WIMAX RECEIVER RF FRONTEND DESIGN FINAL REPORT 1

EECS 411 Receiver RF Frontend Designfor 2.4 GHz carrier, 10 MHz BW WiMAX

Yingying Fan and Wenhao Peng, Student, University of Michigan

Abstract—A dual conversion WiMAX receiver has been im-plemented based on our knowledge gained from lectures andlabs from EECS 411. Techniques for amplifier and filter designsare used, and explorations are made to building good passivemixers. Specifications are made based on the IEEE standards forWiMAX, and characterization is performed in ADS simulations.

Index Terms—WiMAX, RF Frontend, Impedance Matching,RF Filter, Gain, Noise, Distortion, Microstrip Lines, RF Ampli-fier, Frequency Mixer

I. INTRODUCTION

THIS report describes a WiMAX receiver RF frontend im-plemented with commercial packaged amplifiers, lumped

elements, power supply, and microstrip lines, on an RO4003Csubstrate. Designs have been done for amplifier biasing,amplifier stabilization, impedance matching, signal filtering,frequency mixing, and characteristics of bandwidth, gain,noise figure, distortion, are analyzed.

A. Specifications of WiMAX Receiver

The variety version of IEEE standard may results in differ-ent requirements for receivers. Adhering to the philosophy -the latest is always the best, we choose the latest approvedIEEE Std 802.16-2017 as our reference. One the other hand,subcarrier modulation methods and coding rates will alsocorrespond to different SNR requirements. Modulation OFDM16-QAM, coding rate 3/4 is adopted in our project.

1) Noise Figure: The minimum input level sensitivity Rssof the receiver is based on the following equation1[4].

Rss = −101+SNRRx+10log(FsNused

NFFT

Nsubchannels

16) (1)

where SNRRx is 15 dB based on assumption; FS is thesampling frequency which can be calculated by equation2;Nsubchannels is the number of allocated subchannels - 16 asdefault if no subchannelization is used; NFFT is 256; Nusedis 200.

FS = floor(n ∗BW/8000) ∗ 8000 (2)

Yingying Fan was with the Department of Electrical and Computer En-gineering, University of Michigan, Ann Arbor, MI, 48109. USA e-mail:yifa@umich.edu.

Wenhao Peng was with the Department of Electrical and Computer En-gineering, University of Michigan, Ann Arbor, MI, 48109. USA e-mail:pwhum@umich.edu

EECS 411 Project Final Report

For our choice of specs [4], BW = 10 MHz, n = 57/50,so FS = 11.424 MHz, and the minimum input level shouldbe RSS = −74.5 dBm. This results in the noise figure (NF)requirement of 8 dB with 5 dB implemention margin. Thetotal noise figure for cascaded devices is calculated based onequation3.

F = F1 +F2 − 1

G1+

F3 − 1

G1G2+ · · ·+ Fn

G1G2 · · ·Gn−1(3)

2) Dynamic Range: The required dynamic range of thereceiver is defined from 3 dB above the reference sensitivitylevel specified in equation1 to maximum input signal level.The receive should have the capability of decoding a maximumon-channel signal of -30 dBm. Thus,

DR = −30dBm− (−74.5dBm) = 44.5dB (4)

3) IIP3: Input 1 dB compression point is usually 4 dBgreater than the maximum input level, so P1dB = −26 dBmin our case. Then, as a “rule of thumb”, IIP3 is 9 to 10 dBhigher than P1dB , so IIP3 should be -16 dBm [1]. The totalIIP3 for cascaded devices [2] is calculated based on equation5.

1

IIP 23

=1

IIP 23,1

+G1

IIP 23,2

+ · · ·+ G1G2 · · ·Gm−1

IIP3,m(5)

B. Specifications for Each Block

Table IUPDATED RECEIVER SPECIFICATIONS

Gain [dB] NF [dB] IIP3 [dB]Filter 1 -2 2 60LNA 1 14.5 2.5 15Filter 2 -2 2 60HGA 1 14.5 2.5 15Mixer 1 -8 8 3Filter 3 -2 2 60Mixer 2 -12 12 4Filter 4 -6 6 60IF Amp 25 4 7

II. DESIGN AND IMPLEMENTATION

The system begins with a band select filter to filter outunwanted spectrum components. An LNA follows the firstfilter for reducing the noise figure of the system. The secondfilter plays the role of image rejection filter. It also actsas matching network between the LNA and the high gainamplifier to reduce gain loss. The HGA is added to increase thepower for the mixer’s input. We used another image rejection

2 WIMAX RECEIVER RF FRONTEND DESIGN FINAL REPORT

filter to eliminate the effect of the image frequency of thesecond mixer. This passive circuit also makes contributionsto higher linearity. After the second mixer is the narrowestbandpass filter as our channel select filter. The output is finallyamplified by an IF high gain amplifier for following signalprocessing stages. The total block diagram of the receiver isshown in fig.1.

