XR-2212_06 Datasheet, PDF (10/20 Pages) Exar Corporation – Precision Phase-Locked Loop

XR-2212
Design Equations
(See Figure 3 and Figure 10 for definition of
components.)
1. VCO Center Frequency, f₀: f₀ = 1/R₀C₀ Hz
2. Internal Reference Voltage, V_REF (measured at
Pin 11)
V_REF = V_CC/2 - 650mV
3. Loop Low-Pass Filter Time Constant, t : t = R₁C₁
4. Loop Damping, j:
Ǹ j + 0.25
NC₀
C₁
where N is the external frequency divider modular
(See 2). If no divider is used, N = 1.
5. Loop Tracking Bandwidth, $Df/f₀: Df/f₀ = R₀/R₁
6. Phase Detector Conversion Gain, K_O: (K_O is the
differential DC voltage across Pins 10 and 11, per unit
of phase error at phase-detector input)
K_O = -2V_REF/p volts/radian
7. VCO Conversion Gain, K₀: (K₀ is the amount of
change in VCO frequency, per unit of DC voltage
change at Pin 10. It is the reciprocal of the slope of
conversion characteristics shown in Figure 10).
K₀ = -1/V_REFC₀R₁ Hz/V
8. Total Loop Gain, K_T
K_T = 2pK_O K₀ = 4/C₀R₁ rad/sec/volt
9. Peak Phase-Detector Current, I_A; available at Pin 10.
I_A = V_REF (volts)/25mA
APPLICATION INFORMATION
FM Demodulation
XR-2212 can be used as a linear FM demodulator for both
narrow-band and wide-band FM signals. The generalized
circuit connection for this application is shown in
Figure 11, where the VCO output (Pin 5) is directly
connected to the phase detector input (Pin 16). The
demodulated signal is obtained at phase detector output
(Pin 10). In the circuit connection of Figure 10, the op amp
section of XR-2212 is used as a buffer amplifier to provide
both additional voltage amplification as well as current
drive capability. Thus, the demodulated output signal
available at the op amp output (Pin 8) is fully buffered from
the rest of the circuit.
In the circuit of Figure 11, R₀C₀ set the VCO center
frequency, R₁ sets the tracking bandwidth, C₁ sets the
low-pass filter time constant. Op amp feedback resistors
R_F and R_C set the voltage gain of the amplifier section.
Rev. 2.10
10