This document describes research on developing multi-functional and reconfigurable microwave control devices. It discusses a proposed novel broadband tunable rat-race coupler with increased bandwidth and tuning ratio, as well as a compact variable power divider design using integrated transformers for CMOS implementation. It also proposes a varactor-tuned variable attenuator design with wide tuning range and flat insertion loss for applications requiring signal power control. Measurement results demonstrate tuning capabilities and good performance over bandwidth for both designs.
2. Content
Motivation
Part 1 – Tunable hybrids and couplers
Literature review
Proposed Rat-race Coupler with Wide bandwidth
and Tunable Power Dividing Ratio
Proposed CMOS Variable Power Divider Design
Using Integrated Transformer
3. Content (con’t)
Part 2 – Tunable attenuator
Literature review
Varactor-based Microwave Attenuator with Wide
Tuning Ratio and Flat Insertion Loss
High Linearity Varactor-Based Variable Attenuator
Conclusion
4. Motivation
To improve channel capacity and transmission
quality of future communication systems, re-configurability
is an essential feature for
enhanced performance, size and cost reduction.
For example,
Beam steering
Polarization diversity
MIMO
Signal control devices (magnitude and phase)
with compact size and low cost are of prime
interest.
5. Part 1 – Tunable hybrid and
couplers
Basic requirements of tunable couplers:
Continuous tuning
Large tuning range (coupling coefficient)
Minimal control complexity (voltages and
components)
Compact size
Available bandwidth
Insertion loss
6. Literature review: tunable
devices
Based on directional couplers with variable
coupling
Modifying the characteristic impedance of
microstrip branches
7. Literature review (tunable
devices)
Based on Wilkinson Power Divider with
variable dividing ratio
DGS + tuning diodes: to realize transmission line
of variable characteristic impedance
Island Microstrip
Unequal Dividing Ratio [1 : N]
Bias Voltage (V)
8. Conventional Methods: Major
Drawbacks
Variation of insertion loss with frequency
Small tuning range
Poor return loss performance
Limited bandwidth
Complex fabrication (multi-layer, backside
etching)
9. Tunable Rat-race Coupler
(TRRC)
1 Port 2 Port
0 0 Z , 270 @ f
9
At center frequency:
Tunable power dividing ratio 퐾 =
푆43
푆23
= −
푆21
푆41
=
휔0퐶퐷푍0
Ideal port isolation and return loss performance
Single control voltage
CD
VC
Z0 , 90 @ f0
Biasing circuitry
CD
Port 4 Port 3
Cheng, K.M.; and Sung Yeung, "A Novel Rat-Race Coupler With Tunable Power Dividing Ratio, Ideal Port Isolation, and Return Loss Performance,"
IEEE Transactions on Microwave Theory and Techniques, vol.61, no.1, pp.55-60, Jan. 2013
10. K = 0.5
K = 1
K = 2
Simulated Performance (ideal)
10
S11
S22
S31
S21
S41
S23 Port 1
Port 3
Port 1
Port 3
Phase Difference (°)
11. Application Examples
11
Variable Power Divider
(anti-phase output)
Variable Power Divider
(in-phase output)
Variable attenuator
12. Proposed Tunable Rat-race Coupler (New)
Broadband operation (~40%)
Wide tuning ratio
Compact size
Simple control (Single voltage, only two tuning
diodes)
Simple fabrication (single layer)
N1 N2
Port 1
Port 4
Port 2
Port 3
CD
CD
13. Proposed Tunable Rat-race Coupler (New)
N1 and N2 are passive networks with specific
frequency characteristics (Frequency compensation)
dY dY K d bY
d d d Z
2 2
Z
e,1 o,1 0
0 0 0
0 0 0
dY dY
e,2
o,1
d d
0 0
dY dY
o,2
e,1
d d
0 0
N1 N2
Port 1
Port 4
Port 2
Port 3
CD
CD
Cheng, K.M.; and Chik, M.C., "A Novel Frequency Compensated Rat-Race Coupler With Wide Bandwidth and Tunable Power Dividing,"
IEEE Transactions on Microwave Theory and Techniques, vol.62, no.8, pp.55-60, August 2013
14. Circuit diagram and Prototype
Semi-distributed implementation
Avoid lossy lumped inductor
Lower assembly cost
Port1 Port 2
C V bias R
block C
D C
Port 3
D C
Port 4
, B Z
, A Z
, A Z 1 N
, A Z
2 N
Center frequency : 1 GHz
Substrate: Duroid RO4003C
Size: g/5 g/15
Tuning diode: Infineon
BB857
ZA = 86, ZB = 48,
= 30°, = 33°
15. Ideal simulation
Phase Difference (°)
K = 0.5
K = 1
K = 2
S11
S22
S31
S21
S41
S23
Port 1
Port 3
Port 1
Port 3
18. Short Summary
Novel broadband tunable rat-race coupler
Optimal design of N1 and N2 for broadband
operation (analytical formulation)
Increased Bandwidth (from 10% to 40%)
Semi-lumped implementation (internal loss
and size)
19. Proposed CMOS variable power
divider
For CMOS implementation, transmission line is
replaced of LC circuit.
