Multidisciplinary Senior Design ConferenceKate Gleason College of Engineering

Rochester Institute of TechnologyRochester, New York 14623

Project Number: P12312

SINGLE-BALANCED MIXER

Jared P. BurdickElectrical Engineering

ABSTRACTFrequency conversion processes are common-place in most high-frequency systems such as radio

communications and radars. Frequency conversion generally refers to the process decreasing (down-conversion) or increasing (up-conversion) a carrier frequency while maintaining the fidelity of the information contained in the signal (modulations). At the center of these conversion processes is the function referred to as mixing and is accomplished by a class of component know as a mixer. The primary objective of this project was the design, fabrication, and evaluation of a prototype mixer which met customer specifications. For this project several configurations (architectures) of mixers were evaluated before selecting a Single-Balanced Mixer design. The mixer was designed, simulated using RF/Microwave modeling software (AWR), fabricated, and tested. The first iteration passed all specifications, however several improvements are recommended for a final product.

INTRODUCTIONMost high frequency (RF and Microwave, generally between 1 and 100 GHz) electronic systems, which include

wireless communications and radar, must employ some level of frequency conversion, which often involves a chain of conversions to achieve the desired performance. The receiver side of these systems entails down-conversion of high frequency signals into lower intermediate frequencies (IF) which are low enough for appropriate signal processing, primarily analog to digital conversion. The key component in this down conversion process is known as a mixer. Conversely, for IF signals that need to be converted to RF/Microwave frequencies for transmission, the key component is called a modulator, but it is essentially the same device. There are a number of common configurations for mixers, including single-ended, single-balanced, double-balanced, and quadrature IF (QIFM). These configurations offer different advantages (or drawbacks) which can play favorably (or unfavorably) for specific applications or needs. Key performance specifications for mixers often consist of the following:

Parameter DescriptionConversion Loss This is the "loss" defined as the difference in power level from the RF Input to the

fundamental IF Output signal, typically expressed in dB.Conversion Loss Flatness The variation in the conversion loss over frequency, typically expressed in dB.LO to IF Isolation This is the difference in LO power level from the LO input to the IF output, typically

expressed in dB. This is sometimes referred to as LO Leakage.RF to IF Isolation This is the difference in RF power level from the RF input to the IF output, typically

expressed in dB. This is sometimes referred to as RF Leakage.1dB Compression This is a measure of the "saturation level" of the mixer. It is defined as the input RF

level where the conversion loss increases by 1dB.VSWR (Retrun Loss) This is a measure of how well the RF/LO/IF ports are matched to the characteristic

impedance, generally 50Ω, expressed as a ratio. Return Loss is equivalent and is expressed in dB.

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Single-Balanced Mixer Project Page 3

DESIGN PROCESSThe main purpose of this project was the development of a single-balanced mixer which operates over a 400

MHz band centered at 1.0 GHz. A secondary goal was to provide personal development in the areas of project management, RF modeling and simulation, and testing for this future engineer.

The overall effort began with a general investigation of mixers and development of rough initial specifications in conjunction with the sponsors, Dielectric Labs and Anaren Microwave. Investigation of mixer configurations and design was a result of web research including technical papers, product application note, on-line forums, and discussions with other engineers. It was jointly determined to choose a single-balanced mixer configuration as this was neither the simplest of configurations (single-ended), nor the most complex (double-balanced or QIFM) and would fit the scope of the project and the timeframe allowed. The specification was updated several times during the initial design to better reflect achievable results and clarify certain parameters and intended performance.

A large amount of interaction with both Dielectric Labs and Anaren engineers was beneficial in both development of the specifications, understanding of the mixer operation, use of the modeling/simulation tools, manufacturing trade-offs, and test techniques employed.

Customer Needs and SpecificationsThe development of the mixer began with the initial customer needs which are summarized in Table 1. As

mentioned previously, the customer needs focused on the development of both the mixer itself as well as the engineer. The final specifications for the mixer are summarized in Table 2. The functions listed refer to the Final Design Block Diagram of Figure 2, and the Source refers to the Customer Needs Table (Table 1).

