AN0907

TRC105 Discrete L-C RF Tuning Network
Introduction
This application note presents a discrete L-C tuning network for RFM’s TRC105 transceiver IC, as an
alternative to the standard SAW filter tuning network. The tuning network schematic is shown in Figure 1,
and component values for operation at 434 MHz are shown in Table 1.
T R C 1 0 5 L -C R F T u n in g N e tw o r k
V D D _ P A
C 1 5
C 5
R 6
L 1 0
C 1 9
L 2
L 7
L 1 1
T R C 1 0 5 P a d 3 2
L 3
C 4
T R C 1 0 5 P a d 3 1
C 2 4
L 4
C 1 6
L 8
C 1 8
C 2 0
C 1 7
C 2 1
L 1
L 9
5 0 o h m
A n te n n a
C 2 2
V D D _ P A
Figure 1
The tuning network consists of a three-pole band-pass filter followed by a discrete balanced-tounbalanced balun network. L3-C4-C24, L4-C16, and L8-C17 form the three parallel-resonant sections of
the band-pass filter. L2 top-couples resonant section L3-C4-C24 to resonant section L4-C16, which provides a steep roll-off above the filter’s operating frequency. C19 top-couples resonant section L4-C16 to
resonant section L8-C17, which provides a steep roll-off below the filter’s operating frequency. C18, L7
and C20 plus C21, L9 and C22 provide the balanced-to-unbalanced balun function. L11 provides coupling
to a 50 ohm antenna. L10, L1, C15, C5 and R1 provide stable decoupling between the VDD_PA power
supply and the TRC103 differential RF port.
It is important to lay the tuning network out with short circuit board traces to minimize parasitics. Some
adjustment of component values may be required depending on the actual circuit board design.
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Component Values
Typical Tuning Network Component Values for 434 MHz Operation
1
L3, L4, L8
2
SMD Inductor, 6.8 nH ±2%, Q ≥ 75
SMD Inductor, 72 nH ±5%, Q ≥ 75
L2
SMD Inductor, 33 nH ±5%, Q ≥ 50
L7, L9
3
L1, L10
SMD Inductor, 220 nH ±5%, Q ≥ 50
L11 (silkscreened C23)
SMD Inductor, 10 nH ±5%, Q ≥ 50
C4, C16, C17
SMD Capacitor, 15 ±0.25 pF, COG, Q ≥ 100
C19
SMD Capacitor, 2.2 ±0.1 pF, COG, Q ≥ 100
C24 (stacked on C4)
SMD Capacitor, 1.5 ±0.1 pF, COG, Q ≥ 100
C18, C20, C21, C22
SMD Capacitor, 4.3 ±0.25 pF, COG, Q ≥ 100
C15
SMD Capacitor, 470 pF ±20%, Q ≥ 50
C5
SMD Capacitor, 0.047 µF ±20%
R6
SMD Resistor, 1 ohm ±5%
Table 1
1. Coilcraft 0604HQ 1610 Series or equivalent
2. Coilcraft 1008HQ 2520 Series or equivalent
3. Toko LLQ1608-FR22 or equivalent
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Example Circuit Board Layers
Figure 2 - Top Assembly
Figure 2 shows the TRC105 example circuit board top assembly. Note that inductor L11 is placed in the
location labeled C23 on the PCB artwork. PADs PCB design files and Gerber files of the example layout
are available at www.rfm.com/products/apnotes/trc105_pcb2.zip.
Figure 3 - Top Silkscreen
The top silkscreen is shown in Figure 3.
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Figure 4 - Top Solder Mask
The top solder mask is shown in Figure 4.
Figure 5 - Top Etch
The top etch pattern is shown in Figure 5. Using ±5% COG RF capacitors where specified in the bill of
materials is important to performance.
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Figure 6 - Drill & Layer Data
The drill and layer data is provided in Figure 6. The example PCB is implemented on a two-sided 0.031
inch board with 1 ounce copper traces.
Figure 7 - Bottom Etch
Figure 7 shows the bottom etch pattern.
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AN907 - 06/15/11
Figure 8 - Bottom Solder Mask
Figure 8 shows the bottom solder mask.
Figure 9 - Bottom Assembly
Figure 9 shows the bottom assembly.
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Figure 10 - Overall PCB Schematic
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AN907 - 06/15/11