9 frequency synthesizer – Rainbow Electronics ATA5746 User Manual
Page 14
14
4596A–RKE–05/06
ATA5745/ATA5746 [Preliminary]
As can be seen in
, for single devices there is a variance over temperature
and supply voltage range of ±3 dB. The total variance over production, temperature, and supply
voltage range is ±9 dB.
2.9
Frequency Synthesizer
The LO generates the carrier frequency for the mixer via a PLL synthesizer. The XTO (crystal
oscillator) generates the reference frequency f
XTO
. The VCO (voltage-controlled oscillator) gen-
erates the drive voltage frequency f
LO
for the mixer. f
LO
is divided by the factor 24 (ATA5746) or
32 (ATA5745). The divided frequency is compared to f
XTO
by the phase frequency detector. The
current output of the phase frequency detector is connected to the fully integrated loop filter, and
thereby generates the control voltage for the VCO. By means of that configuration, the VCO is
controlled in a way, such that f
LO
/ 24 (f
LO
/ 32) is equal to f
XTO
. If f
LO
is determined, f
XTO
can be
calculated using the following formula: f
XTO
= f
LO
/ 24 (f
XTO
= f
LO
/ 32). The synthesizer has a
phase noise of –130 dBC/Hz at 3 MHz and spurs of –75 dBC.
Care must be taken with the harmonics of the CLK output signal, as well as with the harmonics
produced by a microprocessor clocked using the signal, as these harmonics can disturb the
reception of signals.
3.
XTO
The XTO is an amplitude-regulated Pierce oscillator type with external load capacitances
(2
× 16 pF). Due to additional internal and board parasitics (C
P
) of approximately 2 pF on each
side, the load capacitance amounts to 2
× 18 pF (9 pF total).
The XTO oscillation frequency f
XTO
is the reference frequency for the integer-N synthesizer.
When designing the system in terms of receiving and transmitting frequency offset, the accuracy
of the crystal and XTO have to be considered.
The XTO’s additional pulling (including the R
M
tolerance) is only ±5 ppm. The XTAL versus tem-
perature, aging, and tolerances is then the main source of frequency error in the local oscillator.
The XTO frequency depends on XTAL properties and the load capacitances C
L1,2
at pin XTAL1
and XTAL2. The pulling (p) of f
XTO
from the nominal f
XTAL
is calculated using the following for-
mula:
C
m
, the crystal's motional capacitance; C
0
, the shunt capacitance; and C
LN
, the nominal load
capacitance of the XTAL, are found in the datasheet. C
L
is the total actual load capacitance of
the crystal in the circuit, and consists of C
L1
and C
L2
connected in series.
p
C
m
2
--------
C
LN
C
L
–
C
O
C
LN
+
(
)
C
O
C
L
+
(
)
×
---------------------------------------------------------------
10
-6
ppm
Ч
Ч
=