5

dc1795af

DEMO MANUAL DC1795A

Dc1795a reconfiguration

reFerenCe oPtions

Table 1 details the available reference options and board

modifications for each available option. The CLKSET files

described above assume the noise profile of the default

reference. If a different reference is used, change the

reference noise profile in ClockWizard before simulating

the LTC6950 under the Loop Design tab.

VCo inPut oPtions

Table 2 describes the available VCO input options. Board

modifications and power requirements for each option are

also provided in Table 2. There are two different oscillator

footprints on the board: U3, which accommodates the

popular0.5"×0.5"package,andU4,whichaccommodates

another common 14mm × 9mm package with four or six

pins. U4 also accommodates the smaller 5mm × 7.5mm

package. An external connectorized VCO can also drive the

LTC6950 through SMA connector J15. The CLKSET files

described above assume the noise profile for the default

VCO. If a different VCO is used, change the VCO noise

profile in ClockWizard before simulating the LTC6950

under the Loop Design tab.

CloCk FolloWer

WhenusingtheLTC6950asaclockfollowertheLTC6950’s

PLL circuitry can be powered down by setting the PDPLL

bit in ClockWizard. This will reduce the LTC6950’s overall

power consumption. Table 2 describes how to reconfigure

the board for a differential external input when using the

LTC6950 as a clock follower. Refer to the EZSync Function

section and to the data sheet for more details on using

the LTC6950 as a clock follower.

looP Filter Design AnD instAllAtion

Tables 3, 4 and 5 cross reference the loop filter options

showninClockWizard’sLoopDesigntabwiththeDC1795A

component reference designators. Table 3 is dedicated to

thedefaultoscillator,componentU4.Table4isdedicatedto

anothercommonoscillatorfootprint,componentU3.Table5

can be used with external oscillators. Use ClockWizard to

select, design and simulate different loop filters.

Some VCO tuning voltage ranges are greater than the

LTC6950chargepumpvoltagerange(refertotheLTC6950

data sheet). In such cases, an active loop filter using an

op amp can deliver the required tuning voltage. When

satisfied with the loop filter design, use Tables 3, 4 and 5

to help identify which components should be modified.

The loop filter components are located on the bottom side

of the DC1795A.

table 1. reference options and board Modifications

DeFAult

oPtion

reFerenCe

oPtion

instAll

DePoPulAte

ltC6950 PerForMAnCe

CoMMents

l

On-Board

NA

NA

Limited by on board reference at

frequency offsets <10kHz

U9 can accommodate any 5mm × 7.5mm or

9mm × 14mm oscillator package

External

C67

0402 0.1µF

R73, C71

Best performance when using an

ultralow noise external reference

For improved performance connect J4 to a very low

noise reference, such as the Wenzel 501-04517D