For further size reduction (inductors),
transformer is introduced.
Port 1 Port 2
Port 4
Port 3
C
C
Port 1 Port 2
Port 3
kA
Chik, M.C., Li W.; and Cheng, K.M.; "A compact variable power divider design in CMOS process," Asia-Pacific Microwave Conference, November
2013
20. Simulation Results
kA = kB = 0 (inductor) kA = kB = 0.2
(transformer)
4 4.5 5 5.5 6
0
-2
-4
-6
-8
-10
Frequency (GHz)
(dB)
S
ij
S
21
S
41
S
23
K = 0.5
K = 1
K = 2
4 4.5 5 5.5 6
0
-10
-20
-30
-40
Frequency (GHz)
(dB)
S
ij
S
11
S
22
K = 0.5
K = 1
K = 2
4 4.5 5 5.5 6
0
-2
-4
-6
-8
-10
Frequency (GHz)
(dB)
S
ij
S
21
S
41
S
23
K = 0.5
K = 1
K = 2
4 4.5 5 5.5 6
0
-10
-20
-30
-40
Frequency (GHz)
(dB)
S
ij
S
11
S
22
K = 0.5
K = 1
K = 2
21. Circuit layout and Fabricated
chip
Center frequency: 5 GHz
Die size: 1.2mm × 0.8mm
Tuning ratio of varactor diode : 2 - 3
Vbias VCC
23. Measurement results
5
0
-5
-10
-1 -0.5 0 0.5 1
Control Voltage (V)
Power Dividing Ratio (dB)
Simulated
Measured
100
75
50
25
0
-1 -0.5 0 0.5 1
|2 (%)
31
|2 - |S
21
|2 - |S
11
1 - |S
Control Voltage (V)
24. Short Summary
24
Realization of TRRC in CMOS technology
Chip area reduction by using different transformer
Good performance over 10% fractional bandwidth
Tuning range: 9 dB
Port isolation: > 25 dB
Return loss: > 13 dB
Output phase difference deviation: < ± 5º
Tuning capability of power dividing ratio
Limited by small tuning capacitance ratio (< 3 typically)
of standard CMOS diodes
25. Part 2 – Variable Attenuator
Control of output power level (e.g. AGC)
Conventionally, PIN diodes are used as the
tuning elements
Biasing current required (DC power consumption)
Multiple diodes
Multiple control voltages
Limited tuning range (attenuation level)
Limited dynamic range (power-handling capability)
27. Proposed variable attenuator (New)
Variable power divider with 180° outputs
Power combiner
Varactor-tuned
Chik, M.C., and Cheng, K.M.; "A varactor-tuned variable attenuator design with wide tuning range and flat insertion loss response," International
Microwave Symposium, June 2014
28. Comparison
Attenuator with PIN diodes Proposed attenuator
DC Power
consumption
Increases with number of
diodes
Zero
Bandwidth Wide Moderate
Control method Multiple control voltages Single control voltage
Hybrid design with
varactors
Proposed attenuator
Variation with
