Prof. Yongchae Jeong(E-mail: [email protected])
Overview on Microwave Circuits Design
1. Electronics
2. Radio Wave
3. Comparison between Analog, Digital
and Microwave,
4. Microwave Applications
5. Measurement Systems for Microwave Circuits
6. Curriculum for Microwave Engineering
7. Basic Concepts in Microwave Circuit Design
8. RF Transceiver Architectures
Overview on Microwave Circuits Design
- 어원 : Electronics= Electron ( 전자 )+ics ( 학문명 접미사 )
- 정의 1 : 진공 속이나 기체 , 고체 내에서의 전자의 운동을 연구하는 학문 및 그것을 이용하는 기술
- 정의 2 : 전자기술의 다방면에 걸친 발전과 그 두드러진 유용성으로 인해 생긴 개념으로 초기에는 진공 또는 기체 속에서 이루어지는 전자 운동의 이용을 초점으로 하는 것이었으나 , 1948 년 미국 벨 연구소에서 개발한 트랜지스터에 의해서 질적으로 변화하여 반도체내의 전자의 운동을 이용하는 이론과 기술이 전자공학의 주류로 변화
- 기술적 특징 : 빛 , 열 , 음 , 전자파 등을 전기 신호화해서 전송하고 처리
- 고체 전자공학 ( 반도체 ) 의 발전 과정
1. Electronics
Diode( 진공관 다이오드 , 반도체 다이오드 )
Transistor( 트랜지스터 )
IC(Integrated Circuit: 집적회로 )
VLSI(Very Large Scale Intefration: 초대규모 집적회로 )
Digital ICAnalog IC, RFIC(Radio Frequency IC), MMIC(Monolithic Microwave IC)OEIC (Optoelectronic IC)
1. Electronics
그림 1. 전자 공학의 흐름도
2. Radio Wave
-Radio Wave
- 인공적인 매개물이 없이 공간에 전파하는 3THz 보다 낮은 주파수의 전자파
- 무선통신에 사용되는 무선 주파수를 포함하여 적외선 , 가시광선 , 자외선 , X선 , 우주선 등을 총칭
- 전파의 사용 범위는 대체로 3kHz ~ 3THz 의 주파수를 갖는 전자파
- 무선통신 , 라디오 방송 , TV 방송 , 무선 항해 , 레이더 등은 모두 전파를 이용하는 것으로 , 전파가 점유하는 주파수 범위는 매우 넓고 주파수에 따라 파장이나 전파되는 특성이 다르며 , 현재 국제 전기 통신 협약과 전파법에 의해 관리 , 이용되고 있는 것은 일부분에 불과
그림 2. 전자파의 예
2. Radio Wave
전파의 성질
1) 전파의 직진
: 동일 매개체를 통과할 경우에 직진하는데 주파수가 높을수록 직진성이 강함
2) 전파의 반사 및 굴절
: 빛이 물속을 통과할 때처럼 전파 또한 다른 물질로 구성된 매개체를 통과할 경우에는 그 물질의 경계면에서 일부는 반사되고 일부는 진행방향이 변하여 투과되면서 굴절
3) 전파의 회절
: 전자파는 빛과 마찬가지로 전파 경로상에 산악 또는 건물 등과 같은 장애물이 있는 경우 , 그 뒤쪽에서 전파의 일부가 휘어져 수신
4) 전파의 간섭 ① 시간차에 의한 간섭 : 동일 기지국에서 방사된 동일한 주파수가 여러 경로를 거치면서 전파의
도달 시간에 차이가 생겨 발생 ② 인접 채널 간섭 : 서로 다른 기지국으로부터 발사되는 동일한 주파수로 인해 일어나는
간섭 ③ 동일 채널 간섭 : 여러 단말기가 동시에 통화시도를 하면 같은 채널을 사용하게 되는데 ,
이때 반대쪽에서 나는 간섭
2. Radio Wave
그림 3. 전파의 전파 경로
• 전자파를 이용한 무선장비의 소자 및 시스템
RF : 1GHz• 사전적 의미 Microwave : 300MHz ~ 300GHz
2. Radio Wave
RF 의 정의
-RF (Radio Frequency) : 방사 ( 방파 ) 주파수
- 대략 100 ~300MHz 이상의 고주파 무선통신 및 고주파를 이용하는 소자 , 부품 , 시스템 , 관련 장비 분야 .
