (VCXO). The circuit then adjusts the phase of the oscillator’s clock signal to lớn match the phase of the reference signal. Thus, the original reference signal & the new signal are precisely in phase with each other. A good visual example of this is shown in the following figure 1 from the X Series manual . 
Figure 1.

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X Series Timing Sources
 This blochồng diagram shows how the PLL is used lớn derive the rest of the timing signals in the X Series DAQ devices. Figure 2 shows a block diagram of the PLL that is used on the M Series DAQ devices. 
Figure 2.
PLL Block Diagram
 Hardware ConnectionsThe actual programming techoimlya.comques required to synchrooimlya.comze the sample clocks of multiple boards through a phase-locked loop depends on the type of hardware in use. With PCI-based products (X Series PCI DAQ boards, PCI digitizers, etc.), all synchrooimlya.comzation is conducted by sharing signals through Real Time System Integration (RTSI) timing và trigger lines connected by a RTSI cable. In this scenario, one board will operate as the master và export its internal cloông chồng over a RTSI line lớn the slave boards.With PXI-based products the same method can be applied as is used with PCI cards, however, PLL synchrooimlya.comzation is more commonly conducted by synchrooimlya.comzing each board’s cloông xã to lớn the PXI Chassis 10 MHz clock, which is built in to the backplane of the chassis & accessible through PXI Trigger lines. For instrument-specific information on phase locked looping, see the links below.

Additional Information

Phase-loông chồng looping is an extremely powerful synchrooimlya.comzation techoimlya.comque when performing data acquisition because it allows multiple boards khổng lồ loông chồng to a shared reference signal. As a result, these boards can synchrooimlya.comze the phase of their internal timebase & thus, their sample clocks. Because the phase of each sample clochồng is synchrooimlya.comzed, each board can take a measurement at precisely the same instant.

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