frequency
Large Small
Tuning range
(Attenuation)
Increases with capacitance
ratio (tuning diode)
Independent of
capacitance ratio (tuning
diode)
29. Theory of operation: proposed
design
1 Port
Wilkinson
power
combiner
Broadband
Tunable
Rat -
race coupler
2 Port
A
B
AB
2
A2 B2 1
1 1 1
2 2 2 (1 )
21 BA CA
2
k
S S S
k
31. Circuit diagram and Prototype
Center frequency: 1 GHz
Substrate: Duroid RO4003C
Tuning diode: Infineon BB857
Port 1
Port 2
0 2Z
C
C V
bias R
block C
D C
, A Z
0 Z
0 2Z
g
4
1 Port
Port 2
block C block C
D C
32. Simulation and Measurement
Results
4 dB to 30 dB with a control voltage (reverse-bias)
ranging from 0 to 8.2V
Limited by non-ideal cancellation of signals
0 2 4 6 8 10 12 14 16 18 20
40
30
20
10
0
Attenuation Level (dB)
Control Voltage (V)
EM
Measured
34. Short Summary
Novel Varactor-based Variable Attenuator
Wide tuning (attenuation)
Simple structure
Single control voltage
Zero DC power consumption
Issues need to be addressed
Narrow-band (at large attenuation)
Attenuation is very sensitive to bias (control)
voltage
Limited power handling capability
40. Power performance (attenuator)
Attenuation level = 10 dB) Attenuation level = 25 dB)
0 5 10 15 20 25
20
0
-20
-40
-60
-80
-100
Output Power (dB)
Input Power (dB)
Fundamental
IMD
3
0 5 10 15 20 25
20
0
-20
-40
-60
-80
-100
Output Power (dB)
Input Power (dB)
Fundamental
IMD
3
f1 f2
Fundamental
IMD3
Attenuator
41. Nonlinearity Study
Tuning varactor is the major contributor of IMD
C
j
V
( ) 0
n
C V
1
2
0 1 2 C(v) C C v C v ....
C
j
V
n
C
C
1
0
0
n C
0 1
1
1
n
C
j
V
C
0
n n C
2
2 2
1
2 1
n
C
j
V
C
42. Nonlinearity Study
Output power (nonlinear current method)
P
2 2
1 1 1 2 1 in
OUT in
P A α C
C P
3 2 1 2 2 IMD in P C C P
Reduction in C1, C2
2
A
2
2 3
reduction of IMD and power expansion
43. Proposed linearization method
Original Design Proposed linearization
circuit C(VB)
CP
C(VA)
Additional capacitor with fixed value
Requires minimal modification of the original
design including both layout and choice of
components
44. Proposed linearization method
CP = 0 pF
CP = 1 pF
CP = 2 pF
CP = 2.7 pF
CP = 3 pF
CP = 0 pF
CP = 2.5 pF
VA VB
CD
Cheng, K.M.; and Chik, M.C., "A Novel Varactor-tuned Variable Attenuator Design With Enhanced Linearity Performance,"
IEEE Transactions on Microwave Theory and Techniques, submitted.
