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RF (SPØRGSMÅL 1: ANTENNA MEASUREMENTS, Spørgsmål 4: Body Area Networks,…
RF
SPØRGSMÅL 1: ANTENNA MEASUREMENTS
NEAR AND FAR FIELD PRINCIPLES
Boundary condition
Formula
Far-field approx
Drawing
Amp term, r1 = r2 = r
phase term, more advanced ==> 22,5 deg
§ Phase difference ==> constructive/destructive interference.
hvad adskiller regionerne
FAR FIELD ANCHEOIC CHAMBER SETUP
LIMITATIONS AND CHALLENGES
Physical room
Wedges vs pyramids
no reflections
wedges....
for waves, which travel nearly parallel to their ridges.
Better for not-normal incidence.
pyramids
absorb best the waves at normal (nose-on) incidence.
do not perform well at large angles of incidence.
in effect, as a tapered impedance transition for normal incidence of the EM wave from the intrinsic impedance of 377 to the short of the chamber’s wall. resistance gradually decreases as the pyramid’s cross-section increases.
At normal incidence (pyramid), it should be matched to 377 ohm ==> no reflection
depends on freq
Normal incidence
Bigger room ==> more "normal incidence" compared to non-normal
• Doesn't make sense to have small room if the absorbers need to be absurdly large in comparison to. Room size >< absorber size.
AUT
Rotate antenna under test in azimuth and elevation coordinates
Antenna measures both amp and or phase as it rotates.
near ==> far field transform
compact chamber
amplitude, phase and polarization
magnitude and phase of the tangential electric field are measured at regular intervals over a well-defined surface: a plane or a cylinder (sphere
Planar is simplest, but can't cover all directional angles like spherical can.
Planar
Max angle < ± 70 º
○ ○ If close to antenna ==> just use planar bro
Won't catch back lobe/side
® Phase relationship from antenna ◊ Amplitude & angle scaling (weighted contributions) . } ==> correlation analysis on rows & coloumns of probe antenna. – Then move on to next point and do it over.
Fourier transform to far field.
spherical
Low gain antennas
isoptropic / omni
○ back lobes and the side lobes of the antenna cannot be measured together with the main beam. § But AUT can be rotated in different positions
If we're in near field, we need both phase and magnitude in order to transform
Sampling is typically ½*lambda resolution.
gain measurement
based on friis
need to know distance b/w, G of receiver, P_r, P_t, and f = c / lambda
Polarization measurement
many measurements are required according to the desired degree of polarization description
○ If I want to know if circular Need both horizontal and vertical measurements at SAME TIME, to know phase.
cool points to mention
remember aperture is not only the antenna itself
fix via □ e.g. ferrites. Fiber optic transceivers (electrical to optical conversion
○ Probably the largest issue when dealing with small antennas.
Compact range chamber
reflectors
Spørgsmål 4: Body Area Networks
PROPAGATION CHALLENGES
CENTRAL FADING
UWB frequencies (3.1–10.6 GHz), waves don't penetrate the torso. Instead, they "creep" around the surface of the skin (diffraction).
ATTENUATION EFFECTS
Human tissue has high permittivity and conductivity. It absorbs RF energy, turning it into heat
Unlike open space where signals fade following an inverse-square law, in BANs, the signal fades much faster because the body absorbs RF energy.
"environment" is the human body, which is mostly salt water—a terrible medium for radio waves.
Fading & Shadowing:
If a sensor is on your chest and the receiver is on your back, the signal can't go through you easily.
Creeping Waves: Around the body, signals often travel via "diffraction," where the wave curves around the body surface.
temporal fading
Dynamic Body Movements
signal strength changes rapidly as you move).
Posture changes the Line-of-Sight (LOS) vs. Non-Line-of-Sight (NLOS) conditions
differentb odies / fat, tall, slim etc
IEEE 802.15.4A CHANNEL MODEL CONCEPT/APPROACH
amendment for Ultra-Wideband (UWB).
UWB is great for BANs because it uses very short pulses,
Lower power, better security, and less interference with other devices).
uses a statistical approach (the Saleh-Valenzuela model) to describe how signals arrive at the receiver in "clusters" of multipath components.
Clusters: Instead of one clean signal, the receiver sees "clusters" of reflections.
needs to be small
lcuster ray problem
fir representation and correlation
network of low-power, miniaturized devices (sensors) located on, in, or around the human body.
