XR215A Datasheet, PDF (16/32 Pages) Exar Corporation – Monolithic Phase-Locked Loop

XR-215A
Amplifier Output (Pin 8)
This pin is used as the output terminal for FM or FSK
demodulation. The amplifier gain is determined by the
external feedback resistor, R_F, connected between pins 1
and 8. Frequency response characteristics of the
amplifier section are shown in Figure 11.
Amplifier Compensation (Pin 7)
The operational amplifier can be compensated for unity
gain by a single 300pF capacitor from pin 7 to ground.
(See Figure 11.)
BASIC PHASE-LOCKED LOOP OPERATION
applied to the control terminal of the VCO. If the input
frequency, fs, is sufficiently close to f_o, the feedback
nature of the PLL causes the VCO to synchronize or “lock”
with the incoming signal. Once in lock, the VCO frequency
is identical to the input signal, except for a finite phase
difference.
A Linearized Model for PLL
When the PLL is in lock, it can be approximated by the
linear feedback system shown in Figure 14. O_s and O_o are
the respective phase angles associated with the input
signal and the VCO output, F(s) is the low-pass filter
response in frequency domain, and K_d and K_o are the
conversion gains associated with the phase comparator
and VCO sections of the PLL.
Principle of Operation
The phase-locked loop (PLL) is a unique and versatile
circuit technique which provides frequency selective
tuning and filtering without the need for coils or inductors.
As shown in Figure 13, the PLL is a feedback system
comprised of three basic functional blocks: phase
comparator, low-pass filter and voltage-controlled
oscillator (VCO). The basic principle of operation of a PLL
can be briefly explained as follows: with no input signal
applied to the system, the error voltage V_d, is equal to
zero. The VCO operates at a set frequency, f_o, which is
known as the “free-running” frequency. If an input signal is
applied to the system, the phase comparator compares
the phase and frequency of the input signal with the VCO
frequency and generates an error voltage, V_e(t), that is
related to the phase and frequency difference between
the two signals. This error voltage is then filtered and
DEFINITION OF XR-215A PARAMETERS USED FOR
PLL APPLICATIONS DESIGN
VCO Free-Running Frequency, f_o
The VCO frequency with no input signal is determined by
selection of C₀ across pins 13 and 14 and can be
increased by connecting an external resistor R_X between
pins 9 and 10. It can be approximated as:
ǒ Ǔ f₀
[
220
C₀
1
)
0.6
R_X
where C₀ is in mF and R_X is in kW. (See Figure 8.)
Input
Signal
V_S(t)
f_s
Phase
V_e(t)
Comparator
V_O(t)
Lowpass
Filter
V_d(t)
VCO
f_o
V_d(t)
Figure 13. Block Diagram of a Phase-Locked Loop
Rev. 1.01
16