Freescale Semiconductor, Inc.
24 kΩ
+5V
1 MHz
Y1
20 pF
1 MΩ
MC145170
20 pF
100 pF
8
OUTPUT
DATA OUT
CLK
EN
DATA IN
9
1 kΩ
TEST POINT
(LOCK DETECT)
4 x 5.6 kΩ
2 x 1500 pF
R1/2
CC
R1/2
+ 12 V
–
LF351
+
24 kΩ
4700 pF
4700 pF
ARCHIVED BY FREESCALE SEMICONDUCTOR, INC. 2005
1
16
R1/2
CC
R1/2
Freescale Semiconductor, Inc...
ARCHIVED BY FREESCALE SEMICONDUCTOR, INC. 2005
+5V
+5V
20 nH
1
14
2 x MV2115
R14
10 kΩ
MC1648
1000 pF
7
8
0.1
µF
390 pF
C5
Figure 5. VHF Synthesizer
midpoints to ground to further filter the reference sidebands.
The value of CC is chosen so that the corner frequency of this
added network does not significantly affect the original loop
bandwidth
ω
B.
The rule of thumb for an initial value is CC = 4 / ( R1
ω
RC),
where
ω
RC is the filter cutoff frequency. A good value is to
choose
ω
RC to be 10 x
ω
B, so as to not significantly impact the
original filter.
ω
B =
ω
n
1 + 2ζ2 + 2 + 4ζ2 + 4ζ4
(8)
THE VARACTOR
The MV2115 was selected for its tuning ratio of 2.6 to 1.
The capacitance can be changed from 49.1 pF to 127.7 pF
over a reverse bias swing of 2 to 30 volts. Contact your Moto-
rola representative for information regarding the MV2115 va-
ractor diode.
For example, three parameters are considered.
CT = Nominal capacitance
CR = Capacitance ratio
fR = Frequency ratio
CR=
Cvmin
=
Cvmax
Vmax
Vmin
ρ
(12)
nated by R14 with C5. The capacitance of the varactors (in
series with the rest of the circuit) is much smaller than C5 and
can therefore be neglected for this calculation.
As above, let
ω
RC = 258,600 rad/s be the cutoff of this filter.
R14 is chosen to be 10 kΩ. Therefore,
C5 =
1
1
=
ω
RCR14 (258,600)(10 kΩ)
(11)
= 387 pF
≈
390 pF
= 12,566
1+(2)(0.707)2+ 2+(4)(0.707)2+ (4)(0.707)4
= 25,860 rad/s
ω
RC = 10
ω
B = (10)(25,860) = 258,600 rad/s
CC =
4
4
=
R1
ω
RC (11.23 kΩ)(258,600 rad/s)
(9)
(10)
= 1377 pF
≈
1500 pF
There is also a filter formed at the input to the VCO. Again,
this should be selected to ensure that it does not significantly
affect the loop bandwidth. For this example, the filter is domi-
where
ρ
= the capacitance exponent
AN1207
4
For More Information On This Product,
Go to: www.freescale.com
MOTOROLA
FREESCALE [ FREESCALE SEMICONDUCTOR, INC ]