2. Radio Wave
주파수 (Frequency) 의 정의
• 전자파가 움직이는 보이지 않는 길 ( 지정된 주파수를 통하여 정보를 교환 )
⇒ 파장 또 진동수를 기준으로 한 약속
• 1 초 동안에 일정한 주기로 진동하는 횟수 [Hz]
m/s) 103( Hz 8 cc
f
•
그림 4. 주파수의 개념
2. Radio Wave
표 1. 무선 주파수 대역
* 상업적인 RF 대역
2. Radio Wave
*Microwave 대역표 2. Microwave 대역
3. Comparison between Analog, Microwave, Digital
그림 5. Analog 와 Digital
4. Microwave Applications
Super Heterodyne 방식 : 수신기의 감도를 높이기 위해서 고주파 증폭기의 이득을 크게 한다는 것에 한도가 있으므로 , 고주파 신호를 한번 주파수가 낮은 증간 주파수로 변환시켜 이것을 증폭한 후에 복조하여 저주파 증폭을 하는 방식으로 회로가 복잡하고 가격이 비싸지만 , 감도와 선택도가 향상되고 광대역에 걸쳐 주파수 충실도가 우월
Direct Conversion (Zero IF) 방식 : IF 를 사용하지 않으므로 채널의 선택도와 감도가 떨어지긴 하지만 , IF 단을 사용하지 않기 때문에 가격면에서 저가이고 공간을 절약할 수 있으므로 작고 가벼움 .
IF (Intermediate Frequency) : 주파수변환기에 의해 수신 전파의 주파수와 국부 발진기 주파수 차에 해당하는 주파수 ( 수신측 ), 일반적으로 중간 주파수는 수신 주파수보다 낮게 하여 증폭하기 쉽고 선택도 및 충실도를 높게 하는 것
무선 및 이동 통신에서의 RF[Super Heterodyne 방식 ]
그림 6. Super heterodyne 형태의 AM 수신기의 기본적인 요소
4. Microwave Applications
4. Microwave Applications
일반적인 시스템 구조
4. Microwave Applications
Direct Conversion 방식
4. Microwave Applications
Super Heterodyne 방식
4. Microwave Applications
4. Microwave Applications
RF 와 Microwave 를 사용하는 이유
• 고주파에서 더 넓은 대역으로 전달 ( 정보 운반 능력 )
• 작아지는 시스템에 따르는 소자의 크기 문제
• 동작에 있어 높은 속도를 요구
• 안테나 이득은 안테나의 전기적인 크기에 비례
• 작은 파장에 따른 안테나의 길이 문제 해결
• 신호가 전리층에서 튀지 않으므로 지상과 위성과의 통신이 가능
4. Microwave Applications
*RF 응용분야
표 3. RF 응용분야
4. Microwave Applications
4. Microwave Applications
4. Microwave Applications
4. Microwave Applications
4. Microwave Applications
4. Microwave Applications
4. Microwave Applications
4. Microwave Applications
4. Microwave Applications
5. Measurement Systems for Microwave Engineering
Network Analyzer:
하나의 기계 안에 주파수 Source 와 Spectrum Analyzer 가 들어 있 어 서 , 입력과 출력의 주파수 신호분포결과를 서로 나눔으로써 S 파라미터를 측정하는 장비
그림 7. 8510C Network Analyzer Systems, 45 MHz
to 110 GHz
Scalar Network Analyzer : magnitude
Vector Network Analyzer : magnitude, phase
time domain frequency domain
Linear Device 만 측정 가능 (Frequency Doubler, Mixer 등은 측정 불가능 )
Delay Reflection 측정
(1 port device)SWRS-parameter(S11, S22)Reflection Coefficient Impedance Return Loss
Transmission 측정
(2 port device)Gain or Insertion LossS-parameter(S11, S22)Transmission CoefficientInsertion PhaseGroup Delay
5. Measurement Systems for Microwave Engineering
Spectrum Analyzer:
1-port 측정 장비로 계측기 입력단에서 어느 주파수 성분이 감지되는지를 표시하는장비 , Phase Noise 도 측정 .
