background image

a)

b)

Figure 4.2.  Effect of a Shunt Cap Self-Resonating at the Second Harmonic Together with a Parallel SMD Inductor Resonating

with the Shunt Cap at the Fundamental

The  circuit  shown  in  part  A  of  the  above  figure  has  a  moderate  attenuation  at  the  third  harmonic  frequency.  Up  to  ~13  dBm  output
power level, this limited third harmonic attenuation is enough for the EFR32 to comply with the ETSI and FCC standards. Therefore, a
low BOM two-element match formed by the L0-C0 elements can be used because the transmission line is not required for operation.
With this match, the impedance at the TX pin is usually not the optimum, but rather close to 50 Ω. Fortunately, the L0-C0 ratio variation
allows for limited impedance tuning. Even if some residual impedance does occur, the slight reduction of power can be easily compen-
sated for by higher power state settings with a minimal increase in total IC current at low power levels.

However, at higher power levels, EFR32 requires more harmonic suppression. For example, at a 20 dBm output power level, at least
30 dB third harmonic attenuation is required to comply with the 41.2 dBm limit of the US FCC standard. To resolve this issue, introduce
a second parallel capacitor denoted by C1, which has the self-resonance close to the third harmonic frequency. Unfortunately, the sec-
ond parallel capacitor mistunes the fundamental C0-L0 parallel resonator if it is connected directly parallel to it. Besides, the C1 alone
has  a  shunt  effect,  which  again  increases  the  insertion  loss  at  the  fundamental  frequency.  To  resolve  both  issues,  separate  the  C1
capacitor either by a series inductor or a transmission line to form a ladder network. Now, a series transmission line is used becacuse
of the higher cost and higher loss of applying a series inductor.

However, separation is not the only function of the transmission line. Because the C0-L0 parallel resonant circuit is invisible at the fun-
damental frequency, the series transmission line together with the C1 capacitor should also generate the optimum impedance at the TX
pin. As mentioned previously, the L0-C0 can be tuned in a limited fashion to get the right impedance. However, the main tuning element
is the transmission line because of the C0 and the C1 capacitor value variation limitation, which dictates that self-resonant frequencies
should be close to the critical harmonics. The transmission line is only several mm long, which is much shorter than the wavelength at
the fundamental frequency and can, therefore, be modeled as a lumped inductor with some parasitic parallel capacitance. The value of
C1 adds directly to this parasitic cap. Usually, the width of the transmission line has to be decreased close to the technological mini-
mum (~0.2 mm) to have sufficiently high inductance and to minimize the parasitic cap effect beside the C1. Because the width is more
or less fixed, the residual main tuning possibility is the length of the transmission line.

The matching network with 3.5 mm long and 0.2 mm wide transmission line and with 0.8 pF C1 value is shown in part A of the figure
below. Part B shows the transfer characteristic. The third harmonic is efficiently suppressed with the 0.8 pF C1 value. However, the
input impedance at the TX pin (at Port 2 in the schematic) is quite far from the optimum impedance as shown in 

Figure 4.4 Impedances

of Differently Tuned Tline Matches on the 2G4RF_IOP Pins on page 30

 part A. Here, some tuning is required to shift the impedance

closer to the targeted 23+j11.5 ohm. This can be done either by varying C1 and L0–C0 slightly or by tuning the transmission line length.
For example, with a C0 of 2 pF, L0 of 1.2 nH and C1 of 0.9 pF and with 5 mm long transmission line, the impedance is quite close to
the optimum (see 

Figure 4.4 Impedances of Differently Tuned Tline Matches on the 2G4RF_IOP Pins on page 30

 with good transfer

characteristic.

Based on the above experiences, the design steps of the transmission line match are as follows:

1. Choose a C1 capacitor value, which has its self-resonance, good suppression, and is nearly at the third harmonic. Additional layout

series parasitic inductances, such as via or trace inductance, shift down the resonant frequency.

2. Choose a transmission line with 0.2–0.25 mm width and with a length, which together with C1 tunes nearly the optimum impedance

(~23+j11.5 ohm) at the TX pin.

3. Choose a C0 value that has its self-resonance close to the second harmonic. Again, the series parasitic inductances, such as via

or trace, of the layout decreases the resonant frequency.