46. Circuit Diagram and Prototype
Center frequency: 1 GHz
Substrate: Duroid
RO4003C
Tuning diode: Infineon
BB857
0 2Z
C
CV
bias R
block C
Cp
DC
, A Z
0 Z
0 2Z
g
4
1 P ort
Port 2
block C block C
D C
Cp
Port 1
Port 2
47. Measured results
CP = 0 and CP = 2.2pF
40
20
Reduce attenuation sensitivity
IMD suppression
0
-20
-40
Power expansion improvement
Significant for large CP
0 5 10 15 20 25
0
-10
-20
-30
-40
(dB)
S
21
Bias Voltage (V)
C
P
= 0 pF
C
P
= 2.2 pF
Funndamental
IMD
0 5 10 15 20 25 30
-60
-80
Output Power (dB)
Input Power (dB)
3
C
P
= 0 pF
C
P
= 2.2 pF
0 5 10 15 20 25 30
40
20
0
-20
-40
-60
-80
Output Power (dB)
Input Power (dB)
Funndamental
IMD
3
C
P
= 0 pF
C
P
= 2.2 pF
48. Short Summary
Novel linearization method
Simple to apply
Attenuation level is much less sensitive to control
voltage
Substantial reduction in IMD
49. Conclusion
Several new microwave control devices have
been introduced:
Broadband rat-race with tunable power dividing
ratio
CMOS implementation of variable power divider
Varactor-tuned variable attenuator with high
linearity
They offer enhanced performance:
Wide tuning capability
Wide bandwidth
50. Author’s Publication List
Journal Paper
K. K. M. Cheng, and M. C. J. Chik, “A frequency-compensated rat-race coupler with
wide bandwidth and tunable power dividing ratio,” IEEE Trans. Microw. Theory &
Techn., vol. 61, no. 8, pp. 2841-2847, Aug. 2013.
M. C. J. Chik, and K. K. M. Cheng, “Group delay investigation of rat-race coupler
design with tunable power dividing ratio,” IEEE Microw. Compon. Lett., vol. 24, no. 5,
pp 324-326., May 2014.
K. K. M. Cheng, and M. C. J. Chik, “A varactor-based variable attenuator design with
enhanced linearity performance,” IEEE Trans. Microw. Theory & Techn. (Submitted)
M. C. J. Chik, and K. K. M. Cheng, "A varactor-based variable attenuator with
extended bandwidth by frequency compensation" (In preparation)
Conference Paper
M. C. J. Chik, and K. K. M. Cheng, “A low-profile, compact, mode-decomposition
based antenna array for use in beam-forming application,” 2012 Asia-Pacific Microw.
Conf. Proc., Kaosiung, 2012, pp. 58-60, Dec. 2012.
M. C. J. Chik, W. Li, and K. K. M. Cheng, ‘A 5 GHz, integrated transformer based,
variable power divider design in CMOS process’, in 2013 Asia-Pacific Microw. Conf.
Proc., Seoul, 2013, pp. 366 – 368., Nov. 2013.
M. C. J. Chik, and K. K. M. Cheng, “A novel, varactor-based microwave attenuator with
wide tuning ratio and flat insertion loss response,” presented in Proc. Int. Microw.
Symp. 2014., Tampa Bay, USA., Jun. 2014.
L. P. Cai, M. C. J. Chik, and K. K. M. Cheng, “A compact, linearly-polarized antenna
design with electronically steerable angle of orientation,” 2014 Asia-Pacific Mrcow.
Conf. (Submitted)
Hinweis der Redaktion
Good morning. Today, I would talk about my research in multi-functional and re-configurable microwave control devices.
This is the content of today’s presentation. First, I will brief present the motivation in working this topic. My research is split into two parts, both are related to re-configurable devices. The first one is about tunable hybrids and coupler. I will give a literature review on this topic followed by two developed circuits, namely rat-race coupler with wide bandwidth and tunable power dividing ration and a CMOS variable power divider design using integrated transformer.
In Part 2, the focus is on tunable attenuator. Some typical tunable attenuators design will be introduced, mainly to highlight their characteristics. Then a new varactor based attenuator is proposed. For attenuators in power application, the power handling performance remains a concern. Therefore the power performance of the proposed attenuator is investigated and linearization method is proposed accordingly to enhance the power handling capability. Some concluding remarks will be given at last.
Wireless communication becomes more and more important in recent years. Wireless networks act as a supplement or even replacement of wired network. Many new applications employ wireless technology, such as wireless sensing, smart home, and so on. Due to the proliferation of smartphones and tablets, cellular systems have experienced tremendous growth over the last decade. The emerging 4G technology provides high-speed communication which allows users to transmit/receive large volume of data in a short period of time. Services like video on-demand become realizable. [2] However, these terminals cannot operate at optimal speed due to polarization loss, interference from nearby base stations, and multi-path fading effect. The ever-increasing speed and connectivity is accomplished by the development of sophisticated communication systems. In modern communication systems, to acquire enhanced performance, compact size or even cost reduction, most front-ends would provide re-configurability as a important feature. Typical applications which incorporated re-configurability are beam steering which involve tunable power dividing ratio and phase, polarization diversity in antenna which also consider flexible power distribution. Automatic gain control make use of variable attenuation to control the output power.
