Transcript Space Holder

Appendix
Power Transfer Basics
Low frequencies
+
I
-
wavelengths >> wire length
 current (I) travels down wires easily for efficient power transmission
 measured voltage and current not dependent on position along wire

High frequencies
wavelength  or << length of transmission medium
 need transmission lines for efficient power transmission
 matching to characteristic impedance (Z0) is very important for low reflection and maximum power transf
 measured envelope voltage dependent on position along line

Transmission Line Basics
Zo determines relationship between voltage and current waves
 Zo is a function of physical dimensions and
 Zo is usually a real impedance (e.g. 50 or 75 ohms)
er

Waveguide
a
w
b
Twisted-pair
Coaxial
h
er
h
w1
w
w2
Coplanar
Microstrip
Characteristic impedance for microstrip
transmission lines
(assumes nonmagnetic dielectric)
Power Transfer Efficiency
RS
Load Power (normalized)
RL
1.2
1
0.8
0.6
0.4
0.2
0
0
1
2
3
4
5
6
7
8
9
RL / RS
Maximum power is transferred when RL = RS
10
Power Transfer Efficiency
For complex impedances, maximum power transfer occurs when ZL
= ZS* (conjugate match)
Zs = R + jX
Rs
+jX
-jX
RL
ZL = Zs* = R - jX
At high frequencies, maximum power transfer
occurs when
RS = RL = Zo
Zo
Zo
Smith Chart Review
.
+jX
90
o
Polar plane
1.0
.8
0
.6
+R 
.4
+ 180 o
-
0
o

0
-jX
.2
Rectilinear impedance plane
-90 o
Constant X
Z L = Zo
G=
Constant R
0
Smith Chart maps rectilinear impedanceZ = 0 (short)
G = 1 ±180
plane onto polar plane
L
O
Z L=
G =1
(open)
0
O
Smith Chart
Lightwave Analogy to RF Energy
Incident
Transmitted
Reflected
Lightwave
RF
Transmission Line Terminated with Zo
Zs = Zo
Zo = characteristic impedance of transmission
line
Zo
V inc
Vrefl = 0! (all the incident power is absorbed in the
load)
For reflection, a transmission line terminated in Zo behaves like an infinitely long
transmission line
Transmission Line Terminated with
Short, Open
Zs = Zo
V inc
Vrefl
In phase (0 ) foro open
o
Out of phase (180 ) for short
For reflection, a transmission line terminated in a short or open reflects all power
back to source
Transmission Line Terminated with 25 W
Zs = Zo
ZL = 25 W
V inc
Vrefl
Standing wave pattern does not go to zero as with short
or open
Device Characteristics
Devices have many distinctive characteristics such as:
 electrical behavior
DC power consumption
linear (e.g. S-parameters, noise figure)
nonlinear (e.g. distortion, compression)
 physical specifications
package type
package size
thermal resistance
 other things...
cost
availability
When selecting parts for design, characteristics are traded-off
Let's look at important electrical characteristics for RF design ...
High-Frequency Device Characterization
Incident
Transmitted
DUT
R
B
Reflected
A
TRANSMISSION
REFLECTION
Reflected
Incident
=
SWR
S-Parameters
S11,S22
Reflection
Coefficient
G,r
A
Transmitted
R
Incident
Return
Loss
Impedance,
Admittance
R+jX,
G+jB
=
B
R
Group
Delay
Gain / Loss
S-Parameters
S21,S12
Transmission
Coefficient
T,t
Insertion
Phase
Reflection Parameters
Reflection
Coefficient
G
Vreflected
=
=
Vincident
Return loss = -20 log(r),
r
r
=
F
G
Emax
Emin
No reflection
(ZL = Zo)
0
 dB
1
r
=
ZL - ZO
Z L + ZO
Voltage Standing Wave Ratio
Emax
VSWR =
Emin
=
1+r
1-r
Full reflection
(ZL = open, short)
1
RL
0 dB
VSWR

