1. A Novel, Varactor-based Microwave
Attenuator with Wide Tuning Ratio
and Flat Insertion Loss
Man-chum J. Chik, Kwok-keung M. Cheng
Department of Electronic Engineering,
The Chinese University of Hong Kong
TU1C-2
3. TU1C-2 IMS2014, Tampa, 1-6 June, 2014 3
Introduction
• Re-configurability is an important feature for
increased adaptability, size and cost reduction
• Conventional Tunable Parameters
– Power dividing ratio
– Transmission phase
– Attenuation level (Insertion loss)
• Attenuators’ application
– Automatic gain control
– Circuit protection
– Improve impedance matching
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Variable attenuator
• Typical Design:
– T- / Pi-networks with PIN diodes
• Excessive DC power consumption
• Complex control circuitry
– Shunt-mounted PIN diodes with /4 spacing
• Small tuning range
• Degraded return loss
– Branch-line coupler in reflection-mode
• Degraded return loss
• Severe attenuation variation over frequency
/4@f0
5. TU1C-2 IMS2014, Tampa, 1-6 June, 2014 5
Proposed Variable Attenuator
• Features
– Wide tuning range
– Flat insertion loss
– Good return loss performance
– Zero DC power consumption
– Single control voltage
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Basic Working Principle
• A variable power divider with 180° output
• A Wilkinson power combiner
1Port
combiner
power
Wilkinson
outputswith
dividerpower
Variable
180
2Port
B
A
2
A B2 2
1A B
21
21
0
1
1, 0
2
A B S
A B S
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Variable Power Divider
• A variable power divider with 180° outputs
– Realized by reported broadband tunable rat-
race coupler
DC
DC
C
,AZ
,BZ
,AZ
,AZCV biasR
blockC
1N 2N
1Port 2Port
3Port
0Z
K. K. M. Cheng, and C. M. Chik, “A frequency-compensated rat-race coupler with wide
bandwidth and tunable power dividing ratio,” IEEE Trans. Microwave Theory & Tech., vol. 61, no.
8, pp. 2841-2847, Aug. 2013.
8. TU1C-2 IMS2014, Tampa, 1-6 June, 2014 8
Variable Power Divider
• Features
– Broadband (~40%)
– Wide tuning ratio
– Good return loss and low insertion loss
– Compact size
– Simple biasing circuitry
21
0 0
31
D
S
k ω C Z
S
Biasing circuitry
Varactor
Capacitor
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Variable Power Divider
k = 1
k = 5
k = 1
k = 5
S21
S41
S21
S41
S11
S22
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Insertion loss (Attenuation level)
• Perfect cancellation
k = 1 S21 = 0
• Minimum attenuation (3 dB)
k = 0(∞) S21 = 1/ 2
• Ideal attenuation range: 3 dB – ∞ dB
21
2
1
2 (1 )
k
S
k
1Port
combiner
power
Wilkinson
outputswith
dividerpower
Variable
180
2Port
B
A
2
A B2 2
1A B
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Simulated Ideal Performance
S11
S22
k = 1.2
k = 2
k = 1.2
k = 2
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Circuit diagram and Prototype
• Center frequency: 1 GHz
• Substrate: Duroid RO4003C
• Tuning diode: Infineon BB857
02Z
C
CV
biasR
blockC
,AZ
0Z
02Z
4
g
1Port
2Port
blockC blockC
DC
DC
Port 1
Port 2
13. TU1C-2 IMS2014, Tampa, 1-6 June, 2014 13
Simulation and Measurement Results
• Attenuation versus bias voltage
– 4 dB to 30 dB with a control voltage (reverse-
bias) ranging from 0 to 8.2V
0 2 4 6 8 10 12 14 16 18 20
0
10
20
30
40
AttenuationLevel(dB)
Control Voltage (V)
EM
Measured
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Conclusion
• Novel Varactor-based Variable Attenuator
– Wide tuning capability in attenuation
– Simple structure
– Single control voltage
– Zero DC power consumption
• Limitation
– Very Sensitive to bias voltage at large attenuation
– IMD performance gets worse at large attenuation
– Narrow-band at large attenuation (10%)
Good morning, everyone. It is my pleasure to be here to present my work on “A Novel, Varactor-based Microwave Attenuator with Wide Tuning Ratio and Flat Insertion Loss”
This presentation is divided into 4 parts. First I will give a brief introduction on variable attenuator and some design challenges and limitations of some typical designs. Then I will talk about the proposed variable attenuator design, including the working principle, circuit diagram. In fact, it is formed by the combination of a tunable rat-race coupler and a well-known Wilkinson power combiner. Simulation and measurement results will be provided accordingly. In addition, attenuators are employed mostly in power applications, the IMD performance of the attenuator will be discussed. Finally, I will give a short conclusion.
