RF BASİCS- mikrodalga elektroniği
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Transcript of RF BASİCS- mikrodalga elektroniği
RF/MICROWAVE BASICS
by
Hakan P. Partal
RF BASICS
OUTLINE
•Electromagnetic Wave Properties
•High Frequency Voltage, Current, and Power
•Forward and backward travelling waves
•Impedance
•Network parameters -- S parameters; Reflection, Insertion Loss, Isolation, Phase
•Microwave circuit measurements.
RF BASICSElectrical energy Electrical energy Electrical energy Electrical energy
� Flows as current along a conductor, when a voltage is applied A bunch of electrons (negative charges) move through a conductor toward region of positive potential in response to an electric field. (For an electric current of 1 ampere, 1Coulomb of electric charge (which consists of about 6.242 × 1018
electrons) drifts every second through any imaginary plane, through which the conductor passes. )
� If the applied voltage is sinusoidal the direction of electron flow changes back and forth – alternating current (AC)
� Travels in the air as invisible waves.
� In a typical wireless system, the electrical energy starts out as current flowing along a conductor, gets changed into waves traveling in the air, and then gets changed back into current flowing along a conductor again.
0V
+1V
-1V
time
I
I
RF BASICS
� Moving electrons can be treated as electromagnetic waves
� An electromagnetic wave has
frequency (f),
wavelength (λλλλ), velocity (v)
� A medium has
permitivity or dielectric constant (εεεεr or Dk)characteristic impedance (Z)
εεεεr
� Frequency :cycles per second (Hz).
(F=1/Time)
900 MHz exhibits 900 million
ups and downs in a single second.
Wow!
� Distance = Velocity x Time
⇒ In air, EM waves travel at the speed of light,
⇒ C (3x108 m/sec).
� Wavelength (λλλλ) = C x Time = C x 1/F = C/F (in air)
EM WAVE PROPERTIES
0
+1V
-1V
time
1 sec
2 Hz
EM WAVE PROPERTIES
Wavelength is a function of velocity, frequency, and medium:
Wavelength (λλλλ) = velocity / Frequency = ( C / √√√√Dk ) / F (m) general definition
� Dk=1 in air
� When Dk is high, Wavelength is shorter and more cycles of waves in the same physical distance . EM wave velocity gets slower.
� When Frequency is high, wavelength is shorter
CATS: Higher Frequency ( more movement cycles)
Small in size physically
BUT, large electrically !
ELEPHANTS: Lower Frequency (less movement cycles)
Large in size physically
But, small electrically :–(
EM WAVE PROPERTIES� 1 wavelength : 1λ=360°
� 800MHz signal in a Rogers 3003 (εr=3) board, � λ=8.52”⇒ 24 mils = 1o
� 2GHz signal in a Rogers 3003 (εr=3) board,� λ=3.41”⇒ 10 mils = 1o which is electrically more significant
• Higher the frequency, smaller the wavelength,
can leak thru small gaps !
• Wavelength = c / (F √√√√ εr ) 90o, π/2
0o
180o, π
270o
360o
270
0
+1V
-1V
λ/2
180o
λ
360o
λ/4
90o
MICROWAVE CIRCUIT EXAMPLES
� Couplers are designed based on ¼ wavelength (λ/4). Can be smaller in size if used higher Dk materials,
Smaller in length at higher frequencies
� Also, other resonant circuits such as filters, power combiners, dividers, antennas, etc.
� Delay lines apply electrical delay for wave propagation. They can be smaller with higher Dk materials.
FREQUENCY SPECTRUM
MICROWAVE MEASUREMENTS
S-PARAMETERSCommon way to represent network parameters using complex coefficients:
� magnitude & phase
� Real & imaginary
� dB & phase
2 Port Network
a1
b1
a2
b2
S21
S12
S22S11
Matrix notation: Sij
Port
measured
Port
excited
S11=RL @ port 1
S21=IL from port 1 to 2
S12=IL from port 2 to 1
S22=RL @ port 2
Port 1 Port 2
SCATTERING PARAMETERS
Z02
Z02
Two-port
network
FWD wave
BWD wave
SCATTERING PARAMETERS
S PARAMETERS
SCATTERING PARAMETERS MULTIPLE REFLECTION EFFECTS
b1
RF POWER MEASUREMENTS � dB� At low frequencies, voltage and
current are measured. These parameters are difficult to measure at higher frequencies so power is measured.
� In microwave world the dB scaleis used commonly.
� This scale “compresses” the data range.
0 100 200 300 400 500 600 700 800 900 10000
5
10
15
20
25
30"Com pres s ion" Us ing Logarithm ic S c ale
X
10*Log(x)
Power Out/In
dB
dB
� dB is a relative quantity based on the ratio of two numbers (powers in microwave analysis).