CN1 High Evaluate potential mixer configurations that could be implementedCN2 High Consider performance trade-offs against mixer specificationsCN3 High Simulate mixer performance and compare against requirementsCN4 High Develop design and simulation skillsCN5 Med Manufacture prototype mixer - may not be final production versionCN6 Med Analyze measured performance against specification and simulated performanceCN7 Med Propose potential changes to design that could be implemented in a final production version

Customer Needs TableCustomer

Need #Importance Description

Table 1: Customer Needs

S1 High CN1, CN2, CN3, CN5 1 RF Frequency GHz 0.8 - 1.2 same RF Input frequency rangeS2 High CN1, CN2, CN3, CN5 2 LO Frequency GHz 1 same LO frequency (fixed)S3 High CN1, CN2, CN3, CN5 7 IF Frequency MHz DC - 200 same IF output frequency range

S4 Med CN1, CN2, CN3, CN5, CN71,2,5,7,10,11

Conversion Loss dB 10 sameDifference in power between signal entering RF port and output IF port

S5 Med CN1,CN2, CN3, CN5, CN7 1,2,4,5 RF/IF Isolation dB 30 same Relative to the RF InputS6 Med CN1,CN2, CN3, CN5, CN7 2,4,5,7 LO/IF Isolation dB 35 same Relative to the LO Input Power

S7 High CN1,CN2, CN3, CN5, CN7 2,5 Minimum LO Power dBm 10 sameMinimum LO power level required for all other specs to be met.

S8 High CN1,CN2, CN3, CN5, CN7 1,2,5 Maximum RF Power dBm -10 sameMaximum RF power level required for all other specs to be met.

S9 Low CN1,CN2, CN3, CN5, CN7 1,2,5,7Minimum 1dB Compression

dBm 5 sameThe input RF power level at which conversion loss increases by 1 dB

S10 Low CN1,CN2, CN3, CN5, CN7 1,5,8 RF VSWR Ratio 2.0:1 sameS11 Low CN1,CN2, CN3, CN5, CN7 2,5,8 LO VSWR Ratio 2.5:1 2.0:1

S12 Low CN1,CN2, CN3, CN5, CN72,5,6,7,8,

10,11IF VSWR Ratio 1.5:1 same Measured with LO power at 10 dBm

Indicates Key Specification

Marginal Value

Ideal Value

Comments

Specifications Table

Importance Source Function Specification (metric)Unit of

MeasureSpec

#

Table 2: Final Mixer Specifications

Architecture/Configuration SelectionThe single-balanced mixer configuration takes advantage of the fact that the design inherently rejects even

order spurious responses and inter-modulation products because of the symmetry of the diode configuration. Balanced mixers also provide optimized port to port isolation. The double-balanced approach exhibits the same

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Single-Balanced Mixer Project Page 3

advantages but was rejected due to the added complexity and the risk it would pose to completion of the project on time. The single-ended mixer will have poor port-to port isolation, VSWR and will reject neither the LO or the even-order inter-modulation products. The block diagram for the initial design is shown below in Figure 1.

LPF

Series Diode Pair

RF Choke

RF Choke

90° Hybrid Coupler

RF Input (0.8 - 1.2 GHz)

LO Input (1.0 GHz)

IF Output(DC - 0.2 GHz)

SMA Connector

SMA Connector

SMA Connector

Single-Balanced Mixer Block DiagramInitial Design

Substrate Material

BPF

BPF

Figure 1: Initial Design Block Diagram

The design will consist of the basic mixer blocks which include the 90º hybrid coupler, RF Chokes, and two diodes in series. Additional filtering was included at the RF and LO ports as well as the IF port. The RF and LO port band-pass filters are intended to attenuate undesired frequencies from both entering and exiting the ports (as they may adversely impact other parts of the system). The IF port filtering will be low-pass, its main purpose is to attenuate the LO leakage, as the single-balanced configuration affords no natural suppression (where the double-balanced would). The RF chokes provide a low frequency signal path to ground while appearing as an open-circuit to the higher frequency RF signals.

For the project, primarily due to schedule constraints, it was decided to make use of as many off-the-shelf components as possible, with each element explained in more detail in the following paragraphs.

Substrate Material RO4003C was chosen because it has good RF performance, is inexpensive and readily available. Its

loss is more than acceptable, particularly due to the fact that limited circuitry will be included in the substrate itself (use of purchased components) and the frequency range of the design (around 1 GHz) is not that high.  This material is also well suited for circuit board assembly applications as its thermal coefficient of expansion is well matched to both copper in the z-direction and ceramics (component packages) in the x/y direction.  It is also easier to process than ceramics (less time) and thus better suited for the project.

Parameter SpecificationDielectric Constant εR

3.55Dissipation Factor δ 0.0021Dielectric Thickness .031 inchesCopper Thickness 35 umCoef. of Thermal Expansion X 11 ppm/ºC

Y 14 ppm/ºCZ 46 ppm/ºC

RF Splitting ElementFor this project the 90-degree hybrid coupler was considered the most appropriate since it provided the

simplest implementation while theoretically being able to meet all of the required specifications.  It was decided that the initial design approach will utilize an Anaren XC0900A-03 as it provides good isolation, VSWR, as well as phase and amplitude balance over the desired frequency range. It is also well-suited for mounting on RO4003 material. Key specifications are summarized below.

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Single-Balanced Mixer Project Page 3

Parameter SpecificationInsertion Loss 0.15 dB maxAmp Balance ±0.2 dB maxPhase Balance 90º ± 2º maxIsolation 23 dB minVSWR 1.15:1 max

Switching ElementFor the switching element, the Avago HSMS-2822 silicon Schottky-barrier diode was selected as it is

specifically tailored for mixer and high-speed switching applications.  It comes packaged as a series pair in a SOT (Small Outline Transistor) package which can be easily surface mount soldered to the substrate. The advantage of the series pair is that both diodes are from the same silicon wafer (and same location) which greatly improves the matching of performance, as well as making the printed circuit board layout straightforward.  It has a low forward voltage, a high reverse voltage, low reverse leakage, low total capacitance, and low dynamic resistance which makes it well suited for mixer applications. The package, schematic, and basic specifications are as follows:

Parameter SpecificationMin. Forward Breakdown Volt. VBR 15V @ 100 uAMax. Forward Voltage Vf 340 mV @ 1 mAMin. Peak Reverse Voltage PIV 15 VMaximum Capacitance C 1.0 pF maxDynamic Resistance RD 23 dB min

ConnectorsThe interface to the mixer was accomplished using SMA connectors, which are common, low-cost,

and have good RF performance. The connector termination was required to interface with a printed circuit board. A search for connectors yielded availability from a large number of suppliers.  For this project, connectors already stocked at Dielectric Labs (Gigalane PAF-S05 series) were selected. They are intended for end-launching to printed circuit boards with details as shown below.

Parameter SpecificationFrequency DC - 6 GHzImpedance 50 ΩFinish Gold Plate

IF Low-Pass FilteringThe LPF could be purchased as a component, printed as a distributed element or realized as a lumped-

element design.  A search of potential purchased filters resulted in several issues.  The first being limited number of standard, off-the-shelf filters with the desired (or near enough to desired) specifications.  The second was cost and availability; with common purchase restrictions of minimum quantities or unavailability made this an unattractive solution.  A distributed (printed) design was impractical due to the low frequency as it would take up a significant amount of printed circuit board area to realize. Hence, the Lumped-element approach was chosen as discrete capacitors and inductors of the required values, tolerances, and suitable for high-frequency applications were inexpensive and readily available. Additionally, it would result in the least amount of space on printed circuit board.  Design of a third-order filter was then included as part of the mixer.

LO and RF Band-Pass FilteringThe RF band-pass filtering selection followed the same path as for the LPF above. This was as

problematic, with no good options for purchase (for the same reasons). After discussions with the customer, it was decided to eliminate the requirements for the prototype as they would not impact the mixer performance itself, unlike the LPF which was needed for LO leakage suppression.

RF ChokesThe quarter-wave high impedance lines will be terminated with a radial open. The radial open is a

wider bandwidth structure which appears as a short at the end of the high-impedance quarter wave line. This circuit will be realized as a distributed element and modeled as part of the overall mixer.

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Single-Balanced Mixer Project Page 3

Initial DesignThe first part of the process was selection of the modeling tools to be employed to aid with the design. As

a key consideration of the design is the non-linear analysis (diodes) required where use of passive EM simulation (e.g. HFSS) alone would not be sufficient. Dielectric Labs only possessed linear simulator packages so Anaren resources needed to be used as they had several non-linear options, including Ansoft Designer and AWR. After consulting with several Anaren engineers, it was determined that AWR would provide the most accurate simulation capability. Being unfamiliar with the software, the first part of the detailed design process was gaining competency with this toolset. The initial design activity involved a simple single-balanced mixer that included ideal parts and layout, just to get a basic understanding of the software package. After this was accomplished, ideal components were exchanged with real parts utilizing s-parameter data (90º hybrid coupler), spice model parameters (diode), and real micro-strip transmission line functions (radial RF chokes, lumped element filter with real inductor and capacitor values and characteristics, and interconnection runs).

The initial design yielded a poor results overall. The first design efforts focused on optimizing the conversion loss level and flatness as these were deemed the most sensitive performance parameters, relying on most of the elements in the chain. The first iteration resulted in conversion loss of greater than 15 dB and flatness in the range of 4 dB, with expected conversion loss of closer to 6-7 dB. Review of other parameters led to the conclusion that there were significant VSWR issues in the IF path.

For the second iteration, a quarter-wave impedance transformer was added between the IF filter and the diodes, with the assumption being that the diodes were lower impedance (< 50Ω of the LPF), and the transformer should reduce the mismatch and VSWR. This resulted in improved conversion loss of 10-11dB with a flatness of less than 3 dB, but still not within specification.

The third iteration involved modifying the radial RF chokes. The radial open circuit element, as well as the impedance of the quarter-wave line (width of the line), were both varied, but little improvement was gained. It was decided that a shorted quarter-wave stub would yield a better DC return path for the diodes, and the design bandwidth of the mixer was fairly low (400MHz/1GHz or 40%) so the wide-band radial opens were potentially overkill. This resulted in much improved conversion loss (7-8 dB), flatness (<3 dB), and met customer specification.

The fourth iteration involved increasing the rejection of the Low-pass filter as the simulated LO to RF isolation was marginal. The filter was changed from a 3 rd order to a 5th order which yielded good performance margin with approximately 15dB of specification margin.

The fifth and final iteration was a result of the design review held at Dielectric Labs. Participating engineers suggested the addition of an RF bypass capacitor before the quarter-wave impedance transformer. This provided additional filtering before the transformer and it yielded a much desired simulated conversion loss of around 5-7dB range and flatness of less than 2 dB.

Final DesignThe simulated result of the fifth and final design iteration met all of the customer specifications. A block

diagram can be seen in Figure 1. Figure 2 shows the AWR model of the final design with all of the key circuit values. The 5th order lumped element low-pass filter realization can be seen in Figure 3 with its simulated response in Figure 4. The XC0900A-03 coupler’s response can be found in Figure 5.

6. LPF

11. RF Bypass

10. λ/4 Impedance Transformer

5. Series Diode Pair

3. λ/4 High ImpedanceStub

3. λ/4 High Impedance Stub

4. 90° Hybrid Coupler

1. RF Input (0.8 - 1.2 GHz)

2. LO Input (1.0 GHz)

7. IF Output(DC - 0.2 GHz)

8. SMA Connector

8. SMA Connector

8. SMA Connector

Single-Balanced Mixer Block DiagramFinal Design

9. Substrate Material

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Single-Balanced Mixer Project Page 3

Figure 1: Single Balance Mixer Block Diagram

Figure 2: AWR Final Design Model

0 0.5 1 1.5 2 2.5Frequency (GHz)

LPF IL RL

-80

-60

-40

-20

0

Inse

rtio

n L

oss

-40

-30

-20

-10

0

Ret

urn

Loss

DB(|S(2,1)| ) (L)LPF

DB(|S(1,1)|) (R)LPF

Figure 3: Lumped Element Low Pass Filter Figure 4: Low Pass Filter Frequency Response

0.8 0.9 1 1.1 1.2 1.3Frequency (GHz)

XC0900A3 Coupling

-3.6

-3.4

-3.2

-3

-2.8

Co

uplin

g (d

B)

1.1994 GHz-2.8599 dB

1.1995 GHz-3.5242 dB

1.0006 GHz-3.0466 dB

1.0003 GHz-3.1613 dB

DB(|S(3,1)|)Coupler

DB(|S(4,1)|)Coupler

Figure 5: Anaren XC0900A-03 Coupler Frequency Response

Fabrication and Testing MethodFabrication of the circuit was done using a LPKF prototype circuit milling machine. This process was the

best choice for this project because it allowed the circuit to be made in 1 day rather than 1 week if it were to be built

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Single-Balanced Mixer Project Page 3

using an etch process. The LPKF is a small bench-top router which removes the copper on the substrate material in the areas desired. The interface to the equipment is accomplished by importing a simple CAD (dxf) file. Circuit fabrication and unit assembly were both performed at Anaren. Assembly drawings and instructions, as well as all bill-of-material items, were supplied to the operator, and feed-back on the clarity of the instructions was positive. All components were hand placed and soldered. Two units were assembled, with parts to build an additional 3 available.

SMA Conn Launch(RF In)

SMA Conn Launch(LO In)

SMA Conn Launch (IF Out)

LPF

λ/4 transformerDiode Pair

Coupler

λ/4 shorted stub

λ/4 shorted stub

RF Bypass Cap

Figure 6: Printed-Circuit Board Layout Figure 7: Assembled Unit

Testing was also accomplished at Anaren, utilizing a spectrum analyzer, RF power meter, and 2 RF signal sources to measure the spurious signals, conversion loss, RF to IF isolation, LO to IF isolation and 1-dB compression performance. VSWR (Return Loss) measurements were performed using a Network Analyzer. For all measurements, the set-ups employed were calibrated (including cables and adapters), so the true losses due to the mixer were as accurate as possible. Both assembled units were fully tested.

RESULTS AND DISCUSSIONBoth prototypes met all the customer specifications listed in Table 2. A summary table comparing measured

results of both units to the specification as well as simulated performance can be seen in Table 3.

Unit 1 Unit 2S1 RF Frequency GHz 0.8 - 1.2 OK YesS2 LO Frequency GHz 1 OK YesS3 IF Frequency MHz DC - 200 OK YesS4 Maximum Conversion Loss dB 10 7.2 7.6 8.0 YesS5 RF/IF Isolation dB 30 42 37.0 38.5 YesS6 LO/IF Isolation dB 35 50 41.0 43.0 YesS7 Minimum LO Power dBm 10 OK YesS8 Maximum RF Power dBm -10 OK YesS9 Minimum 1dB Compression dBm 5 8 8.6 8.6 YesS10 Maximum RF VSWR Ratio 2.0:1 1.40:1 1.29:1 1.25:1 YesS11 Maximum LO VSWR Ratio 2.5:1 1.09:1 1.05:1 1.08:1 YesS12 Maximum IF VSWR Ratio 1.5:1 - 1.46:1 1.43:1 Yes

Confirmed as part of other Tests

Meet Spec ?

Performance SummarySpec

#Specification (metric)

Unit of Measure

Specifi-cation

Simulation Result

Measured Results

Confirmed as part of other Tests

Table 3: Performance Summary

The mixer performed well for the two units that were tested. No additional tuning, modifications, or enhancements were made to the assembled units. Their performance was very similar indicating that both the design and fabrication processes may be repeatable, but not a given considering the limited sample. Selected plots for Unit 1 can be seen in Figures 8-13. Simulated performance is either superimposed or alongside for comparison.

There were several slight differences between the measured and simulated performance. Some discrepancies to consider include LO to IF isolation, RF to IF isolation, and conversion loss. The measured LO to IF isolation was approximately 7-9 dB less than expected. This may have to do with the fact that the LPF may not roll off as sharply as simulated, allowing less rejection at the higher frequencies. This could be a consistent theme with the RF to IF

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Single-Balanced Mixer Project Page 3

isolation (3-5 dB lower than simulated) as well. Due to time constraints, a prototype of the filter itself was not produced and evaluated. This would be a relatively straightforward investigation and would be useful to look at in the future. The conversion loss in the mixer was slightly higher than modeled, which may have to do with connector losses which were not included in the model.

There are other factors that could contribute to not meeting simulated results such as process tolerances (both in purchased components and the final prototype), modeling limitations, and tolerances of the tolerances (estimated at approximately ±0.2 dB in uncertainty).

To further improve this mixer, adding band-pass filters to the RF and LO input paths would make this more practical for ultimate utilization in a system with non-ideal signal environments. It would also be useful to take time to model the connectors, RF chokes, and even the micro-strip runs in HFSS to get an accurate representation of their response to put back into the AWR model.

0

10

20

30

40

50

60

0.800 0.900 1.005 1.100 1.200Is

olati

on (d

B)

RF Frequency (GHz)

RF - IF Isolation - Unit #1

Measured Simulated

Figure 8: Conversion Loss Actual vs. Simulated Figure 9: RF to IF Isolation Unit 1

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Conv

ersio

n Lo

ss (d

B)

RF Input Power Level (dBm)

Conversion Loss vs. RF Input PowerUnit #1

0.8 GHz 0.9 GHz 1.005 GHz 1.1 GHz 1.2 GHz

Figure 10: 1dB Compression Unit 1 Figure 11: Spurious Output Levels Unit 1

0 1 2 3 4 4.85Frequency (GH z)

IF Output P o w e r S pe c trum

-80

-60

-40

-20

0

Po

wer

(d

Bm

)

0.7 GHz-77.27 dBm

0.3 GHz-72.65 dBm 0.85 GHz

-54.26 dBm

1 GHz-40.24 dBm

0.15 GHz-16.27 dBm

RF = 0.85 GHZLO = 1.0 GHz

Copyright © 2008 Rochester Institute of Technology

Unit #1IF Output SpectrumLO = 1000 MHzRF = 850 MHzHoriz. Scale: 200 MHz/div

Single-Balanced Mixer Project Page 3

Figure 12: Typical Measured IF Output Spectrum Figure 13: Simulated IF Output Spectrum

CONCLUSIONA single-balanced mixer was designed, fabricated, and evaluated for prototype purposes which complied with

all final customer specifications. This project considered several design configurations and trade-offs in order to produce a mixer that performed as desired. The prototype units passed all specifications, however it was concluded that further investigation in several areas would be valuable before finalizing the electrical design for a production version. Additionally, mechanical packaging of the mixer would be a key consideration in the design of the final product. This would take into account installation requirements (size, connector locations, mounting considerations, etc.), environmental concerns (temperature ranges, mechanical shock and vibration, etc.), and cost.

Finally, the project was successful in its goal of being a valuable learning and development experience for this future engineer.

REFERENCES

References:[1] Henderson, B., "Mixers: Part 1 Characteristics and Performance" Watkins-Johnson Application Notes, pp.752-

758.[2] Henderson, B., "Mixers: Part 2 Theory and Technology" Watkins-Johnson Application Notes, pp.759-766.[3] “Balanced and Double-Balanced Mixers” Anaren Application Notes, pp. 113-127.[4] “Hybrid Couplers” Anaren Application Notes, pp. 45-53.[5] Sayre, C., "Chapter 7: Mixer Design" Complete Wireless Design, pp.379-401.[6] Agilent Technologies Semiconductor Products Group, 2003. “A 5-6GHz Schottky Diode Single Balanced

Mixer.” White Paper pp 2-12.

ACKNOWLEDGMENTS I would like to acknowledge the employees at both Dielectric Labs and Anaren who helped immensely in the

success of this project. Specifically, I would like to thank Steve Randall and Manish Chugh of Dielectric Labs and Paul Stockwell, Chong Mei, Mark Bolan and Mark Burdick of Anaren.

Copyright © 2008 Rochester Institute of Technology