In modern communication systems, re-configurability is also important for increased adaptability, size and cost reduction. To attain re-configurability, we usually have a control signal. This control signal usually takes the form of either voltage or current, depending on the characteristics of the active device used. For re-configurable devices, these are some common parameters which are tunable to fit different circumstances, such as, power dividing ratio in power divider, transmission phase in phase shifter, and also attenuation level in attenuator. Today, my presentation is focused on attenuation level in attenuator.
Attenuators are being widely employed in microwave systems, for use in automatic gain control circuit, circuit protection, improving impedance matching.
For variable attenuators, engineers paid much effort on improving the performance in terms of attenuation range, size, complexity and so on. The typical designs usually make use of PIN diodes as the tuning elements. By varying bias voltage over PIN diode, the current thus the resistance would be altered, acting as a variable resistor to provide variable attenuation. Some transmission mode designs include the adoption of T-/Pi-network with PIN diodes, and shunt-mounted PIN diodes with /4 spacing. Later, engineering proposed to exploit hybrid coupler in reflection-mode operation. However, these proposed designs has various drawbacks, like excessive DC power consumption and complex control circuitry when multiple PIN diodes are used, small tuning range, degraded return loss, and severe attenuation variation over frequency.
In contrast, my proposed variable attenuator features wide tuning range, flat insertion loss over frequency, ideal return loss performance, zero DC power consumption and just only one control voltage. It seems there are lots of feature, however, the working principle is much simpler.
The proposed variable attenuator in general consists of a variable power divide with 180° outputs and a Wilkinson power combiner. Here shows the block diagram of the device. The working principle of this attenuator is signal cancellation. Considering the variable power divider, at center frequency, It splits the input power into two parts of ratio A and B. Also the ant-phase characteristic is added to the output. These two anti-phase output will go into the Wilkinson power combiner with equal ratio combining. The 2 is introduced due to property of Wilkinson power combiner. Therefore when A is equal to B, the final output/ insertion loss will be zero; when A=1 and B=0, the output power will be halved.
The variable power divider with 180° outputs is realized by previously reported broadband tunable rat-race coupler. To operate as a power divider, the unused port is terminated by reference impedance. By varying the diode capacitance through biasing, we can obtain different power dividing ratios at Port 2 and Port 3 values.
This reported rat-race coupler features wide bandwidth of 40%, the tuning ratio k is solely depending on the diode capacitance. In addition, it has good return loss and low insertion loss with compact size. It adopts simple biasing circuitry. Here is a fabricated prototype of the rat-race coupler.
This slide shows the ideal performance of the variable power divider. Two sets of power dividing ratio (1, 5) are shown here. Good return loss and flat insertion loss response can be observed in both cases. When k varies, the proportion of S21 and S41 also vary.
We can eventually combine the transmission coefficients of variable power divider and Wilkinson power combiner, and obtain the equation of the overall insertion loss, which is dependent on k only. Referring to the equation, perfect cancellation can be achieved by setting k = 1, that is A = B. In this case, The insertion loss will become zero. In contrast, when k = 0(∞), that means all power are directed to A or B. Due to inherent insertion loss of Wilkinson power combiner, the overall insertion loss is 3dB which is the minimum attenuation. Therefore, ideal attenuation range is from 3dB to ∞dB.
These are the simulated ideal performance. Here two k values which correspond to 10dB and 20dB attenuation are used. Flat insertion loss can be observed over fractional bandwidth of 20%. The variation of attenuation level over k is plotted. As expected, when k = 1, the attenuation level goes to infinity and decreases as k varies.
This circuit diagram illustrated the connection and all the components required. The device is then designed at 1GHz and prototyped on Duroid substrate. The variable capacitance is realized by a tuning diode, Infineon BB857 Some transmission lines are meandered to save space.
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%.
In addition, the extracted group delay response (Fig. 10) is observed to be only mildly affected by the selected attenuation level (5 and 20 dB) over the frequency range from 0.9 to 1 GHz.
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.
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 limitations. 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. Also, the IMD performance gets worse at large attenuation as highlighted in previous slide. Finally, the bandwidth of attenuator gets narrower at the attenuation goes up.
This is the end of my presentation. Thank you for listening.
