IDT8N4Q001 REV G Data Sheet
QUAD-FREQUENCY PROGRAMMABLE-XO
Principles of Operation
The block diagram consists of the internal 3rd overtone crystal and
oscillator which provide the reference clock fXTAL of either 114.285
MHz or 100MHz. The PLL includes the FemtoClock NG VCO along
with the Pre-divider (P), the feedback divider (M) and the post divider
(N). The P, M, and N dividers determine the output frequency based
on the fXTAL reference and must be configured correctly for proper
operation. The feedback divider is fractional supporting a huge
number of output frequencies. The configuration of the feedback
divider to integer-only values results in an improved output phase
noise characteristics at the expense of the range of output
frequencies. In addition, internal registers are used to hold up to four
different factory pre-set P, M, and N configuration settings. These
default pre-sets are stored in the I2C registers at power-up. Each
configuration is selected via the the FSEL[1:0] pins and can be read
back using the SCLK and SDATA pins.
As identified previously, the configurations of P, M (MINT & MFRAC)
and N divider settings are stored the I2C register, and the
configuration loaded at power-up is determined by the FSEL[1:0]
pins.
Table 4. Frequency Selection
Input
FSEL1
FSEL0
Selects
Register
0 (def.)
0 (def.)
Frequency 0
Frequency 1
Frequency 2
Frequency 3
P0, MINT0, MFRAC0, N0
P1, MINT1, MFRAC1, N1
P2, MINT2, MFRAC2, N2
P3, MINT3, MFRAC3, N3
0
1
1
1
0
1
The user may choose to operate the device at an output frequency
different than that set by the factory. After power-up, the user may
write new P, N and M settings into one or more of the four
configuration registers and then use the FSEL[1:0] pins to select the
newly programmed configuration. Note that the I2C registers are
volatile and a power supply cycle will reload the pre-set factory
default conditions.
Frequency Configuration
An order code is assigned to each frequency configuration
programmed by the factory (default frequencies). For more
information on the available default frequencies and order codes,
please see the Ordering Information Section in this document. For
available order codes, see the FemtoClock NG Ceramic-Package
XO and VCXO Ordering Product Information document.
If the user does choose to write a different P, M, and N configuration,
it is recommended to write to a configuration which is not currently
selected by FSEL[1:0] and then change to that configuration after the
I2C transaction has completed. Changing the FSEL[1:0] controls
results in an immediate change of the output frequency to the
selected register values. The P, M, and N frequency configurations
support an output frequency range 15.476MHz to 866.67MHz and
975MHz to 1,300MHz.
For more information and guidelines on programming of the device
for custom frequency configurations, the register description, the
selection of fractional and integer-feedback configurations and the
serial interface description, see the FemtoClock NG Ceramic 5x7
Module Programming Guide.
The devices use the fractional feedback divider with a delta-sigma
modulator for noise shaping and robust frequency synthesis
capability. The relatively high reference frequency minimizes phase
noise generated by frequency multiplication and allows more efficient
shaping of noise by the delta-sigma modulator.
The output frequency is determined by the 2-bit pre-divider (P), the
feedback divider (M) and the 7-bit post divider (N). The feedback
divider (M) consists of both a 7-bit integer portion (MINT) and an
18-bit fractional portion (MFRAC) and provides the means for
high-resolution frequency generation. The output frequency fOUT is
calculated by:
1
MFRAC + 0.5
(1)
-----------
----------------------------------
f
= f
MINT +
OUT
XTAL
P N
18
2
The four configuration registers for the P, M (MINT & MFRAC) and N
dividers which are named Pn, MINTn, MFRACn and Nn with n = 0 to
3. “n” denominates one of the four possible configurations.
IDT8N4Q001GCD REVISION A FEBRUARY 21, 2012
4
©2012 Integrated Device Technology, Inc.
