SP16160CH1RB Reference Design Board User’s Guide
- 18 -
www.national.com
•
Program the LMK04031B with the PIC Loader
board set with Switch 1 = ON, Switch 2 = OFF
Perform the following changes to modify the board for
a
differential CMOS clock
:
•
Remove R171, C174, C176 and C177
•
Populate R152, R153, R159 and R160 with 0
ohm, 0402 resistors
•
Populate C172 and C173 with 0.1uF, 0402
capacitors
•
This
mode
requires
programming
the
LMK04031B via the CodeLoader software
Perform the following changes to modify the board for
an
External signal generator clock source
:
•
Remove R171, C174, C176 and C177
•
Populate R151 and R158 with 0 ohm, 0402
resistors
•
Populate T4 with a MA/COM MABA007159 or
equivalent transformer
•
Populate C172 and C173 with 20pF, 0402
capacitors
•
Populate C174 with a 5 pF, 0402 capacitor
•
Populate SMA_CLK with a SMA connector
•
Provide a low-jitter, +18 dBm sinusoidal signal
through a bandpass filter.
9.3 Varactor Controlled Crystal Oscillator
Very low-jitter clock performance is in-part achieved
by using low noise oscillators for the frequency
reference and VCXO. The reference board comes
standard with Crystek CCHD-950 and CVHD-950
model oscillator modules that achieve low noise
performance with -162 dBc/Hz broadband phase
noise. Low phase noise comes with an associated
high cost.
A lower cost solution is achieved for this reference
board design with a varactor controlled crystal at the
expense of lower noise performance at high input
signal frequencies due to jitter. This circuit can be
installed on the back side of the board to replace the
VCXO module.
Please contact National Semiconductor for more
information on configuring the reference board if this
option is desired.
9.4 Bypassed Switching Regulators
LM2734 Switching regulators are utilized in this
reference board design to minimize LDO regulator
losses for the high current ADC supplies.
Noise on the supplies, especially the sensitive 1.8 V
analog supply to the ADC, can cause a performance
degradation in the form of small spurs offset ~500
kHz from a large input tone as shown in Figure 21.
These spurs may be insignificant in many
applications but can be reduced further if needed by
reducing the supply ripple at the output of the
switching regulators or eliminated completely by
bypassing the switching regulators.
Bypassing the switching regulators reduces the noise
on the ADC supplies at the expense of expending
more power in the LDO regulators. Perform the
following changes to bypass the switching regulators.
•
Remove the resistor jumpers on L22 and L24.
•
Remove R212 and R232.
•
Populate R211 and R231 with 0805 0 ohm
resistors.
-115
-110
-105
-100
-95
-90
M
a
g
n
it
u
d
e
[
d
B
F
S
]
Figure 22: The on-board switching regulators
can cause small spurious tones offset ~500
kHz from a large input signal
Summary of Contents for LMH6517
Page 24: ...SP16160CH1RB Reference Design Board User s Guide 24 www national com Figure 28 Layer 2 Ground...
Page 25: ...SP16160CH1RB Reference Design Board User s Guide 25 www national com Figure 29 Layer 3 Ground...
Page 26: ...SP16160CH1RB Reference Design Board User s Guide 26 www national com Figure 30 Layer 4 Power...
Page 27: ...SP16160CH1RB Reference Design Board User s Guide 27 www national com Figure 31 Layer 5 Ground...
Page 28: ...SP16160CH1RB Reference Design Board User s Guide 28 www national com Figure 32 Layer 6 Signal...