그림 8. 8563EC Portable Spectrum Analyzer, 9 kHz to 25.6 GHz
5. Measurement Systems for Microwave Engineering
Noise Figure Meter (or Analyzer):
잡음을 임의로 발생시키는 Noise Source 와 잡음지수를 측정하는 Noise Figure meter 로 구성 , 회로와 수신부 시스템의 잡음지수를 측정 , Tuner 를 사용하여Noise Figure Parameter 를 추출 가능 , 저잡음 증폭기의 잡음지수 연구와 수신부
의 잡음지수 측정에필수적인 장비
그림 9. N8975A Series Noise Figure Analyzer
5. Measurement Systems for Microwave Engineering
Power Meter: Power 측정
그림 10. E4418B Single-Channel Power Meter
5. Measurement Systems for Microwave Engineering
Probe Station:
Wafer 및 Chip sample 등 전자 소자들의 전자 , 전기적 특성 및 물성 연구에 주로 사용하는 소자 탐침용 장비 . 주로 I-V, C-V, 각종 파라미터 및 Wafer 의 신뢰성을
테스트
그림 11. Cascade Microtech Probe System
5. Measurement Systems for Microwave Engineering
전자기학 (Electromagnetics):
Vector 및 scalar, 정전계 , 유전체의 정전용량 , 자성체와 인덕턴스 , 정자계의 특성을 익히고 시변계에서 Maxwell 방정식을 통해 기본적인 전자기적 현상을 이
해
회로이론 (Circuit Theory):
키르히호프법칙 , RLC 응답 , Laplace 변환 , Fourier 변환 등의 여러회로 이론들 에 대한 이해
물리전자 (Solid State Electronic Device):
반도체 소자의 특성과 동작의 원리 , 다이오드와 트랜지스터의 이해
전자회로 (Electronic Circuit):
다이오드 , 바이폴라 트랜지스터 , FET 와 같은 전자소자의 동작원리를 습득하며 , 이들의 소신호 모델을 이용한 증폭회로의 해석과 설계 기법을 학습
초고주파공학 (Microwave Engineering):
전송선이론 , 초고주파 회로망분석 , 정합이론 및 각종 초고주파 소자 및 증폭기 에대한 이해
무선통신회로 및 실험 (Wireless Communication Circuits and Experiments):
무선통신시스템의 구성하고 있는 주요 회로의 동작 및 설계 방법을 학습
전파공학 (Wave Propagation Engineering):
대기 중에서의 전파의 전파 과정과 안테나의 설계이론 학습
6. Curriculum for Microwave Engineering
Microwave Circuits Design Lab. 37
Memoryless system
A system is called “memoryless” if its output does not depend on the past values of its input.
For memoryless linear system,
y(t)=x(t)
where is a function of time if the system is time variant
For a memoryless nonlinear system, the input-output relationship can be approximated with a polynomial,
where j are in general functions of time if the system is time invariant
For memoryless and time-variant systems,
7. Basic Concepts in Microwave Circuit Design
txtxtxty 3
3
2
210
txtxtxty 33
221
Microwave Circuits Design Lab. 38
7. Basic Concepts in Microwave Circuit Design
Harmonics If a sinusoid is applied to a nonlinear system, the output generally exhibits frequency components that are integer multiples of the input frequency.
if x(t)=Acost, then
where the input frequency (): “fundamental”
the higher-order terms(n, n:integer): “harmonics.”
Even-order harmonics result from j with even j and vanish if the system has
odd symmetry, i.e., if it is fully differential.
