5
dc1649afa
DEMO MANUAL DC1649A
DC1649A RECONFIGURATION
DC1649A is flexible and allows the use of a variety of VCOs
and the choice to employ either an active or passive loop
filter. The following covers the hardware reconfiguration of
DC1649A. Refer to PLLWizard’s Help and the LTC6945 data
sheet to better understand how to change programmed
parameters on the DC1649A.
Installing Different Tunable Devices
DC1649A permits the use of different tunable oscillators,
like VCOs and VCXOs. There are two different oscillator
footprints on the board: U3, which accommodates the
popular 0.5in
×
0.5in package, and U4, which accom-
modates another common 14mm
×
9mm package with
four or six pins. An external connectorized VCO can also
drive the LTC6945 through J5. Table 2 gives options to
customize the DC1649’s VCOs. When using an active loop
filter, an additional pole is required in the loop filter to limit
the op amp’s noise contribution to the VCO phase noise.
Selecting Loop Filter Type
DC1649A can use either an active or a passive loop filter,
depending upon the application. Some VCO tuning volt-
age ranges are greater than the LTC6945 charge pump
voltage range (refer to the LTC6945 data sheet). In such
cases, an active loop filter using an op amp can deliver
the required tuning voltage. Table 3 summarizes these op-
tions. PLLWizard helps determine the component values
listed in this table.
Table 2. Oscillator Options
TUNABLE
DEVICE
SELECTION
COMPONENTS
RESISTOR FOR
ADDITIONAL POLE
CAPACITOR FOR
ADDITIONAL POLE
POWER
ENABLE PIN OR
REMARKS
U3
C26
≅
100pF, depopulate
R17, C25 and C28
R10 = 0Ω for passive
loop filter
R10
≅
100Ω for active
loop filter
C27 = Open for passive
loop filter
Pick C27 such that
1/(2π • R10 • C27)
≅
15
•
the loop bandwidth in Hz
for active loop filter
Determined by U3
device power supply
specification, up to 24V
applied to “V+VCO” turret
U4
C28
≅
100pF, depopulate
R10, C25 and C26
R17 = 0Ω for passive
loop filter
R17
≅
100Ω for active
loop filter
C32 = Open for passive
loop filter
Pick C32 such that
1/(2π • R17 • C32)
≅
15
•
the loop bandwidth in Hz
for active loop filter
Determined by U4
device power supply
specification, up to 24V
applied to “V+VCXO”
turret
Use “EN” turret to control
pin 2 of U4
External
C25
≅
100pF, R15 = 0Ω,
depopulate R13, C26
and C28
Selection components
are for an active loop
filter. Connect the external
VCO output to J5. Use
“CPGAIN” to tune the
external device
Table 3. Loop Filter Options
LOOP FILTER
TYPE
SELECTION
RESISTORS
RZ FROM
PLLWizard
CI FROM
PLLWizard
CP FROM
PLLWizard
POWER
CPINV BOX, SYSTEM
TAB IN PLLWizard
Passive
R14 and R16 = 0Ω,
depopulate R13 and
R15
RZ_P
CI1_P in parallel
with CI2_P
CP_P
None
Unchecked
Active
R13 (except when
using an external
VCO) and R15 = 0Ω,
depopulate R14 and
R16
RZ_A
CI1_A in parallel
with CI2_A
CP_A
Determined by the
maximum allowed
tune voltage of the
populated VCO (U3
or U4), up to 24V
applied to “V+OA”
turret
Checked
Downloaded from
Downloaded from
Downloaded from
Downloaded from
Downloaded from
