SM1211E915TOPIC Semtech, SM1211E915TOPIC Datasheet - Page 14

• Manufacturer Part Number: SM1211E915TOPIC
• Description: KIT PICTAIL EXT FOR SX1211
• Manufacturer: Semtech
• Type: Transceiver, UHFr

Specifications of SM1211E915TOPIC

• Frequency: 915MHz
• For Use With/related Products: SX1211
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

The crystal oscillator (XO) forms the reference oscillator of an Integer-N Phase Locked Loop (PLL), whose
operation is discussed in the following section. Figure 5 shows a block schematic of the SX1211 PLL. Here the
crystal reference frequency and the software controlled dividers R, P and S determine the output frequency of the
PLL.
The VCO tank inductors are connected on an external differential input. Similarly, the loop filter is also located
externally. However, there is an internal 8pF capacitance at VCO input that should be subtracted from the desired
loop filter capacitance.
The output signal of the VCO is used as the input to the local oscillator (LO) generator stage, illustrated in Figure 6.
The VCO frequency is subdivided and used in a series of up (down) conversions for transmission (reception).
With an integer-N PLL architecture, the following criterion must be met to ensure correct operation:
Rev 7 – Sept 2
ADVANCED COMMUNICATIONS & SENSING
The comparison frequency, Fcomp, of the Phase Frequency Detector (PFD) input must remain higher than six
times the PLL bandwidth (PLLBW) to guarantee loop stability and to reject harmonics of the comparison
frequency Fcomp. This is expressed in the inequality:
However the PLLBW has to be sufficiently high to allow adequate PLL lock times
Because the divider ration R determines Fcomp, it should be set close to 119, leading to Fcomp≈100 kHz
which will ensure suitable PLL stability and speed.
3.2.3. PLL Architecture
3.2.4. PLL Tradeoffs
nd
, 2008
XT_M
VCO Output
XO
LO
XT_P
Figure 5: Frequency Synthesizer Description
÷(R
i
+1)
÷8
÷8
Fcomp
Figure 6: LO Generator
PLLBW
PFD
Page 14 of 92
Fcomp
90°
90°
90°
÷75.(P
6
LF_M
i
+1)+S
Vtune
Q
Q
I
I
Q
I
i
LO1 Rx
LO2 Rx
LO1 Tx
LO2 Tx
LF_P
VCO_M
Receiver
LOs
Transmitter
LOs
VCO_P
VR_VCO
LO
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SX1211