xr215a Exar Corporation, xr215a Datasheet - Page 15

• Manufacturer Part Number: xr215a
• Description: Monolithic Phase-locked Loop
• Manufacturer: Exar Corporation

Range-Select (Pin 10)
The frequency range of the XR-215A can be extended by
connecting an external resistor, R
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 I
and I
chip. These transistors are biased from a fixed internal
reference. The current I
by the external resistor R
VCO frequency can be expressed as:
where f
R
recommended for operation at frequencies in excess of
5MHz.
X
Rev. 1.01
is in k . External resistor R
2
through transistors T
1
is the frequency with pin 10 open circuited and
f
0
Internal
Bias
1
f
1
is set internally, whereas I
X
. Thus, at any C
1
1
1.3V
Figure 12. Explanation of VCO Range-Select Controls
and T
T
1
0.6
R
X
X
, between pins 9 and
2
9
600
on the monolithic
I
1
X
( 750 ) is
0
I
2
T
setting, the
2
2
is set
10
1
15
Range Select
The range select terminal can also be used for fine tuning
the VCO frequency, by varying the value of R
the VCO frequency can be changed in discrete steps by
switching in different values of R
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 T
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.
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 k
R
X
Input
Fo = F1(1+(0.6/R
external resistor (see Figure 3 or Figure 4. )
2
is turned off, and R
0V, Fo
> 3V, F
X
))
1
X
X
XR-215A
between pins 9 and 10.
is effectively switched
X
. Similarly,