4. Choose a parallel L0 value that resonates with the C0 at the fundamental frequency and provides a low insertion loss. Slight tuning

of L0 can further improve the impedance optimization.

AN930: EFR32 2.4 GHz Matching Guide

Transmission Line (Tline) Match for Minimal BOM Solutions (U.S. Patent US9780757B1)

silabs.com

 | Building a more connected world.

Rev. 0.4  |  29

Summary of Contents for EFR32

Page 1: ...er 2 4 GHz operation refer to AN928 EFR32 Layout Design Guide Refer to AN933 EFR32 2 4 GHz Minimal BOM for information on minimizing the bill of materials For information on the matching pro cedure fo...

Page 2: ...aders simply seeking recommended values topologies and RF performance data should skip to Appendix 2 2 4 GHz RF Network Schematics and Technical Data The presented matches are tested with three EFR32...

Page 3: ...GND vias Radio Transciever 2G4RF_IOP 2G4RF_ION RF Frontend PA I Q LNA BALUN RFSENSE Frequency Synthesizer DEMOD AGC IFADC CRC BUFC MOD FRC RAC PGA Figure 2 1 2 4 GHz Front End Configuration The on chi...

Page 4: ...harmonic content in TX mode or the sensitivity in RX mode As part of the design process the goal is to deliver maximum power to a 50 output termina tion e g to a 50 antenna in TX mode In addition prop...

Page 5: ...Bm with FCC compliance It applies a series film type SMD LQP15TN series from Murata inductor and so has a higher cost than the paral lel LC match Moreover it has slightly higher insertion loss The adv...

Page 6: ...utions U S Patent US9780757B1 The matching circuit should show the Zload_opt 23 j11 5 impedance at the input while it is terminated by 50 at its output The general lumped two reactive element matching...

Page 7: ...ccount the parasitics of the discrete components For Silicon Labs reference designs 0402 or 0201 sized surface mount device SMD elements are used With 0201 elements one can expect lower parasitics but...

Page 8: ...st so they are a good compromise at 2 4 GHz in terms of price and Q SMD parasitics are investigated in the ladder two element match employing 0402 SMD elements and a film type inductor Simplified equi...

Page 9: ...pass prototype matching elements series in ductance and parallel capacitance There are three approaches to simulating PCB parasitics lumped element distributed element and EM based Since the trace len...

Page 10: ...ired The gap to the side ground metal are given by the G parameters in the figure The easi est way to make that estimation is to use a grounded coplanar calculator which computes the unit parallel cap...

Page 11: ...onic restrictions of the ETSI and FCC standards However below 10 dBm power levels the ladder two element match is advantageous because it has lower cost and lower insertion loss compared to the ladder...

Page 12: ...ement and four element matches Here the 2G4RF_IOP pin of the EFR32 chip is connected to Port 2 The L0 inductor is connected between Ports 3 and 4 the C0 capacitor be tween Ports 5 and 6 the L1 inducto...

Page 13: ...ement Matching Network Schematic Reference Designator Component Value Tolerance Part Number Manufacturer LH0 1 9 nH 0 05 nH LQP15MN1N9W02 Murata CH0 1 5 pF 0 1 pF GRM1555C1H1R5BA01D Murata Figure 3 10...

Page 14: ...lement match is as expected only proper for the lower 10 dBm power regimes Figure 3 12 Measured S11 and S21 Transfer Characteristic of the Ladder 2 element SMD Match The measured ladder two and four e...

Page 15: ...much larg er even at the highest 20 dBm power level It should be noted that in the spectrum measurements the Spectrum Analyzer is used as a wideband 50 termination With a real antenna the termination...

Page 16: ...amples and are typical Large test sample sensitivity results are only avail able for the 7x7 mm dual band package with a four element lumped element match and are provided in the data sheet Table 4 1...

Page 17: ...wer variation is usually less than 0 5 dB which is less than the chip to chip variation 5 All matches with all EFR32 package versions have less than 0 5 dB power variation across the entire 2 4 GHz ba...

Page 18: ...ure rf 3 http wcalc sourceforge net cgi bin coplanar cgi Copyright 2001 2009 Dan McMahill CGIC copyright 1996 1997 1998 1999 2000 by Thomas Boutell and Boutell Com Inc Permission is granted to use CGI...

