XR-215A Datasheet, PDF (15/32 Pages) Exar Corporation – Monolithic Phase Locked Loop

XR-215A
Internal
Bias
I₁ I₂
T₁
T₂
1.3V
600
Á⁹
Range Select
10
> 3V, F₁
Input
R_X
0V, Fo
Fo = F1(1+(0.6/R_X))
Figure 12. Explanation of VCO Range-Select Controls
Range-Select (Pin 10)
The frequency range of the XR-215A can be extended by
connecting an external resistor, R_X, between pins 9 and
10. With reference to Figure 12, the operation of the
range-select terminal can be explained as follows: The
VCO frequency is proportional to the sum of currents I₁
and I₂ through transistors T₁ and T₂ on the monolithic
chip. These transistors are biased from a fixed internal
reference. The current I₁ is set internally, whereas I₂ is set
by the external resistor R_X. Thus, at any C₀ setting, the
VCO frequency can be expressed as:
ǒ Ǔ f₀ + f₁
1
)
0.6
R_X
The range select terminal can also be used for fine tuning
the VCO frequency, by varying the value of R_X. Similarly,
the VCO frequency can be changed in discrete steps by
switching in different values of R_X between pins 9 and 10.
Digital Programming
Using the range select control, the VCO frequency can be
stepped in a binary manner, by applying a logic signal to
pin 10, as shown in Figure 12. For high level logic inputs,
transistor T₂ is turned off, and R_X is effectively switched
out of the circuit. Using the digital programming capability,
the XR-215A can be time-multiplexed between two
separate input frequencies, as shown in Figure 19 and
Figure 20.
where f₁ is the frequency with pin 10 open circuited and
R_X is in kW. External resistor R_X (750W) is
recommended for operation at frequencies in excess of
5MHz.
Amplifier Input (Pin 1)
This pin provides the inverting input for the operational
amplifier section. Normally it is connected to pin 2 through
a 10 kW external resistor (see Figure 3 or Figure 4.)
Rev. 1.01
15