The amplitude of the nth harmonic consists of a term proportional to An and
other terms proportional to higher powers of A.
tA
tA
tA
AA
ttA
tA
tA
tAtAtAty
3cos4
2cos2
cos4
3
2
3coscos34
2cos12
cos
coscoscos
33
22
33
1
22
33
22
1
333
2221
Microwave Circuits Design Lab. 39
7. Basic Concepts in Microwave Circuit Design
Gain Compression
The small signal gain (1)of circuit is usually obtained with the assumption that harmonics are negligible.
In most circuits of interest, the output is a “compressive” or “saturating” function of input. At high input level, gain is a decreasing function of A.
smallnot is @cos4
3
small is @cos
3cos4
2cos2
cos4
3
2
3
31
1
3
3
2
2
3
31
2
2
AtA
A
AtA
tA
tA
tA
AA
ty
0,0,04/3 31312
31 A
Microwave Circuits Design Lab. 40
7. Basic Concepts in Microwave Circuit Design
1-dB compression point(P1dB): The input signal level that causes the small signal gain to drop by 1dB.
Fig. 7 Definition of 1dB compression point
To calculate the 1-dB compression point,
3
1
3
11 145.0
3
41087.0
dBA
Microwave Circuits Design Lab. 41
7. Basic Concepts in Microwave Circuit Design
Desensitization and blocking
When the desired signal is fed to circuit with a strong interferer, the “average” gain of the circuit is reduced because of a large interferer : “desensitization”
interferer:cos signal,:coscoscos 22112211 tAtAtAtAtx
tAAtA
tA
ttAAttAAtt
A
tAttAAttAAtA
tAtAtx
1
2
211
3
13
2
33
2
21
2
2121
2
2
2
111
3
13
2
33
22
2
1
2
2121
2
2
2
11
33
13
3
22113
3
3
cos2
3cos
4
3
cos
2
2cos1cos3coscos33coscos3
4
coscoscos3coscos3cos
coscos)(
tAAAAty 1
2213
31311 cos
2
3
4
3
Microwave Circuits Design Lab. 42
7. Basic Concepts in Microwave Circuit Design
For A1 << A2,
For 3<0 and sufficiently large A2, the overall gain drops zero, and we
say the signal is “blocked” in RF design.
Many RF receivers must be able to withstand blocking signals 60 to 70dB
greater than the wanted signal. Filter, Matching circuits, etc.
tAAty 11
2231 cos
2
3
Microwave Circuits Design Lab. 43
7. Basic Concepts in Microwave Circuit Design
Cross Modulation
When a weak signal and a strong interferer pass through a nonlinear system, the transfer of modulation on the amplitude of the the interferer to the amplitude of the weak signal is occurred.
The desired signal at the output contains amplitude modulation at m and 2m.
)indexmodulation,1(cos)cos1(cos 2222 mttmAtA m
ttmtmm
AAA
ttmAtAtAty
mm
m
1
22221311
222113111
coscos22cos22
12
3
coscos1cos3cos
Microwave Circuits Design Lab. 44
7. Basic Concepts in Microwave Circuit Design Intermodulaton
When two signals with different frequencies are applied to a nonlinear system, the output in general exhibits some components that are not harmonics of the input frequencies. Intermodulation distortion(IMD)
Fundamental components
Intermodulation products:
tAtAtx 2211 coscos
3
22113
2
22112
22111
coscoscoscos
coscos
tAtAtAtA
tAtAty
tAAAA
tAAAA
2
2
123
3
2321
1
2
213
3
131121
cos2
3
4
3
cos2
3
4
3: ,
2cos4
32cos
4
3: 2
2cos4
32cos
4
3: 2
coscos:
121
2
23
121
2
23
12
212
2
1321
2
2
1321
212122121221
tAA
tAA
tAA
tAA
tAAtAA
Microwave Circuits Design Lab. 45
7. Basic Concepts in Microwave Circuit Design
The interest IM products are the third-order IM products at 22-1 and 21-
2.
If the difference between 1 and 2 is small, the components at 21-2 and
22-1 appear in the vicinity of 1 and 2 .