When we talk about tunable hybrid and couplers, which property is variable? Obviously it is the power dividing ratio or coupling factor for couplers. And tuning can be discrete or continuous. Discrete tuning has limited applications therefore it is not under today’s scope of discussion. Continuous tuning in power dividing ratio or coupling factor are of greater interest. However, for continuous tuning, we usually encounter design issue such as complicated tuning mechanism and limited tuning range. In following slides, some typical design of tunable couplers will be briefly discussed.
Here shows a conventional design which makes use of a branch-line coupler and varactor inserted into two branches. By varying the capacitance of varactors, the characteristic impedances of microstrip branches are modify, and the coupling ration is altered as a result. The coupling ratio varies about 2dB over 1GHz. However, not shown here, this design would possess poor return loss due to unmatched input impedance, which would limits its useable bandwidth.
Another design attempted to realize microstrip line with variable characteristic impedance (30Ω to 100Ω) for the provision of tunable power dividing ratio. A defected ground structure is added underneath the transmission line. By varying the bias voltage of the varactor diodes, it is possible to change the effective inductance of the structure, and subsequently the characteristic impedance of the DGS line. Ultimately, the power dividing ratio can be varied. Practically, a tuning range of 8dB was achieved. However, proper terminations (R2 and R3) are needed for different power dividing ratios. Return loss performance may degrade if the terminations are fixed.
Another design attempted to realize microstrip line with variable characteristic impedance (30Ω to 100Ω) for the provision of tunable power dividing ratio. A defected ground structure is added underneath the transmission line. By varying the bias voltage of the varactor diodes, it is possible to change the effective inductance of the structure, and subsequently the characteristic impedance of the DGS line. Ultimately, the power dividing ratio can be varied. Practically, a tuning range of 8dB was achieved. However, proper terminations (R2 and R3) are needed for different power dividing ratios. Return loss performance may degrade if the terminations are fixed.
Rat-race couplers are being employed extensively in microwave systems, for example, balanced-mixers and feeding network for antenna array. Many researches has been made to enhance its performance. A recently proposed rat-race coupler demonstrates excellent performance at center frequency. It consists of two tuning diodes (variable capacitor) and two transmission line sections of 90° and 270°. It offers tunable power dividing ratio which depends on the capacitance of tuning diode CD instead of the characteristic impedance ratio between branch-lines. Therefore, the power dividing ratio is solely limited by the range of capacitance achieved. Also, ideal port isolation and return loss performance is observed at center frequency. Only a single control voltage is needed to tune the power ratio.
Here shows the simulated performance of the reported rat-race. Ideal return loss and port isolation at center frequency is observed for all power dividing ratios. It demonstrates ideal phase difference characteristic for both input ports, that is 180deg and 0deg. However, the phase difference quickly deviates from ideal value which is not favourable. the power dividing ratio deviates from desired value over frequency. The practical fractional bandwidth based on +/-0.5dB is only about 10%.
The tunable rat-race coupler is multi-functional. This slide shows some application examples and its corresponding connection. It can used as a variable power divider when one of input ports is terminated by reference impedance. When port 3 is terminated by Z0, the device would give anti-phase output; when port 1 is terminated by Z0, it would give in-phase output. When two ports are termination by Z0, it becomes a variable attenuator.
It should be noted that the serial line in N1 and shunt stubs in N2 are having the same characteristic impedance ZA and electrical length . This will enable the odd-mode admittance of N1 to be equal to the even-mode admittance of N2 (i.e. Yo,1 = Ye,2).
