National Semiconductor
Application Note 2028
Gerrit Segers and Jaimy de Koning
July 21, 2010
Evaluation Board for the
LMH2191
General Description
Detailed Operating Description
The evaluation board is designed to help the evaluation of the
LMH2191 Dual Channel 52 MHz Clock Tree Driver. The
LMH2191 is ideal suitable for the distribution of a system clock
to peripheral devices in mobile handsets. It provides a solu-
tion to clocking issues such as limited drive capability, pro-
tection of the master clock from varying loads and frequency
pulling effects, isolation from noisy modules, and crosstalk
isolation. It has very low phase noise which enables it to drive
sensitive modules such as Wireless LAN and Bluetooth.
The LMH2191 evaluation board is designed to have maxi-
mum flexibility in evaluating the LMH2191 in various config-
urations. The schematic, Bill of material and board layout can
be found at the end of this document. In the following sections
a description will be given on how the circuit works and how
to setup the measurement bench. For the factory default
jumper setting, please refer to the Configuration section.
POWER SUPPLY
The board is powered by the use of banana connections,
which is mostly a preferable connector type rather than a multi
pole connector. The DUT (LMH2191) uses a single supply
called VBAT while the output buffer, used for monitoring the
CLK outputs and the TCXO voltage, is powered by a dual
supply (called V+ & V-). The supply for the DUT ranges from
2.5V to 5.5V while the supply for the output buffer (LMH6559)
has a fixed voltage of +5V and –5V. The output voltage of the
LDO is 1.8V and can be monitored on an extra banana con-
The setup of the evaluation board consist of three distin-
guished segments.
The first part is the TCXO oscillator that generates the clock
signal that is fed to the clock buffer.
The second part is the circuit around the LMH2191 itself. The
DUT needs a stable supply to be able to operate adequately.
Furthermore there is the possibility to switch each CLK output
ON and OFF separately using a CLK request jumper setting
or an external connector. Each clock output has several dif-
ferent capacitive load settings which are selectable by using
jumper shorts. Each clock output is also fed to an SMA con-
nector.
nector called VOUT
.
TCXO SUPPLY
The supply for this on-board oscillator is done by the
LMH2191 via the VOUT pin. The VOUT voltage is decoupled by
C13 and C14 and is connected to jumper J8 pin 2. For normal
operation pin 1 and pin 2 are shorten and the LDO voltage is
connected to the supply pin of the TCXO. If, for any reason,
an external supply is needed, this supply voltage can be con-
nected to J8 at the pins 3 and 4. Pin 3 is the supply of the
TCXO and pin 4 is the ground connection. (NOTE: The jumper
short for the pins 1 and 2 must be removed then).
The third part is the output buffer, that makes it possible to
monitor both clock outputs or the TCXO voltage one by one
by jumper selection. The output buffer is capable to drive a
50 ohm cable in order to connect the test board to measuring
equipment.
CLOCK BUFFER LMH2191
The oscillator frequency of the TCXO or any external input
frequency is fed to the SCLK_IN pin of the LMH2191 (DUT).
The input of the DUT is AC coupled because of an internal
coupling capacitor of around 30 pF. This means that any DC
voltage added to the input frequency doesn't disturb the DUT
from functioning.
The DUT has two clock outputs which can be switched ON or
OFF independently. When both outputs are switched OFF the
part is in shutdown mode and that means that the LDO is
switched OFF too. In case the TCXO is powered by the on
board LDO of the DUT, the input frequency is switched OFF
too. The current consumption drops down to zero. ON and
OFF switching of the outputs is performed by both CLK_REQ
inputs. Any of the clock outputs is OFF when the matching
request input is low, thus connected to ground or a voltage
below 400 mV.
The testboard provides two jumpers J5 and J6 and the SMA
edge launch connectors CON9 and CON10 connected to the
CLK_REQ inputs, to switch the clock outputs ON or OFF.
Each clock request input has an internal pull down resistor of
200kΩ which means that when no jumper is placed the clock
output is OFF. To force a clock output to ON the voltage on
the request pin must raise above 1.4V. The voltage used to
toggle the outputs ON or OFF is VBAT or an external voltage
applied to the connectors CON9 and CON10. If an external
voltage is used, the jumpers for J5 and J6 must be removed.
30113705
FIGURE 1. LMH2191 Evaluation Board
© 2010 National Semiconductor Corporation
301137
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