Spørgsmål 2: Patch Antennas
GEOMETRY
opbygning (tegning)
½ bølge
tan delta
• Skal finde substrat med lavt tab, da alt felt er koncentreret under pladen (skvulper) ○ Ift. Tråd-antenne, hvor stor del af felt er I luft, så er patch bundet tæt med substrat, så tan delta betyder meget.
TM100 mode
betydning
feed
impedansvalg
E field at short ends = max (looks like an open circuit
mikrostop vs. probe fed
:( ikke ideel ved mikrostirp
But the way you feed power --> poorer radiation structure from that side (interferes with radiation), since the E-field is opposite
TRANSMISSION LINE MODEL
2 radiating slots
almost no thru GND plane + almost no side lobes (ideally)
two radiating narrow apertures (slots), § each of width W and height h, separated by a distance L with low impedance Z_c
antenna array
kredsløb ækvivalent
Each radiating slot is represented by a parallel equivalent admittance Y (with conductance G and susceptance B (each slot is #1 or #2 slot)
see notes antenna
finite dimensions == fringing
eff eps_r and L
§ smaller the dielectric constant of the substrate, the larger is the fringing (larger fringing, length shorter)
larger the dielectric constant, the more tightly the fields are held within the substrate (fringing smaller, length longer)
when no radiation is desired, as in transmission lines, a high value of 𝜀r is desired.
Open-ended TL.
Belastet med Z_L = inf. ==> Standing waves med L = ½Lambda
CAVITY MODEL
normalized fields within the dielectric substrate (between the patch and the ground plane) can be found more accurately by treating that region as a cavity bounded by electric conductors (above and below it) and by magnetic walls (to simulate an open circuit) along the perimeter of the patch
it is the fringing fields that are responsible for the radiation.
"Transmissionsliniemodellen er god til et hurtigt overblik over impedans, men hvis vi vil have den præcise feltfordeling og indregne alle tabstyper, bruger vi cavity-modellen, hvor vi ser patchen som et dielektrisk hulrum med magnetiske vægge langs kanterne."
CIRCULAR POLARIZATION CONCEPTS
hvorfor vil vi have det
2 måder
Singly feed patch
Dual-orthogonal fed patch
splitter
kombinerer 2 patches med pi/2
2x2 array of linearly polarized patches -
Udstråling
Spørgsmål 5: RF measurement techniques
S-PARAMETERS MEASUREMENTS
s parameter def
tegning + b1, b2 etc + s11, s22
TOSM
TOSM
Thru
Open Circuit
Short Circuit
Matching
NOISE FIGURE MEASURE
reference tmep T_0
F ==> SNR_i / SNR_o
Cascaded systemsd
Formula
Y-factor method
N1, N2 (on off power)
Y is ratio
F formula =ENR / Y -1
ENR
definition
P_noise
T_R
MEASUREMENT OF NON-LINEAR EFFECTS
ideal amp = linear (Pin / Pout)
nonideal ==> saturation (compression) ==> 1 dB point
Looking at gain analysis, apply small /large signals to the amp and then we crawl up until deviation between lowsignal amplification gain and large signal amp gain.
$P1dB$: This is the point where the gain (for amplifiers) or insertion loss (for passive devices) drops by $1\text{ dB}$ compared to its low-power linear performance.
how 2 measure
P-IIP3
Two-Tone Third-Order Intermodulation
how a device handles multiple signals simultaneously,
how 2 measure
The Setup: Two separate RF generators produce "tones" (usually $1\text{ MHz}$ apart). These are merged using a power combiner before entering the D.U.T.
The Measurement: The analyzer looks for "intermodulation products"—spurious signals created by the D.U.T. that appear at specific intervals near the original tones.
Spørgsmål 6: RF system level
RF-RECEIVER FRONT-END DIAGRAM
LNA
BASIC FUNCTIONALITY
SYSTEM-LEVEL IMPLEMENTATIONS / LIMITAITONS
NON-LINEAR EFFECTS
Spørgsmål 3: Interfacing
S PARAMETERS
DEFINITIONS
definition af "s params"
CALCULATIONS ON LUMPED ELEMENT MODELS
RC filter eksempel
MEASUREMENTS
TOSM
Thru
Open Circuit
Short Circuit
Matching
S-parameter boks model
b1, b2 etc
betingelse
s11,22,12 osv def
gamma
in/out
load og source
SIMPLE LUMPED ELEMENT MATCHING NETWORKS
SMITH CHART APPROACH
eksempel (e.g. givet ud fra enten gamma eller Z-L ==> tegn smith og bevæg og fortæl om hvad man kan gøre
Q
