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.

1. MULTI-FUNCTIONAL AND
RE-CONFIGURABLE
MICROWAVE CONTROL
DEVICES
CHIK Man Chum, Jonathan
Department Electronic Engineering

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

16. Measurement Results (K = 0.5)
0
-10
-20
-30
-40
0
-2
-4
-6
-8
-10
| (Measured) |S
| (Measured) |S
| (Measured) |S
| (Measured) |S
| (EM)
| (EM)
| (EM)
| (EM)
| (Measured) |S
| (EM)
| (Measured) |S
| (EM)
| (Measured) |S
| (EM)
0.75 0.875 1 1.125 1.25
Frequency (GHz)
(dB)
(dB)
S
ij
|S
21
21
|S
41
41
|S
23
23
|S
43
43
200
180
160
0.75 0.875 1 1.125 1.25
Frequency (GHz)
S
41
- S
21
(Measured)
S
41
- S
21
(EM)
20
0
-20
0.75 0.875 1 1.125 1.25
Frequency (GHz)
Phase Difference ()
S
43
- S
23
(Measured)
S
43
- S
23
(EM)
0.75 0.875 1 1.125 1.25
Frequency (GHz)
S
ij
|S
11
11
|S
22
22
|S
31
31

17. Measurement Results
/S
| (EM)
/S
| (EM)
/S
| (Measured)
/S
| (Measured)
1 4.5 8 11.5 15
10
5
0
-5
-10
Control Voltage (V)
Power Dividing Ratio (dB)
|S
21
41
|S
43
23
|S
21
41
|S
43
23

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

22. Measurement Results (K = 0.5)
| (Simulated)
| (Simulated)
| (Measured)
| (Measured)
4 4.5 5 5.5 6
0
-2
-4
-6
-8
-10
Frequency (GHz)
(dB)
S
ij
|S
21
|S
31
|S
21
|S
31
4 4.5 5 5.5 6
200
180
160
Frequency (GHz)
Phase Difference ()
S
41
- S
21
(Simulated)
S
41
- S
21
(Measured)
| (Simulated) |S
| (Simulated) |S
| (Simulated)
| (Measured) |S
| (Measured) |S
| (Measured)
4 4.5 5 5.5 6
0
-10
-20
-30
-40
(dB)
S
ij
Frequency (GHz)
|S
11
22
33
|S
11
22
33
| (Simulated)
| (Measured)
4 4.5 5 5.5 6
0
-10
-20
-30
-40
Frequency (GHz)
(dB)
S
ij
|S
23
|S
23

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)

26. Literature Review: variable
attenuators
PIN diode based Varactor
based

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

  


30. Simulated performance

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

33. Simulation and Measurement
Results
0
-5
-10
-15
-20
-25
-30
VC = 0.66V
VC = 7.12V
0.8 0.85 0.9 0.95 1 1.05 1.1
| (dB)
21
|S
Frequency (GHz)
0
-10
-20
-30
-40
|S11|, VC = 7.12V
|S22|, VC = 7.12V
|S11|, VC = 0.66V
|S22|, VC = 0.66V
0.8 0.85 0.9 0.95 1 1.05 1.1
Reflection Coefficient (dB)
Frequency (GHz)

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

35. Issues: Narrowband operation
 Insertion loss flatness degrades with
increasing attenuation  Smaller bandwidth

36. Issues: Narrowband operation
S21 of TRRC
S21 of Wilkinson
power combiner
S41 of TRRC
S31 of Wilkinson
power combiner

37. Proposed solution (Frequency
compensation)
 WPC is replaced by rat-race coupler with k = 1
S21 of TRRC S23 of RRC
S41 of TRRC S43 of RRC

38. Circuit diagram and Prototype
 Broadband rat-race coupler (TRRC)
 Fixed rat-race coupler
Port 1
VC
Cblock
Rbias
Cblock
C
ZA, 
ZB, 
Z0
Z0
C
Port 2
C
C
Center frequency: 1 GHz
Substrate: Duroid RO4003C
Tuning diode: Infineon BB857

39. Measured results
0
-10
-20
-30
-40
|S22|, VC = 7.2V
0.8 0.85 0.9 0.95 1 1.05 1.1
Reflection Coefficient (dB)
Frequency (GHz)
0
-5
-10
-15
-20
-25
-30
| (dB)
21
0 2 4 6 8 10 12 14 16 18 20
40
30
20
10
0
Attenuation Level (dB)
Control Voltage (V)
Measurement
Simulation
|S11|, VC = 4.5V
|S11|, VC = 7.2V
|S22|, VC = 4.5V
VC = 4.5V
0.8 0.85 0.9 0.95 1 1.05 1.1
|S
Frequency (GHz)
VC = 7.2V

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.

45. Simulation Results
20
0
-20
-40
-60
-80
-100
-120
0 5 10 15 20 25
Output Power (dB)
Input Power (dB)
Fundamental
IMD3
CP = 0 pF
CP = 1 pF
CP = 2 pF
CP = 2.7 pF
CP = 3 pF

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)