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MC74HC4046ADTR2
Phase Locked Loop
-"--
A versatile device for 0.1 to 16MHz
frequency synchronization
Prepared by: Cleon Petty, Gary Tharalson & Marten Smith
Logic Application Engineers
AbstractThe MC74HC4046A (hereafter designated HC4046A)
phase–locked loop contains three phase comparators, a
voltage–controlled oscillator (VCO) and an output
amplifier. The user of this document should have a copy of
the HC4046A data sheet in ON Semiconductor Data Book
DL129 available for details of device operation and
operating specifications. The user should also be aware that
the following information is useful for approximating a
design but, because of process, layout and other variables,
there can be substantial deviation between theory and actual
results. Therefore,
prototypes be built and checked before committing a
design to production.Typical applications for the HC4046A usually involve a
configuration such as shown in Figure 1.
Figure 1. Typical Phase–Locked LoopRef Osc
Feedback
VCO/OUTPUT FREQUENCYThe output frequency, Fo, is calculated as a function of the
Ref Osc input and the ÷N feedback counter:
Fo = Ref Osc * N ( 1 )
The ability of the loop to emulate the above formula
makes it ideal for multiplying an input frequency by any
number up to the maximum of the VCO. The HC4046A
VCO frequency is controlled by the equation:
VCO freq = f(I * C) ( 2 )
where I is controlled by the external resistors R1 and R2 and
C by external capacitor Cext .
Frequency of oscillation is calculated by starting with the
familiar equation:
( 3 )I- cdV
and reworking it to obtain a formula that incorporates all the
detail to fit the HC4046A. First, the charge time of the device
for half–cycle time is obtained as follows:- dVor, Fo
where I and dV must be obtained for the HC4046A.
There are two components that comprise the I charge for
the HC4046A VCO, I1 and I2. I1 is the current that sets the
frequency associated with the VCO input and is a function
of R1, VCOin, and an internal current mirror that is ratioed
at 120/5 ≈ 24, resulting in the equation:I1- VCOin