Figure 1. Block Diagram of the receiver

A. Filter1: Band-Select Filter

Our pass-band is set to be 2.3 GHz - 2.5 GHz. The imagefrequency may appear at 3.048 GHz - 3.248 GHz due to thefirst mixer. The band select filter is the first stage of ourwhole receiver chain, so it plays significant role on the totalnoise figure. For better image rejection for the whole receiver,we push the first band-select filter hard to have good roll-off performance. We chose 0.5 dB equal-ripple bandpass filterwith 5th order. And we implemented it using the coupled linestructure. The layout is shown in fig.29.

By Momentum simulation we got Gain > −1.3 dB and NF< 1.225 dB in frequency range 2.3 GHz to 2.5 GHz, andImage Rejection (at 3.048 GHz) -57.3 dB, which meets ourspecification pretty well.

Figure 2. Band-select filter gain and noise figure

B. Filter2:Image Rejection Filter

The second filter in our receiver system was chosen to be alow pass filter for better gain in the passband. The closestimage frequency to the passband is 3.048 GHz. Lumpedelement filters are not suitable for GHz frequency rangefor their bad quality factor. Compared with stepped filters,periodic stubs filters are more complex to implement but own

better roll-off property. We chose 5th order 3 dB equal-ripplelowpass filter structure with cut-off frequency at 2.75 GHz.The layout is shown in fig.5.

By Momentum simulation we got Gain > −0.324 dB andNF < 0.348 dB in frequency range 2.3 GHz to 2.5 GHz,and Image Rejection (at 3.048 GHz) -18.203 dB, which meetsour specification pretty well. It also roughly helps us meetthe requirements of -10 dB adjacent channel interference, -29dB nonadjacentchannel rejection and 50 dB minimum imagerejection

Figure 3. Image rejection filter gain and noise figure

C. 2.4 GHz RF Low Noise Amplifier

The LNA is implemented with an ATF 541 M4 transistor.DC biasing points are selected based on the recommendedtypical biasing current and voltage. As shown in fig.22, thegate is biased at 0.8 V, drain at 3 V. And a series 11 Ohm anda shunt 120 Ohm stabilization resistors are used at the gatebiasing and at the output. The amplifier topology is the same aswe have in the labs, where radial stubs are used for connectingthe biasing voltage sources that makes the transistor a commonsource amplifier. Unconditional stability is first achieved byadding stabilization resistors. Then the input port is matchedfor optimal noise figure. After that, the output port is matchedfor maximal return loss. With this procedure, the followingperformance is achieved.

LNA achieves Frequency 2.3 to 2.5 GHz of operation, Gain> 14.7 dB, NF < 0.53 dB. And Inputs and outputs arematched to 50 Ohms. 10 dB S11 BW is 2.1 to 2.7 GHz,and 10 dB S22 BW is 0.5 to 3.3 GHz.

D. 2.4 GHz RF High Gain Amplifier

The HGA uses the same transistor and biasing and stabi-lization network as the LNA. Simultaneous input and outputmatching is performed for maximal return loss at both theinput and the output. The gain and noise figure informationare shown in fig.7. Fig. 8 displays the information of IIP3.With this procedure, the following performance is achieved.

HGA achieves frequency 2.3 to 2.5 GHz of operation, Gain> 14.8 dB, NF < 0.75 dB, and Inputs and outputs are matched

EECS 411 MICROWAVE CIRCUITS I 3

Figure 4. LNA is unconditionally stable before matching from 0.1 to 4.5GHz.

Figure 5. LNA S-parameters and noise figure

to 50 Ohms. 10 dB S11 BW is 2.0 to 2.6 GHz, and 10 dBS22 BW is 0.5 to 2.65 GHz.

E. Mixer1: RF to 374 MHz IF

The first mixer is implemented with a 90 degree hybridebalanced mixer which improves the RF input matching andRF-LO isolation.[3] The used LC matching are placed at theinput of the diodes and the IF output port, and microstripmatching is placed before the branch line for the RF port.

F. Filter3: Image Rejection Filters II

The second image rejection filter is a low pass filter whichhave lower passband loss. This filter is used to eliminate theinfluence caused by the closest image frequency to the secondmixer which is 514 MHz. In MHz frequency range, lumpedelement filters are chosen to reduce the area cost. We used highquality factor inductors and capacitors from Murata company.The loss at 374 MHz is 1.24 dB. The attenuation at 514 MHz isnear 20 dB. It meets the specifications we set in our proposal.

Figure 6. IIP3 measures 24 dBm for LNA.

Figure 7. HGA S-parameters and noise figure

G. Mixer2: 374 MHz IF to 70 MHz IF

The second mixer is implemented with a double-balanceddiode ring mixer. A simple LC balun is used to convert singleended signals to differential signals. The LC values are chosenso that their resonant frequency is the input RF frequency. LOpower is swept to find the best choice. LC lumped elementsare used for matching the RF port.

H. Filter4: Channel Select Filters

The last filter in our receiver chain is the channel selectfilter. The local oscillator for the second mixer is 444 MHzand the input frequency is 374 MHz so Our system has centerfrequency of 70 MHz and a 10 MHz bandwidth. Based onthis, the passband of the filter is set as 65 MHz to 75 MHz.The conversion loss in the passband is around 4 dB. Noisefigure is around 4.5 dB.