Fig. 8 Intermodulation in a nonlinear system
If a weak signal accompanied by two strong interferers experiences third-
order nonlinearity, then one of the IM products falls in the band of interest,
corrupting the desired component.
Fig. 9 Corruption of a signal due to intermodulation between two interferers
Microwave Circuits Design Lab. 46
7. Basic Concepts in Microwave Circuit Design
IP3 This parameter is measured by a two-tone test in which A is chosen to be
sufficiently small so that higher-order nonlinear terms are negligible and the gain is relatively constant and equal to 1.
As A increases, the fundamentals increase in proportion to A, whereas the third-order IM products increase in proportion to A3.
Fig. 10 Growth of output components in an intermodulation test
Horizontal coordinate : Input IP3(“IIP3”)
Vertical coordinate: Output IP3(“OIP3”)
IP3 is used as a measure of linearity and a unique quantity that by itself can serves as a means of comparing the linearity of different circuits.
3
4
3
13
IPA
Microwave Circuits Design Lab. 47
7. Basic Concepts in Microwave Circuit Design
Fig. 11 (a)Calculation of IP3 without extrapolation, (b)graphical interpretation of (a)
The actual value of IP3, however, must still be obtained through accurate extrapolation to ensure that all nonlinear and frequency-dependent effects are taken into account.
only) caseA (Class 23 dBmin
dBdBm
PP
IIP
Microwave Circuits Design Lab. 48
7. Basic Concepts in Microwave Circuit Design
Calculation of an overall input third intercept point in terms of the IP3 and gain of the individual stage.
Two nonlinear stages in cascade
Fig. 12 Cascaded nonlinear stages
The overall OIP3:
, 3
13
2
12112
3
3
2
211 tytytytytxtxtxty
33
3
2
213
23
3
2
212
3
3
2
2112
txtxtx
txtxtxtxtxtxty
2 3
3
3
122113112 txtxty
23
4
3
3
122113
113
IPA
Microwave Circuits Design Lab. 49
7. Basic Concepts in Microwave Circuit Design
The alternate overall OIP3:
where AIP3,1 and AIP3,2 represent the input IP3 points of the 1st and 2nd stages.
From the result, 1 increases, the overall IP3 decreases. This is because with higher gain in the first stage, the second stage senses larger input levels producing greater IM3 products.
31
34
3
34
1
2
4
31
2
IP3,2
2
12
2
IP3,1
3
1
2
12
3
111
3
3
122113
2
3
AA
AIP
Microwave Circuits Design Lab. 50
7. Basic Concepts in Microwave Circuit Design
Noise Thermal noise (or Johnson noise, Nyquist noise)
- The agitated charge carrier random motion noise being caused by thermal
vibration of bound charge
- White noise up to 1013 Hz
- Noise power: P=kTB
where k: Boltzman constant (1.3810-23 J/ºK)
T: Absolute temperature
B: System bandwidth
Ex.]The available power in a 1Hz bandwidth from a thermal noise source
P=kT=410-23 [W/Hz]=-174dBm/Hz @room temperature
Shot noise (or Schottky noise)
- The transfer noise of charge across an energy barrier (ex. A PN junction,
IDS in MOSFET)
-
where q=1.6 10-19[C] (electron charge), Idc:dc current through the device
BqIii dcdcsS 22
,
2
Microwave Circuits Design Lab. 51
7. Basic Concepts in Microwave Circuit Design
Flicker noise - Random trapping noise of charge at the oxide-silicon interface of MOSFETs
- Dominant at low frequencies in the semiconductor devices
- Must be considered in the design ultra wideband amplifiers (dc~10GHz) and
microwave oscillator
Plasma noise
- Random motion noise of charges in an ionized gas as a plasma, the
ionosphere, or sparking electrical contacts
Quantum noise
- The quantized nature of charge carriers and photons
- Often insignificant relative to other noise sources
Microwave Circuits Design Lab. 52
7. Basic Concepts in Microwave Circuit Design
Input-Referred Noise The noise of a two-port system can be modeled by two input noise
generators: a series voltage source and a parallel current source. In general, the correlation between the two sources must be taken into account.