� dB = 10Log10(Pout/Pin)
� Log10(AxB) = Log10 (A)+Log10 (B)
� Ex. A coupler has 1W applied to input (port 1) and 1/2W measured at Output Ports (Ports 2&3). What is the output power in dB?
P(dB) = 10Log10(0.5W / 1W)
= 10Log10(0.5) = -3dB
Ratio dB Value
1/1000 -30
1/100 -20
1/10 -10
½ -3
1 0
2 3
10 10
100 20
1000 30
100x 2x1mW 100mW 200mW
20dB 3dB+ = 23dB
dBm
� dBm = 10Log10(Pout/1mW)
where Pin is defined as 0.001W=1mW
� This allows us to represent an actual power using the dB scale as opposed to relative powers.
� When a system has 1mW at the input, the output power is described in dBm.
1mW Pout
= 10Log10(Pout/1mW)
Pout is represented in dBm.
Ex. –Convert 200mW to dBm:
10Log10(200mW/1mW) = 23dBm
Power(mW) dBm
0.001 -30
0.01 -20
0.1 -10
1 0
10 10
100 20
1000 30
SYSTEM IMPEDANCE
� System impedance in microwave
circuits is generally 50ohm.
� Components and cables that
connect them have characteristic
impedance of 50ohm.
� Equipment is generally designed
with 50ohm interfaces.
� If all components of the system
are NOT 50ohm, the system will
not operate optimally.
Test
Equipment.
Microwave
Device
50ohm cable
50ohm ports
50ohm ports
CHARACTERISTIC IMPEDANCE
� Electric field is generated by charges � E-field (V/m)
� Magnetic field is generated by current (moving charges)-
H-field (A/m) (Right hand rule)
� Characteristic Impedance of a medium “Z” is defined as E / H
� Free Space intrinsic impedance, Zair = η = 377ohm.
� In transmission lines, Z is determined by geometry and materials used.
ε2
ε1
Top GND plane
Bottom GND plane
TL
TRANSMISSION LINES (TL)
� A TL carries a microwave signal in a guided medium
� EM waves can travel in the air (antenna radiation)
� Some TL examples are Coaxial cables, Striplines, Microstrip lines, waveguides, etc.
� TL parameters: Frequency range, Bandwidth, power handling, loss, size, manufacturing process, etc.
Stripline Microstrip line Coaxial cable
TL
Dielectric substrate
GND plane
ε1
E-lines
H-lines
ε2
ε1
Top GND plane
Bottom GND plane
TL
ε1
Outer cond.
IC
air
MICROWAVE MEASUREMENT PARAMETERS
Return Loss
Insertion Loss
Isolation
Phase
MICROWAVE MEASUREMENT PARAMETERS
Network Analyzer:
-Applies a signal to port#1
-Measures reflected power at port#1
-Measures transmitted power at port#2
Incident
ReflectedTransmitted
Two Port DevicePort 1 Port 2
RETURN LOSS
� It is a relative measure of reflected power
� If characteristic impedance of a transmitted medium different, reflection occurs!
RL (dB) = |10Log10(Preflected/Pincident)|
= -20 log10|S11|
� The larger (Negative) number means better RL (less reflection)
� 20dB of Return Loss means
1% of incident power reflected back.
10Log10(1/100)= -20dB Incident
Wave
Air
Sea
εr=1
ε r=85
Reflected
Transmitted
INSERTION LOSS
� Loss caused by Reflection
� TLs get hot when power pass through them, some power is lost converted to heat energy
� Losses in the dielectric and copper,
� Losses due to radiation (leakages)
� Some power is intentionally lost due to design needs (e.g attenuators)
IL (dB) = |10Log10(Ptransmitted/Pincident)|
ISOLATION
• When EM wave travels in a poorly confined medium, some power could sneak out.
• If wavelength is small (higher frequency or higher Dk), this leakage is more pronounced.
• Must shield and use GND vias to eliminate leakages.
• Consider a 4 port coupler, some power from Input 1 will show up at Input 2
Isolation (dB) = 10Log10(P2 / Pincident)
• When there is a Good Isolation, P2=0 and ISO= -Infinity
• The larger (Negative) number represents the better isolation
4 Port
Coupler
Input 1
Input 2
OUT
Load
PHASE
� Corresponds to the time or distance travelled
� Gives information about the length of a transmission line
� Can measure the time difference between two signals at the same frequency.
Phase = 360 x(∆t / t(1cycle) ) (degrees)1 wavelength = 360o = 2π
0V
+1V
-1V
∆t
time0V
+1V
-1V
Phase
2π360o
π180o