Transmission Parameters
V Incident
V Transmitted
DUT
Transmission Coefficient =
T
VTransmitted
=
V Incident
V
Insertion Loss (dB) = - 20 Log
V
V
Gain (dB) = 20 Log
V
Trans
Trans
=
= - 20 log
Inc
= 20 log
t
Inc
V
Insertion Phase (deg) =
V
Trans
Inc
t
=

t
Group Delay (GD)
Frequency
w
Dw
Phase

-d 
dw

w

=
Group delay ripple
to
Average delay
D
Group Delay (t g)
tg
=
Frequency
-1
360 o
*
d
df
in radians
in radians/sec
in degrees
f in Hertz (w=2p f)
average delay indicates electrical length
 GD ripple indicates distortion
 aperture of measurement is very important
aperture is frequency-delta used to calculate GD
wider aperture: lower noise / less resolution
narrower aperture: more resolution / higher noise

Phase versus Frequency
R
50 W
50 W
A
Phase
Difference
between
A and R
Frequency
Phase versus Frequency
R
50 W
50 W
DUT
Phase
Difference
between
A and R
Frequency
A
Phase versus Frequency
R
50 W
50 W
DUT
Phase
Difference
between
A and R
Frequency
A
T/R Versus S-Parameter Test Sets
Transmission/Reflection Test Set
Source
S-Parameter Test Set
Source
Transfer switch
R
R
B
A
Port 1
Port 2
Fwd



Port 2
Port 1
DUT
RF always comes out port 1
port 2 is always receiver
response, one-port cal available
B
A
Fwd



DUT
Rev
RF comes out port 1 or port 2
forward and reverse measurements
two-port calibration possible
Response Calibration
DUT
THRU
Source
Load
Source
Reference
DUT
Measurement
errors due to
mismatch
Load
Two-Port Calibration
Two-port calibration corrects for all major sources of systematic
measurement errors
R
Directivity
A
B
Crosstalk
DUT
Frequency response


reflection tracking (A/R)
transmission tracking (B/R)
Source
Mismatch
Load
Mismatch
Six forward and six reverse error terms yields 12 error terms for two-port
devices
Thru-Reflect-Line (TRL) Calibration
TRL calibration was developed for non-coaxial microwave
measurements
Advantages
microwave cal standards easy to make (no open or load)
 based on transmission line of known length and impedance
 do not need to know characteristics of reflect standard

Disadvantages
impractical length of RF transmission lines
 fixtures usually more complicated (and expensive)
 8:1 BW limitation per transmission line

Characterizing Unknown Devices
Using parameters (H, Y, Z, S) to characterize devices:
gives us a linear behavioral model of our device
 measure parameters (e.g. voltage and current) versus frequency under various source and load conditi
(e.g. short and open circuits)
 compute device parameters from measured data
 now we can predict circuit performance under any source and load conditions

H-parameters
V1 = h11I1 + h12V2
I2 = h21I1 + h22V2
h11 = V1
I1
V2=0
(requires short circuit)
h12 = V1
V2
I1=0
(requires open circuit)
Why Use S-Parameters?
relatively easy to obtain at high frequencies
measure voltage traveling waves with a vector network analyzer
don't need shorts/opens which can cause
active devices to
oscillate or self-destruct
 relate to familiar measurements
(gain, loss,
reflection coefficient ...)
 can cascade S-parameters of multiple
devices to predict system
S 21
Incident
Transmitted
performance
a1
b2
S11
 can compute H, Y, or Z parameters
from S-parameters
if
DUT
Reflected
S 22
desired
Port 2
Port 1
Reflected
b
1
 can easily import and use S-parameter files in our electronica2
Incident
S 12
Transmitted
simulation tools