Page 19: ...x7 mm 48 pin dual band EFR version at the TX pin Here the optimum impedance is approximately 23 j11 5 At 2 45 GHz the optimum is slightly lower 21 j10 4 Figure C below shows the measured 20 dBm 2 4 GH...

Page 20: ...23 j11 5 Further the optimum impedance does not depend much on the power level At a power level of 10 dBm the optimum termination is only slightly off Zload_opt_10dBm 20 j10 6 for the 7x7 mm dual band...

Page 21: ...ches use GRM1555 type SMD capacitors from Mura ta The ladder two element and four element matches use film type LQP15MN series inductors while the hybrid Tline and parallel LC matches apply low cost m...

Page 22: ...2 Low Second harmonic strongly violates FCC at high power levels 2 element ladder 10 dBm 9 7 42 8 63 5 Low Up to 10 dBm ETSI FCC compliant Hybrid LC Tline C 19 1 51 5 60 Low Middle ETSI FCC Compliant...

Page 23: ...Ladder Four Element Match with the Three EFR Package Versions d T line Match with 7x7 mm Dual Band EFR Version AN930 EFR32 2 4 GHz Matching Guide 2 4 GHz RF Network Schematics and Technical Data sila...

Page 24: ...to 20 dBm power levels 2 The LC parallel minimal BOM match is ETSI and FCC compliant up to 13 dBm power level while the ladder two element lumped element match complies up to 10 dBm 3 Due to the appl...

Page 25: ...o the load impedance that the match should show at the IC side is Zload_opt 23 j11 5 if the antenna side is terminated by 50 A ladder four element match can be designed in two ways using the ladder tw...

Page 26: ...yout Parasitics on page 10 are used The figure below shows the tuned ladder four element match locus and schematic with parasitics The endpoint at the IC side denoted by TP11 in the Smith chart and by...

Page 27: ...Element and PCB Parasitics Table 3 1 Final SMD Values for the Ladder Four Element Match Ladder Four Element Matching Network Schematic refer ence designator Component value Tolerance Part Number Manu...

Page 28: ...unt SMD Capacitor Self Resonating Close to the Second Harmonic The transfer characteristic is shown in Figure 4 1 Effect of a Shunt SMD Capacitor Self Resonating Close to the Second Harmonic on page 2...

Page 29: ...critical harmonics The transmission line is only several mm long which is much shorter than the wavelength at the fundamental frequency and can therefore be modeled as a lumped inductor with some par...

Page 30: ...ents PCB Parasitics are Neglected a b Figure 4 4 Impedances of Differently Tuned Tline Matches on the 2G4RF_IOP Pins AN930 EFR32 2 4 GHz Matching Guide Transmission Line Tline Match for Minimal BOM So...

Page 31: ...s shown in Figure 4 5 EM Simulation of the Tline Match on page 31 part a value is chosen which resonates with C0 close to the fundamental frequency results in low insertion loss and if possible also t...

Page 32: ...d Setting the Tline Length to Get Good Impedance at the 2G4RF_IOP Pin Figure 4 7 Characteristic of the Final EM Tuned Tline Match AN930 EFR32 2 4 GHz Matching Guide Transmission Line Tline Match for M...

Page 33: ...F GRM1555C1H1R2BA01 Murata As shown in part A of the figure below the impedance at the fundamental frequency is 20 j12 which is close to the optimum 23 j11 5 Also second and third harmonic suppression...

Page 34: ...in TX Mode with Different EFR32 Packages The fundamental power second and third harmonic variations across the 2 4 2 48 GHz band are also shown in Appendix 1 According to Appendix 1 PA Optimum Impedan...

Page 35: ...20 dBm Power Level a b Figure 4 12 Measured Spectrum Plots of the Single Band EFR32 Versions with Tline Match at 20 dBm Power State a 7x7 mm BRD4101A b 5x5 mm BRD4111A AN930 EFR32 2 4 GHz Matching Gu...

Page 36: ...ply or express copyright licenses granted hereunder to design or fabricate any integrated circuits The products are not designed or authorized to be used within any Life Support System without the spe...

Reviews: