International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
Single-ended FET Mixer Design for 36MHz
Bandwidth C-band Satellite Transponder
Yi Yi Aye, Chaw Myat Nwe

Abstract—The design and performance of an active
single-ended FET mixer is presented. A single gate GaAs
MESFET (Gallium Arsenide Metal Epitaxial Semiconductor
FET) mixer is used for 36MHz bandwidth C-band satellite
transponder between 6GHz and 4GHz frequency down
conversion. The mixer is designed 6GHz RF input signal
frequency with 2GHz local oscillator (LO) frequency and 4GHz
IF (intermediate frequency) frequency. In this mixer design, DC
biasing, input and output matching are considered. The IF filter
is designed at the output of mixer to get the desired frequency
and reject the unwanted signal frequency. IF filter is designed
with 5th order Chebyshev response. Agilent’s Advance Design
System (ADS 2009) software has been used for simulation and
optimization of the circuits.
Index Terms—single-ended mixer, GaAsFET, DC biasing,
Input/output matching and IF filter.
I. INTRODUCTION
Mixers are key devices for front-end components in any
transceiver of a communication system. A mixer is a
three-port device that uses a nonlinear or time varying
element to achieve frequency conversion. Firstly choose
passive or active and down or up frequency conversion. The
down conversion mixer is used to convert the RF signal down
to an intermediate frequency by mixing the RF signal from the
Low Noise Amplifier (LNA) with the local oscillator (LO)
signal. Active mixers can supply a conversion gain instead of
loss. They require less LO drive power and much less
sensitive to port terminations. Active mixers also possess a
more noise figure than passive mixer [1].
this paper, mixer and filter designs are presented. The input of
frequency converter is low noise amplifier (LNA) and the
output is high power amplifier. The input signal frequency is
6GHz and 2GHz (LO frequency) at the output of (LNA). The
IF filter is designed at the output of mixer at 4GHz frequency.
II. PROCEDURE FOR MIXER DESIGN
A. Design Selection
Mixer types are single-ended (unbalance), single balance,
double- balance and image rejection. Single-ended mixer is
presented in this paper. Single-ended mixer type can have
spurious outputs than other types. This type is simple, low
cost, low power consumption and low isolation. Balance
mixers have high power, high isolation and additional
components [2].
B. Component Selection
Microwave mixer can be designed by using Schottky
barrier diode or FET, either MESFET or HEMT. Using a FET
rather than a diode as the non-linear element in a mixer has
several advantages. Some of these include the possibility of
achieving a conversion gain, using lower LO drive power and
obtaining isolation between the signal ports of the FET. This
mixer design uses Gallium Arsenide Metal Semiconductor
FET (GaAsMESFET) component. The MESFET is biased by
the two sources, the drain-to-source voltage and the
gate-to-source voltage. These voltages control the channel
current by varying the width of the gate-depletion region and
the longitudinal electric field. This device provides low noise
and higher gain in established solid state application. It also
provides high frequency characteristics unavailable from
bipolar transistors. The electron mobility of gallium arsenide
is five to seven times than of silicon [3].
III. MIXER PERFORMANCE PARAMETERS
A. Conversion Loss
Fig.1 Block diagram of C-band satellite transponder
C-band satellite transponder model is shown in Fig.1. Band
Pass Filter, Low Noise Amplifier, Frequency Converter and
High Power Amplifier sections are included in this model. In
Yi Yi Aye, Department of Electronic Engineering, Mandalay
Technological University, Mandalay, Myanmar, (yiyiaye57@gmail.com),
Myanmar, +95-94300272.
Chaw Myat Nwe, Department of Electronic Engineering, Mandalay
Technological University, Mandalay, Myanmar,+95-9259034924,
(chawmyatnwe77@gmail.com).
Mixer design requires impedance matching at three ports,
complicated by the fact that several frequencies and their
harmonics are involved. Each mixer port would be matched at
its particular frequency (RF, LO, or IF), and undesired
frequency products would be absorbed with resistive loads, or
blocked with reactive terminations. There are inherent losses
in the frequency conversion process because of the generation
of undesired harmonics and other frequency products. An
important figure of merit for a mixer is therefore the
conversion loss, which is defined as the ratio of available RF
input power to the available IF output power, expressed in dB.
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All Rights Reserved © 2012 IJSETR
International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
Conversion loss applies to both up-conversion and
down-conversion.
Practical diode mixers typically have conversion losses
between 4 and 7 dB in the 1–10 GHz range. Transistor mixers
have lower conversion loss, and they may even have
conversion gain of a few dB. The conversion gain of the FET
mixer can be found as
I D  I DSS (1 
VGS 2
)
Vp
(2)
VDD =5V (supply voltage)
I DSS =60mA and Vp =-2V (from datasheet)
VDS =3V, I D =30 mA
2
Gc 
gm R d
2
2
4 ω RF Cgs R i
(1)
VGS  VP (1 
The quantities g m , R d , R i and C gs are all parameters of the
selected FET. The
g m value is 60mS (max) and 20mS (min),
R d is 2Ω and R i is 2 Ω for selected component in this
design. Conversion gain is -6dB (min) and 3dB (max). One
factor that strongly affects conversion loss is the LO power
level; minimum conversion loss often occurs for LO powers
between 0 and 10 dBm. This power level is large enough that
the accurate characterization of mixer performance often
requires nonlinear analysis.
ID 2
)
I DSS
(3)
VGS =-0.6V
VDD  R D I D  VDS
(4)
R D =66 Ω
R G = 100 K Ω
B. Noise Figure
Noise is generated in mixers by the diode or transistor
elements, and by thermal sources due to resistive losses.
Noise figures of practical mixers range from 1 to 5 dB, with
diode mixers generally achieving lower noise figures than
transistor mixers. The noise figure of a mixer depends on
whether its input is a single-sideband signal or a double
sideband signal [4].
C. Port Isolation
The isolation between LO and RF of the mixer is important
as LO-to-RF feedthrough results in LO signal leaking through
the antenna. The leaked LO signal should be small enough to
avoid corrupting the desired signals of other RF systems.
LO-to-IF and LO-to-RF isolation are not important because
the high-frequency feedthrough signals can be rejected by the
high-Q IF filter easily [5].
Fig.2 Schematic diagram of DC biasing
B. For input and output matching
In the input matching, RF and LO ports matching are
designed at 6GHz and 2GHz. The input impedance can be
estimated from S-parameters as follows:
Γ in  S11 
IV. DESIGN CALCULATION
S21S12
1  S22
(5)
There are several FET parameters that offer nonlinearities
that can be used for mixing, but the strongest is the
transconductance g m , when the FET is operated in a common
source configuration with a negative gate bias. When the gate
bias is near the pinch-off region, where the transconductance
approaches zero, a small positive variation of gate voltage can
cause a large change in transconductance, leading to a
nonlinear response. Thus the LO voltage can be applied to the
gate of the FET to pump the transconductance to switch the
FET between high- and low-transconductance states, thus
providing the desired mixing function.
The S-parameters of the transistor for 6 GHz (RF frequency)
and 2GHz (LO frequency) are selected and input impedances
are calculated. For RF matching,
Γ in =0.81<-99.8
A. For DC Biasing
Generally, two methods (dual power source and self-bias)
can be used to bias a GaAs FET. Dual power source method is
used in this design for DC biasing. This method is appropriate
for use in the higher frequencies. When directly connecting
the source to the ground terminal, source inductance can be
made relatively small. By using this method, higher gain can
be obtained and a lower noise factor anticipated in the higher
frequencies [6].
Zin =0.32-2.8j.
Zin 
1  Γ in
1  Γ in
(6)
Zin =0.2-0.8j
For LO port matching,
Γ in =0.93<-39.39
The values are set on combined Smit-Chart and series and
shunt reactive components are read from Chart. The actual
component values are gained by using equation (7) and (8).
1
ωXN
N
L1 
ωB
C1 
(7)
(8)
2
All Rights Reserved © 2012 IJSETR
International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
Where,
ω  2 πf
X =the reactance as read from the chart
B =the susceptance as read from the chart
N =the number used to normalize the original impedances
that are to be matched
The series capacitor and shunt inductor values for RF port
matching are 0.53pF and 1.1nH. C=0.88pF and L=11.36nH
are the impedance values for LO port matching.
The output impedance can be estimated from S-parameters
as follows:
Γ out  S22 
S21S12
1  S11
R D are used for DC bias. L1 , C1 and L 3 , C3 are
used for input matching. L 2 and C2 are used as output
matching. The extra capacitors C 4, and C 5 are used IF short
resistance
and bias. Supply voltage
VDC is 5V. A capacitor C6 is used
at IF output port to suppress the high frequency feedthrough
signals. At mixer output, IF frequency is (RF freq-LO freq)
and IF filter is designed at 4GHz. The expected results are
gained by designing IF filter at mixer output.
(9)
The S-parameters from the datasheet of the selected
transistor at 4 GHz (IF frequency) are achieved and calculate
the output impedance.
Γ out =0.72  -19.7
Z out 
1  Γout
1  Γout
(10)
Zout =2.96-2.7j
This value is set on Smit chart. Series inductance can be
read from Z-chart and shunt capacitance can be get from
Y-chart. The actual component values are found using eq (11)
and eq (12). This completes the out put matching network.
L2 
XN
ω
(11)
=4.38nH
C2 
B
ωN
(12)
Fig.4 Complete Mixer Circuit with Bias and matching circuit
C. IF filter design
IF filter is designed at mixer output. This filter is designed
with 5th order Chebyshev response. In this design, centre
frequency is 4GHz, 0.5dB ripple and 500MHz bandwidth.
=0.44pF
Firstly the impedance ratio (
output. Load resistance
Fig.3 Input and Output matching circuit diagram
The schematic diagram for matching is shown in Fig.2.
The shunt inductor L1 and series capacitor C1 are used for
RF matching. The values are 1.1nH and 0.53pF respectively.
At the output, the shunt inductor does short the RF signal
feed-through. For output matching, establish an equivalent
LC parallel circuit with L 2 and C2 . At IF frequency, the
values are 4.38nH and 0.44pF [7]. For LO matching,
L 3 =11.36nH and C 3 =0.88pF are used at the gate of the
transistor.
In the complete mixer circuit, DC bias tees, input and output
matching circuit are included. Gate resistance R G , drain
Rs
) is gained at the mixer
RL
R L is 50 Ohm and R s is the source
impedance from the output of mixer. The element values of
Chebyshev low-pass prototype are gained from table 3-6B in
“RF Circuit Design”(second edition) book [8]. The actual
transformation from the low-pass to the band-pass
configuration is accomplished by resonating each low-pass
element of the opposite type and the same value. All shunt
elements of the low-pass prototype circuit become parallel
resonant circuits, and all series elements become
series-resonant circuits [8].
To complete the filter design, the transformed filter is then
scaled using the following formulas. For the parallel-resonant
branches,
C
Cn
2 πRB
(13)
L
RB
2
2 πf 0 L n
(14)
For the series-resonant branches,
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All Rights Reserved © 2012 IJSETR
International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
C
B
2
2 πf 0 C n R
(15)
L
RL n
2 πfB
(16)
power is (-10dBm) and maximum is +10dBm. The mixer
performances (Conversion gain, port-to-port isolation and
output spectrum) are shown with RF frequency and LO power
sweep.
Where, in all cases,
R=the load impedance,
B=the 3-dB bandwidth of the final design,
f 0 =the center frequency of the final design,
Ln =the normalized inductor band-pass element values,
Cn = the normalized capacitor band-pass element values.
Fig.8 Mixer complete circuit without IF filter
Fig.5 Fifth-order Chebyshev Low pass prototype
Fig.6 Fifth-order Chebyshev Band-pass prototype
Fig.7 Fifth-order Chebyshev Bandpass Filter for 4 GHz
Center frequency
V. SIMULATION RESULTS
The present design is simulated using the Advanced
Design System 2009. An ADS (Advanced Design System) is
a fast general purpose RF and microwave circuit design.
Harmonic balance simulation makes possible the simulation
of circuits with various types.
Out put spectrums of mixer without IF and with IF filter
are shown in Fig.9. In mixer output spectrum include spurious
signal outputs and neglect them. Conversion gains with RF
frequency and LO power sweep are described in Fig. 11(a)
and (b). The maximum conversion gain of single-ended FET
mixer can gain 2dB with RF power of -20dBm. The
calculated conversion gain is between -6dB and 3dB, so this
simulated result is good result. The results of Port-to-Port
isolation versus RF frequency and LO power are described in
Fig.12 (a) and (b). Port-to-Port isolation (dB) between RF and
IF is between 10 and 5dB. LO-to-RF and LO-to-IF isolation is
between -10 and -20 dB. Isolation of single-ended mixer type
is lower than balance mixer design. This isolation results are
satisfied.
In the simulation, RF frequency range is (5 to 6) GHz with
-20dBm power and LO frequency is 2 GHz. Minimum LO
Fig.9 Mixer output spectrum without and with IF filter
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All Rights Reserved © 2012 IJSETR
International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
Fig.12(b) Port-to-Port isolation with RF frequency
Fig.10 Mixer complete circuit with IF filter
VI. CONCLUSION
A single gate GaAs MESFET mixer using a NE72000
microwave transistor was designed. The performances of
mixer were presented with RF frequency and LO power.
Calculated conversion gain of single-ended FET mixer with
RF frequency is between -6dB and 3dB. Down conversion
gain versus RF frequency obtained from simulation is
between 2 and -2dB. So simulation results are found with
good performances. The performance of the mixer was
simulated and compared with the required specifications.
Good agreement was found.
Fig.11(a) Mixer conversion gain versus RF frequency
Typically Port-to-Port isolation of single-ended mixer type
is poorer than balance type. In this design, good isolation is
found between RF-to-IF port. So port-to-port isolation is also
good performance.
ACKNOWLEDGMENT
The author would like especially thank to Dr.Chaw Myat
Nwe, for her encouragement and suggestions. The author
wishes to express her special thanks to, Dr. Kyaw Soe Lwin
for his kindness, helpful suggestions for this paper. I would
like to thank all teachers in MTU. Especially, I would like to
express my special thanks to my parents for their noble
support and encouragement.
REFERENCES
[1]
Nuriha Abd Rahman, Burhanuddin Yeop Majlis, “A GaAs PHEMT
single-ended mixers for 28 GHz applications”, The 4th Annual
Seminar of National Science Fellowship 2004.
[2]
steve Long, “Fundamentals of Mixer Design” Agilent EEsoft customer
Fig.11(b) Mixer conversion gain with LO power
[3]
[4]
[5]
[6]
[7]
[8]
Fig. 12(a) Port-to-Port isolation with LO power
Education and Applications, Design Seminar, April 2001.
Stephen A. Maas, “Nonlinear Microwave and RF Circuits”, Second
Edition.
David M.Pozar, University of Massachusetts at Amherst “Microwave
Engineering” Fourth Edition.
Keng Leong Fong, Member, IEEE and Robert G. Meyer, Fellow,IEEE,
“Monolithic RF Active Mixer Design,” IEEE Transaction on circuits
and Systems-II: Analog and Digital signal processing,
vol.46,No.3,March 1999.
Application Note, “Application of Microwave GaAs FETs”,
California Eastern Laboratories.
Esmat A.F. Abdalah, “Computer Aided Analysis and Design of Single
Gate MESFET Mixer”, Electronics research Institute, Dokki,
Cairo,Egypt.
Chris Bowick with John Blyler and Ajluni, “RF Circuit Design”
Second Edition.
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