LMX9830SMX National Semiconductor, LMX9830SMX Datasheet - Page 19

• Manufacturer Part Number: LMX9830SMX
• Manufacturer: National Semiconductor

Specifications of LMX9830SMX

• Lead Free Status / Rohs Status: Not Compliant

10.6.1 IQ-DA Converters and TX Mixers
The ROM output signals drive an I- and a Q-DA converter.
Two Butterworth low-pass filters filter the DA output signals.
The 6 MHz clock for the DA converters and the logic circuitry
around the ROM tables are derived from the autotuner.
The TX mixers mix the balanced I- and Q-signals up to 2.4-2.5
GHz. The output signals of the I- and Q-mixers are summed.
10.7 CRYSTAL REQUIREMENTS
The LMX9830 contains a crystal driver circuit. This circuit op-
erates with an external crystal and capacitors to form an
oscillator. shows the recommended crystal circuit. Table 21
specifies system clock requirements.
The RF local oscillator and internal digital clocks for the
LMX9830 is derived from the reference clock at the CLK+ in-
put. This reference may either come from an external clock
or a dedicated crystal oscillator. The crystal oscillator con-
nections require an Xtal and two grounded capacitors.
It is also important to consider board and design dependant
capacitance in tuning crystal circuit. Equations that follow al-
low a close approximation of crystal tuning capacitance re-
quired, but actual values on board will vary with capacitive
properties of the board. As a result, there is some fine tuning
of crystal circuit that has to be done that can not be calculated,
must be tuned by testing different values of load capacitance.
Many different crystals can be used with the LMX9830. Key
requirements from Bluetooth specification is + 20ppm. Addi-
tionally, ESR (Equivalent Series Resistance) must be care-
fully considered. LMX9830 can support maximum of 230 Ω
ESR, but it is recommended to stay <100 Ω ESR for best per-
formance over voltage and temperature. Reference Figure 9
for ESR as part of crystal circuit for more information.
10.7.1 Crystal
The crystal appears inductive near its resonant frequency. It
forms a resonant circuit with its load capacitors. The resonant
frequency may be trimmed with the crystal load capacitance.
1.
Load Capacitance
For resonance at the correct frequency, the crystal
should be loaded with its specified load capacitance,
which is the value of capacitance used in conjunction with
the crystal unit. Load capacitance is a parameter
specified by the crystal, typically expressed in pF. The
crystal circuit shown in Figure 5 is composed of:
— C1 (motional capacitance)
— R1 (motional resistance)
— L1 (motional inductance)
— C0 (static or shunt capacitance)
The LMX9830 provides some of the load with internal
capacitors Cint. The remainder must come from the
external capacitors and tuning capacitors labeled Ct1
and Ct2 as shown in Figure 4. Ct1 and Ct2 should have
the same the value for best noise performance.
The LMX9830 has an additional internal capacitance
CTUNE of 2.6pF. Crystal load capacitance (C
calculated as the following:
C
The C
capacitance C
capacitance is:
C
L
total
= C
= C
L
int
above does not include the crystal internal self-
L
+ C
+ C
TUNE
0
0
as shown in Figure 5, so the total
+ Ct1//Ct2
L
) is
19
2.
3.
FIGURE 4. LMX9830 Crystal Recommended Circuit
Based on crystal spec and equation:
C
C
16.6pF is very close to the TEW crystal requirement of
16pF load capacitance. With the internal shunt
capacitance Ctotal:
Ctotal = 16.6pF + 5pF = 21.6pF
Crystal Pullability
Pullability is another important parameter for a crystal,
which is the change in frequency of a crystal with units of
ppm/pF, either from the natural resonant frequency to a
load resonant frequency, or from one load resonant
frequency to another. The frequency can be pulled in a
parallel resonant circuit by changing the value of load
capacitance. A decrease in load capacitance causes an
increase in frequency, and an increase in load
capacitance causes a decrease in frequency.
Frequency Tuning
Frequency Tuning is achieved by adjusting the crystal
load capacitance with external capacitors. It is a
Bluetooth requirement that the frequency is always within
±20 ppm. Crystal/oscillator must have cumulative
accuracy specifications of ±15 ppm to provide margin for
frequency drift with aging and temperature.
TEW Crystal
The LMX9830 has been tested with the TEW TAS-4025A
crystal, reference Table 18 for specification. Since the
internal capacitance of the crystal circuit is 8 pF and the
load capacitance is 16 pF, 12 pF is a good starting point
for both Ct1 and Ct2. The 2480 MHz RF frequency offset
is then tested. Figure 6shows the RF frequency offset
test results.
Figure 6 shows the results are -20 kHz off the center
frequency, which is –1 ppm. The pullability of the crystal
is 2 ppm/pF, so the load capacitance must be decreased
by about 1.0 pF. By changing Ct1 or Ct2 to 10 pF, the
total load capacitance is decreased by 1.0 pF. Figure 7
shows the frequency offset test results. The frequency
offset is now zero with Ct1 = 10 pF, Ct2 = 10 pF.
Reference Table 19 for crystal tuning values used on
Mesa Development Board with TEW crystal.
L
L
= C
= 8pF + 2.6pF + 6pF = 16.6pF
int
+ C
TUNE
+ Ct1//Ct2
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