For illustration, simulated response of the network using ideal components are presented. Two values of k, 0 and 0.2 are chosen to demonstrated the effect of coupling within the differential inductor. The upper graph is the insertion loss and the bottom one is the return loss. The left column shows the response when there is zero coupling and the right one shows results when the coupling factor equals 0.2. We can see that the coupling would lead to variation in insertion losses especially in higher frequency band. Also, the return loss and isolation bandwidth becomes narrower although they still remained to an acceptable level. In short, the larger the coupling, the narrower the fractional bandwidth.
The proposed device is prototyped at 5 GHz. The left diagram is the circuit layout and the right one is the micrograph of the fabricated chip. The die size including both bond pads and biasing circuitry is about 1.2mm x 0.8mm, which is compact. These two are the differential inductors employed. The tuning ratio of the varactor is about 2-3. The corresponding use for each bond pad is labeled. P1,2,3 correspond to Port1,2,3 and G stands for ground. Vbias is the control voltage and Vcc is for biasing the guard ring for better noise protection.
These are the measurement results of the fabricated chip for K=0.5. There is about 1dB difference between simulated and measured insertion losses S21 and S41. The measured results follows the trend of simulated results. The phase deviation is about +/-5deg from 4.5GHz to 5GHz. More than 13dB Return loss and 20dB port isolation are achieved. However, the S21 is not as flat as expected. It is believed that the coupling within the transformers lead to this variation which is not favourable.
At center frequency, the device exhibits power dividing ratio ranging from -6dB to 3dB with a control voltage of +/-0.5V. Referring to the upper graph, there exist small discrepancy between simulated and measured performance. It is observed from the bottom graph, the internal loss of the device is approximately 35% of incident power. This is mainly caused by the series resistance of the tuning diode and the conductor loss of the transformers.
The discrepancies between e simulated and measured results were mainly attributed to the process variation and inaccurate device model used in simulation. Good performance is observed over 10% fractional bandwidth with 9dB tuning range, port isolation better than 25 dB, return loss betters than 13dB, and output phase deviation smaller than +/-5deg.
Recalling the range of power dividing ratio K is determined by the capacitance of tuning diode. Standard CMOS diodes can only provide a small tuning capacitance ratio, smaller than 3 typically. As a result, this ratio limits the tuning capability of power dividing ratio of the device.
Upon measurement, the fabricated device demonstrated 4dB to 30dB attenuation with a control voltage ranging from 0 to 8.2V.
For comparison purposes, two sets (VC = 0.66V and 7.12V) of measured results were given. In both cases, excellent performances in insertion loss flatness and return loss were achieved (in the vicinity of 0.93 GHz). At a control voltage of 0.66V (7.12V), the variable attenuator was found to exhibit an insertion loss of 5 dB (20dB) and minimum return loss of 13 dB (18 dB), over a fractional bandwidth of about 20%.
To conclude a novel varactor-based variable attenuator is introduced, featuring wide tuning capability in attenuation, simple structure, single control voltage as well as zero DC power consumption. However, there are also some issues that should be addressed. The bandwidth of attenuator gets narrower at the attenuation goes up. The attenuation variation is very sensitive to bias voltage at large attenuation. Slight change in voltage would lead to large change in attenuation which is not favourable.
Based on s-parameter measurement, the attenuation level variation is extracted over 0 to 20V. It covers the attenuation range from about 5dB to 23dB. The discrepancy is mainly attributed to the phase unbalance of two signals, resulting incomplete cancellation.
Being mostly employed in power applications, the IMD performance of the attenuator is worthwhile to be investigated. Conventional two-tone measurement is adopted for the characterization of IMD performance. This is the diagram of the setup. Two tones centered at 950MHz with 20MHz spacing are used. This two diagrams compare the fundamental and IMD output power. It can be observed that the third-order power becomes more significant as the attenuation level increases. In other words, the unwanted distortion is more severe if we use large attenuation. It is ironic that we usually need high attenuation for a high signal power. This characteristic may hinder the application of this attenuator. Further research will be focus on linearizing the IMD performance using circuit approach.