Fig. 13 Representation of noise by input noise generators
Fig. 14 (a)MOS amplifier, (b) equivalent input noise generators
Microwave Circuits Design Lab. 53
7. Basic Concepts in Microwave Circuit Design
Noise Figure Signal-to-noise ratio(SNR): The ratio of the signal power to the total noise
power.
where SNRin : The SNR measured at the input
SNRout : The SNR measured at the output
Friis equation:
The noise contributed by each stage decreases as the gain preceding the
stage increases, implying that the the first few stages in a cascade are the
most critical.
Figure)(Noiseout
in
SNR
SNRNF
111
)1(2121
3
1
21
mppp
m
ppp
tot AAA
NF
AA
NF
A
NFNFNF
Microwave Circuits Design Lab. 54
7. Basic Concepts in Microwave Circuit Design
Noise Sensitivity of RF receiver
The minimum signal level that the system can detect with acceptable signal-to-noise ratio.
where Psig: The input signal level per unit bandwidth
PRs: The source resistance noise power per unit bandwidth
The overall signal power is distributed across the channel bandwidth, B :
The minimum signal level that the system can detect with acceptable SNR:
where Pin,min: The minimum input level that achieves SNRout,min
B: Bandwidth [Hz]
outout
in
SNR
PP
SNR
SNRNF Rssig
outSNRNFPP Rssig
out, BSNRNFPP Rstotsig
log10dBmin,outdBdBm/Hzmin,in BSNRNFPP RsdBm
Microwave Circuits Design Lab. 55
7. Basic Concepts in Microwave Circuit Design
In dB scale,
Dynamic Range The ratio of the maximum input level that the circuit can tolerate to the
minimum input level at which the circuit provides a reasonable signal quality.
DR bases the definition of the upper end of the dynamic range on the
intermodulation behavior and the lower end on the sensitivity.
“Spurious-free dynamic range”(SFDR)
min
mindBmmin,in log10dBm/Hz174
SNRF
SNRBNFP
min
3
min3
min,inmax,in
3
2
3
2
SNRFP
SNRFFP
PPSFDR
IIP
IIP
Microwave Circuits Design Lab. 56
8. RF Transceiver Architectures Primary criteria in selecting transceiver architectures: Complexity Cost Power dissipation Number of external components But IC technologies makes once seemed impractical design to return as plausible solutions.
RF Transceiver Architecture Heterodyne Homodyne Image-reject Digital-IF Subsampling receivers Direct-conversion and two-step transmitters
Transmitter: Narrowband modulation, amplification, and filtering to avoid leakage to adjacent channels Receiver: Able to process the desired channel while sufficiently rejecting strong neighboring interferers. Fig. 15 a)Transmitter and b)receiver front ends of a wireless transceiver
Microwave Circuits Design Lab. 57
8. RF Transceiver Architectures
Terminology Band: The entire spectrum in which the users of a particular standard are allowed to
communicate (e.g., the GSM receive band spans 935 MHz to 960 MHz) Channel: The signal bandwidth of only one user in the system (e.g. 200KHz in GSM) Band selection: The operations that reject out-of-band interferers
Channel selection: The operations that reject out-of-channel(usually in-band) interferers.
Isolation between TX and RX
Finite attenuation of the transmitted signal in the receive band
Desensitization of LNA by PA output leakage
NADC and GSM systems avoid by offsetting the
transmit and receive time slots, but analog FDD
standards (e.g., AMPS, CDMA) require high
isolation.
Fig. 16 Desensitization of LNA by PA output leakage
Microwave Circuits Design Lab. 58
8. RF Transceiver Architectures
Heterodyne receiver (or Downconversion mixing, Downconversion) Primary the signal band is translated to much lower frequencies
Relax the Q required of the channel-select filter.
The translation is carried out by means of a mixer.