4 WIMAX RECEIVER RF FRONTEND DESIGN FINAL REPORT

Figure 8. IIP3 measures 24 dBm for HGA.

Figure 9. Noise figure and conversion gain of the first mixer VS LO power

I. 70 MHz IF Amplifier

The IF Amp is implemented with an ATF 511 P8 transistor.Biasing is chosen such that it works at low frequencies. Sta-bilization resistors are added to ensure unconditional stability.Then simultaneous input and output matching are performed.With matching networks optimized for matching, the followingperformance is achieved.

III. OVERALL SYSTEM PERFORMANCE

We connected all the subblocks into a whole system asshown in Fig. 20. In Fig. 18Gain measures 30 dB, Noise Figuremeasures 5.3 dB, and IIP3 measures -25 dBm. Spurious-FreeDynamic range, according to our lecture, is found as

SPDR =2

3(IIP3−N0B)

=2

3(−25− (−174 + 70))

=2

3(79)

= 52.7dB

which is greater than the required 44.5 dB of dynamic rangerequired by WiMAX standards.

IV. SCHEMATICS

For clarity, one column schematics are placed here.

Figure 10. IIP3 of the first mixer

Figure 11. Gain, noise figure and input matching results of the second imagerejection filter

Table IIRECEIVER CHARACTERIZATION

Gain [dB] NF [dB] IIP3 [dB]Filter 1 -1.3 1.3 60LNA 1 14.7 0.6 24.5Filter 2 -0.4 0.3 60HGA 1 14.8 0.75 24.5Mixer 1 -6.8 6 5.3Filter 3 -1.3 0.1 60Mixer 2 -10.5 8.6 4.1Filter 4 -5.5 5 60IF Amp 26.3 3.7 8.2

Total 30 5.3 -25

V. LAYOUT

Layouts are performed for Momentum EM simulations.They are included at the end of this report for the coherenceof our discussion on design, performance and specifications.

EECS 411 MICROWAVE CIRCUITS I 5

Figure 12. Noise figure and conversion gain of the second mixer VS LOpower

Figure 13. IIP3 of the second mixer

VI. CONCLUSION

We have gained a lot of practical experience in designing asystem of RF circuits. By making a reasonable budget on gain,noise figure, and linearity, we have achieved the specificationsrequired by the WiMAX communication standards.

ACKNOWLEDGMENT

The authors would like to thank the EECS 411 faculty andstaff for the superior learning experience.

REFERENCES

[1] Hsiao-Chin Chen et al. “CMOS RF circuits for 5-GHzBWA”. In: (2007), pp. 70–73.

[2] Prof. Ali M. Niknejad. Lecture 9: Intercept Point, GainCompression and Blocking.

[3] David M Pozar. Microwave engineering. John Wiley &Sons, 2009.

Figure 14. Conversion gain and noise figure of the channel select filter

Figure 15. IF Amp is unconditionally stable before matching from 10 to 500MHz.

[4] IEEE Computer Society, the IEEE Microwave Theory,and Techniques Society. IEEE Standard for Air Interfacefor Broadband Wireless Access Systems. IEEE, 2017.

6 WIMAX RECEIVER RF FRONTEND DESIGN FINAL REPORT

Figure 16. IF Amp operates at Frequency 70 MHz, Gain > 26 dB, NF < 4dB, and Inputs and outputs are matched to 50 Ohms. 10 dB S11 BW is 41to 78 MHz, and 10 dB S22 BW is 47 to 70 MHz.

Figure 17. IIP3 measures 8 dBm for the IF amp.

Figure 18. Gain of 30 dB is achieved, and Noise Figure of 5.3 dB is achieved.

Figure 19. IIP3 measures -25 dBm. At input -80 dBm, the third orderintermods are 110 dB lower than the fundamentals, so the IIP3 is 55 dBhigher than -80 dBm, which is -25 dBm.

EECS 411 MICROWAVE CIRCUITS I 7

Figure 20. Total Block Schematic

Figure 21. LNA Microstrip Design.

8 WIMAX RECEIVER RF FRONTEND DESIGN FINAL REPORT

Figure 22. LNA Biasing Network.

Figure 23. Schematic of the first mixer

EECS 411 MICROWAVE CIRCUITS I 9

Figure 24. Schematic of the second image rejection filter

Figure 25. Schematic of the second mixer.

10 WIMAX RECEIVER RF FRONTEND DESIGN FINAL REPORT

Figure 26. IF Amp schematic. A shunt 50 Ohm resistor is placed at the output.

Figure 27. Schematic of the channel select filter

Figure 28. Layout of the image rejection filter

EECS 411 MICROWAVE CIRCUITS I 11

Figure 29. Layout of the band select filter

12 WIMAX RECEIVER RF FRONTEND DESIGN FINAL REPORT

Figure 30. Layout of LNA Matching Network.

Figure 31. Layout of HGA Matching Network.

Figure 32. Layout of the branch line coupler in the first mixer