Texas Instruments CDC7005, User Manual, Page: 17 / 37

Hardware Configuration
3-5 EVM Hardware
3.3.4 Loop Filter (J29 − J35)
The loop filter is one of the key elements determining the loop bandwidth of
the PLL. The loop filter converts the charge pump current into the control
voltage for the voltage controlled oscillator. The phase difference between the
input clocks of the phase frequency detector determines the width of the
charge pump output current pulses. These high frequency pulses are
transformed into a voltage to control the oscillator.
Basically, three types of loop filters are implemented on the EVM.
-
Passive loop filter
-
Active loop filter using an operational amplifier (OPA) implemented on the
CDC7005.
-
Active loop filter using an external low-noise OPA.
Filter types can be selected by soldering the bridges J29 − J30 and J32 − J34,
see Table 3 − 2. Control voltage of the VCXO can be measured at J35. If an
external OPA is used, it needs to be switched on by connecting J31. For
example, passive filter operation is provided when pads 1 and 3 of J29 are
solder bridged and pads 1 and 3 of J34 are solder bridged.
Default setting: Passive Loop Filter
Note:
Using an active loop filter requires changing of the charge pump direction via
SPI programming.
Table 3 − 2.Filter Configurations
Bridge Passive Filter Active With An Internal OPA Active With An External OPA
J29 1 − 3 1 − 2 1 − 2
J30 Don’t care 1 − 3 1 − 2
J31 Open Open Closed
J32 Don’t care 1 − 2 1 − 3
J33 Don’t care 1 − 2 1 − 3
J34 1 − 3 1 − 2 1 − 2
3.3.5 High-Speed Outputs and Inputs (J6 − J15 and J19 − J20)
The CDC7005 drives five differential LVPECL outputs. All PECL outputs are
ac-coupled and terminated with 150
Ω to GND. This is in contrast to typical
LVPECL termination, which requires V
CC
− 2 V as termination voltage. The
reason is to simplify the power supply scheme. The device output’s trace
impedance is 50
Ω and traces are matched in length. All outputs have options
for pull-up and pull-down resistors.