RF signal: Bocos1t
LO signal: Aocosot o=1- 2
Some of output signals(IF):
1o=1(1-2)=2 or 21-2
Output of LPF: 2 (a)
Fig. 17 (a)Simple heterodyne downconversion
(b)inclusion of an LNA to lower the
noise figure
(b)
RF IF
LO
Microwave Circuits Design Lab. 59
8. RF Transceiver Architectures Problem of Image
- For x1(t)=A1cos1t and x2(t)=A2cos2t, the low pass filtered product of x1(t) and x2(t) is of the form cos(1-2)t, no different form cos(2-1)t
- In a heterodyne architecture, the bands symmetrically located above and below the LO frequency are downconverted to the same center frequency.
Image frequency
- If RF signal is centered around 1 (= LO- IF), the image is around 2LO- 1(= LO+ IF) and vice versa.
Image rejection filter in front of mixer is
designed to have a relatively small loss in
the desired band and a large attenuation
in the image band
Fig. 18 Problem of image in heterodyne reception Fig. 19 Image rejection by means of a
filter
Microwave Circuits Design Lab. 60
8. RF Transceiver Architectures Two cases corresponding to high and low values of IF
1) High IF Leads to substantial rejection of the image 2) Low IF High Q Allows great suppression of nearby interferers.
The trade-offs parameters in choice of IF
- Amount of image noise - The spacing between the desired band and the image - The loss of the image-reject filter Trade-off between image rejection and channel selection.
Fig. 20 Rejection of image versus suppression of
interferers for (a)high IF and (b)low IF
An important drawback of the heterodyne architecture - The image reject filter is realized as a passive, external component because of high Q.
- This requires input/output matching of LNA to 50, where LNA is inevitable more severe trade offs between the gain, noise figure, stability, and power dissipation in the amplifier.
Microwave Circuits Design Lab. 61
8. RF Transceiver Architectures
Dual IF topology
Multiple downconversion technique performs partial channel selection at progressively lower center frequencies, thereby relaxing the Q required of each filter.
Most of today’s RF receivers : 2-stages of downconversion(“Dual-IF”)
Fig. 21 Dual-IF heterodyne receiver
Microwave Circuits Design Lab. 62
8. RF Transceiver Architectures
Homodyne Receivers (or Direct–conversion, Zero IF)
The LO frequency is equal to the input carrier frequency. Channel selection requires only a low pass filter with relatively sharp cutoff characteristics.
Fig. 12(a) operates properly only with double-sideband AM signals because it overlaps positive and negative parts of the input spectrum.
For frequency and phase modulated signals, the downconversion must provide quadrature output so as to avoid loss of information. This is because the two sides of FM or QPSK spectra carry different information and must be separated into quadrature phases in translation to zero frequency.
Fig. 22 (a) Simple homodyne receiver,
(b) homodyne receiver with
quadrature downconversion
Microwave Circuits Design Lab. 63
8. RF Transceiver Architectures Two advantages over a heterodyne counterpart.
1)The image problem is circumvented because IF=0. As a result, no image filter is required, And the LNA need not drive a 50-Ohm load.
2)The IF SAW filter and subsequent downconversion stages are replaced with low pass filters and base band amplifiers are amenable to monolithic integration.
Direct conversion has number of issues do not exist or are not as serious in a
heterodyne receiver.
Channel selection: Rejection of out-of-channel interferers by an active low-
pass filter is more difficult than by a passive filter, fundamentally active
filters exhibit much more severe noise-linearity-power trade-offs than do
their passive counterparts.
Microwave Circuits Design Lab. 64
8. RF Transceiver Architectures DC offsets
- Since in a homodyne topology the downconverted band extends to zero
frequency, extraneous offset voltages can corrupt the signal and saturate the
following stages.
- LO leakage: From capacitive and substrate coupling and, if the LO signal is
provided externally, bond wire coupling, the isolation between the LO port
and the inputs of the mixer and the LNA is not infinite.
- Self-mixing: The leakage signal
appearing at the inputs of the LNA
and the mixer from LO signal is
mixed with LO signal, thus producing
a DC component at C.
- A large interferer leaks from the LNA
or mixer input to the LO port and is
multiplied by itself.
