LMX2434EVAL National Semiconductor, LMX2434EVAL Datasheet - Page 2

• Manufacturer Part Number: LMX2434EVAL
• Description: EVALUATION BOARD FOR LMX2434
• Manufacturer: National Semiconductor
• Series: PLLatinum™r

Specifications of LMX2434EVAL

• Main Purpose: Timing, Frequency Synthesizer
• Utilized Ic / Part: LMX2434
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Secondary Attributes: -
• Embedded: -
• Primary Attributes: -
• Lead Free Status / Rohs Status: Not Compliant
• Other names: *LMX2434EVAL

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1.0 Optimizing Sensitivity at 5.0
GHz
Suppose the requirement is to optimize the performance at
5.0 GHz for an LO to be used in a particular 802.11a WLAN
application. According to Figure 2, the FinRF input imped-
ance lies on the upper half of the Smith Chart. This means
that the FinRF input impedance at 5.0 GHz can be repre-
sented by a series combination of a resistor and an inductor.
From the LMX243x UTCSP FinRF Input Impedance Table,
ZFinRF = 4.92 + j19.79Ω at 5.0 GHz.
To move closer to 50Ω and to an ideal match, a 1.0 pF shunt
capacitor is placed between the VCO output and the FinRF
input of the PLL as shown in Figure 3. The closer the
impedance gets to 50Ω, the better the sensitivity is. The
impedance at 5.0 GHz is then illustrated in Figure 4.
The capacitor used should exhibit a self resonace frequency
that’s higher than the frequency of interest. High Q (low
ESR), temperature compensating (NPO) ceramic capacitors
are recommended here. Today, it is not too difficult to find 1.0
pF capacitors that have self resonance frequencies well
above 5.0 GHz, such as the 600S series capacitors from
American Technical Ceramics.
FIGURE 4. LMX243x UTCSP FinRF Input Impedance
FIGURE 3. 1.0 pF Shunt Match at 5.0 GHz
with 1.0 pF Shunt Match
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Figure 5 shows that an approximate 3.4 dB improvement in
sensitivity is achieved at 5.0 GHz with the addition of the 1.0
pF shunt capacitor. Figure 5 also demonstrates that the
sensitivity gets worse at frequencies other than 5.0 GHz.
This evaluation is performed under typical conditions, with
Vcc = 2.50V and T
Note: A DC blocking capacitor is typically included at the
PLL’s FinRF input. For all the evaluations performed in this
note, the impedance and loss characteristics of the DC
blocking capacitor are included in the calibration and re-
moved from any test results.
FIGURE 5. LMX243x UTCSP FinRF Input Sensitivity
Improvement at 5.0 GHz
A
= 25˚C.
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