b 1 = S11 a 1 + S 12 a 2
b 2 = S21 a 1 + S 22 a 2
Measuring S-Parameters
a1
b1
S 21 =
Reflected
Incident
Transmitted
Incident
b
a2 = 0
a2 = 0
S 22 =
2
= a
1
a2 = 0
S 12 =
a1 = 0
Z0
Transmitted
Incident
S 12
b
a1 = 0
1
= a
2
a1 = 0
Reverse
a2
Incident
b2
= a
2
b2
Reflected
Load
Transmitted
Reflected
Incident
S 22
DUT
b1
Load
DUT
Reflected
b1
= a
1
b2
Transmitted
21
Z0
S 11
Forward
S 11 =
S
Incident
Equating S-Parameters with Common Measurement Terms
S11 = forward reflection coefficient (input match)
S22 = reverse reflection coefficient (output match)
S21 = forward transmission coefficient (gain or loss)
S12 = reverse transmission coefficient (isolation)
Remember, S-parameters are inherently linear quantities -- however, we
often express them in a log-magnitude format
Going Beyond Linear Swept-Frequency Characterization
So far, we've only talked about linear swept-frequency characterization (used for passive an
devices).
Two other important characterizations for active devices are:
nonlinear behavior
 noise figure

Linear Versus Nonlinear Behavior
A * Sin 360° * f ( t - t )
°
A
Linear behavior:
input and output frequencies are the same (no
additional frequencies created)
output frequency only undergoes magnitude and
phase change

Time
to
Sin 360° * f * t
A
Time
f
1
Input
Frequency
Output
DUT
Nonlinear behavior:
f
1
output frequency may undergo frequency shift (e.g.
with mixers)
additional frequencies created (harmonics,
intermodulation)

Frequency
Time
f
1
Frequency
Measuring Nonlinear Behavior
Most common measurements:
 using a spectrum analyzer + source(s)
harmonics, particularly second and third
intermodulation products resulting from two or more RF carriers
 using a network analyzer and power sweeps
gain compression
RL 0 dBm
AM to PM conversion
8563A
LPF
LPF
SPECTRUM ANALYZER
ATTEN
10 dB
10 dB / DIV
9 kHz - 26.5 GHz
DUT
CENTER 20.00000 MHz
RB 30 Hz
VB 30 Hz
SPAN 10.00 kHz
ST 20 sec
Noise Figure (NF)
Gain
So/No
DUT
Si/Ni
Measure of noise added by amplifier
 NF = 10 log [(Si/Ni) / (So/No)]
 Perfect amp would have 0 dB NF

Y-factor Technique for NF Measurements
Nout = Na + kTsBG
G, Na
Amplified Input Noise
Added Noise
Zs @ Ts = kTsB
+ 28 V
Th (noise source on) => N2 (at amplifier
Excess Noise
Source
output)
Noise Power Output
(Nout)
Tc (noise source off) => N1 (at amplifier
ENR (dB)
output)
Y = N2/N1
NF (dB) = ENR (dB) - 10 log (Y-1)
N2
N1
Slope = kGB
Na
Th
Tc
Source impedance temperature
AM to PM Conversion
u
n
d
e
s
i
r
e
d
A
M
:
s
u
p
p
l
y
r
i
p
p
l
e
,
f
a
d
i
n
g
,
t
h
e
r
m
a
l
d
e
s
i
r
e
d
A
M
:
m
o
d
u
l
a
t
i
o
n
(
e
.
g
.
Q
A
M
)
Q
I
Measuring AM to PM Conversion
CH1
CH2
S21
S21
log MAG
phase
1 dB/
1
/
REF 25 dB
REF 174
PRm
C2
1_: 0
dB
1_ 0
REF=1
1
1
2
3
1
3
2
2
use transmission setup with a
power sweep
display phase of S21

AM to PM = 0.727deg/dB

PRm
2_ 351.38 m
.5 dBm
C2
3_ 727.45 m
1.0 dBm
START -15.0 dBm
CW 1.880 000 000 GHz
STOP 5.0 dBm
Heat Sinking
for power devices, a heat sink is essential to keep Tjunction low
 heat sink size depends on material, power dissipation, air flow, and T ambient
 ridges or fins increase surface area and help dissipate heat
 usually device attaches directly to heat sink (flange mounts help)


bolt device in place first, then solder
heat sink