Fig. 23 Self mixing of (a) LO signal , (b) a strong interferer
Microwave Circuits Design Lab. 65
8. RF Transceiver Architectures I/Q Mismatch - For phase and frequency modulation schemes, a homodyne receiver must incorporate quadrature mixing. - Either the RF signal or the LO output by 90o phase shifting
The shifting the RF signal generally entails severe noise-power-gain trade-offs, making it more desirable to use the topology of quadrature generation in LO path.
Fig. 24 Quadrature generation in
(a) RF path,
(b) LO path
Fig. 25 Effect of I/Q mismatch on a demodulated QPSK waveform; (a)gain error (b)phase error
Microwave Circuits Design Lab. 66
8. RF Transceiver Architectures Even-Order distortion - Two strong interferers close to the channel of interest experience nonlinearity such as in the LNA. - Mixers exhibit a finite direct feedthrough from the RF input to the IF output.
Thus, a fraction of vRF(t) appears at the output with no frequency translation. (Ex.: 30 ~ 40 dB in typical differential mixers)
- Even order distortion demodulates AM components.
Fig. 26 Effect of even-order distortion on interferers
- Differential LNAs and mixers can suppress even-order distortion. 1) Balun (single ended ant. to differential LNA) (difficult!!) 2) NF increasing due to insertion loss of balun
tAtAtx 2211 coscos
tAA 21212 cos
Microwave Circuits Design Lab. 67
8. RF Transceiver Architectures Flicker noise - Flicker noise arises from random trapping of charge at the oxide-silicon interface of MOSFETs. Represented as a voltage source in series with the gate, the noise density is
where K: A process-dependent constant and negligible at high frequencies.
- In particular, since the downconverted spectrum extends to zero frequency, the 1/f noise of devices substantially corrupts the signal, a severe problem in MOS implementations.
LO leakage - Leakage of the LO signal to the antenna and radiation creates interference in
the band of other receivers using the same wireless standard. - The design of the wireless standard and the regulations of the Federal
Communications Commission(FCC) impose upper bounds on the amount of in-band LO radiation, typically between –50dB and –80dBm.
12
fWLC
KV
OXn
Microwave Circuits Design Lab. 68
8. RF Transceiver Architectures
Hartley Architecture The RF input is mixed with the quadrature phases of the local oscillator
(cosLOt and sinLOt), low-pass filters the resulting signals and shifts one by 90o before adding them together.
Fig. 27 Hartley image-reject receiver
Key point: The signal components at B and C have same polarity, whereas the image components have opposite polarities.
The input signals: x(t)=ARFcosRFt+ Aimcosimt
where ARFcosRFt : The desired channel signal
Aimcosimt : The image channel signal
Microwave Circuits Design Lab. 69
8. RF Transceiver Architectures
Signals of at point A and B
Signals of at point C and output port
The RF signal is down-converted with no corruption by the image.
tAt
Atx
tA
tA
tA
tA
ttA
ttA
ttAttAttAtAtx
imLOim
RFLORF
B
imLOim
LORFRF
imLOim
RFLORF
FPL
imLOimLOim
RFLORFLORF
imLOimRFLORFLOimimRFRFA
cos2
cos2
sin2
sin2
sin2
sin2
sinsin2
sinsin2
cossincossinsincoscos
..
tAtxtxtx
tA
tA
tx
LORFRFCBIF
imLOim
LORFRF
C
cos)()()(
cos2
cos2
Microwave Circuits Design Lab. 70
8. RF Transceiver Architectures
Weaver Architecture
The weaver architecture replaces the 90 stage of the Hartley architecture by a second quadrature mixing operation.
Assume 2<< 1
Fig. 28 Weaver image-reject receiver
Fig. 29 Graphical analysis of Weaver architecture
Microwave Circuits Design Lab. 71
8. RF Transceiver Architectures
Digital-IF Receivers
The 1st IF signal is digitized, and “mixed” with the quadrature phases of a digital sinusoid, and low-pass filtered to yield the quadrature baseband signals.
Digital processing avoids the problem of I and Q mismatch.
Fig. 30 Digital-IF receiver