Download MICROWAVE SYSTEM DESIGN CONSIDERATIONS

MICROWAVE SYSTEM
DESIGN
CONSIDERATIONS
Misunderstanding of
complete system
System will surely fail
Without a solid understanding of complete communications system
from the transmitter’s modulator input to the receiver’s modulator
output, including everything in between, and how the selection of
various components, circuits, and specifications can make or break
an entire system, any wireless design will surely fail.
Block Diagram of Simple One Way Microwave
Communication System
IF
Data in
Mixer
PA
Mod
LO
IF Amp
Data out
IF
Noise
Mixer
Dem
LO
LNA
Baseband:
Data, Voice, Video,….etc
BW?
Modulation: Digital or Analog
Transmitter Components: IF Filters
Filter
PA RF Filters
Mixer (up conversion)
Antenna
Link Calculation:
Satellite
Terrestrial Radar Wireless Mobile, …..etc
Receiver Components: Antenna RF Filters LNA
Mixer (down conversion) IF Filters
IFAmplifier
Demodulator
Other Components:
Control System
& Power Supply
Monitoring: Test and measuring components & Measuring tools
Receiver Calculations
Receiver
To demodulator
Signal + Noise
Noise
(So/No)min
Internal Noise
Noise is added into an RF or IF passband and degrades system
sensitivity
The receiving system does not register the difference between signal
power and noise power. The external source, an antenna, will deliver
both signal power and noise power to receiver. The system will add
noise of its own to the input signal, then amplify the total package by
the power gain
Noise behaves just like any other signal a system processes
Filters:
will filter noise
Attenuators:
will attenuate noise
Noise Sources
External
Manmade
• Switching equipment
• Power generating equipment.
• Ignition Noise
• Interference
Internal
Natural
• Atmospheric
• Cosmic
• Galactic
• Thermal Noise
• Shot Noise
•Gen./Recomb.
•Flicker Noise
(Mod. Noise)
…………….
Thermal Noise
The most basic type of noise being caused by thermal vibration
of bound charges. Also known as Johnson or Nyquist noise.
R
R
Vn = 4KTBR
T
Available noise power
Pn = KTB
Where, K = Boltzmann’s constant (1.3810-23J/oK)
T
Absolute temperature in degrees Kelvin
B
IF Band width in Hz
At room temperature 290o K:
For 1 Hz band width,
Pn = -174 dBm
For 1 MHz Bandwidth
Pn = -114 dBm
Shot Noise:
Source:
random motion of charge carriers in electron tubes
or solid state devices.
Noise in this case will be properly analyzed on based
on noise figure or equivalent noise temperature
Generation-recombination noise:
Recombination noise is the random generation and
recombination of holes and electrons inside the
active devices due to thermal effects. When a hole
and electron combine, they create a small current
spike.
Antenna Noise
In a receiving system, antenna
positioned to collect
electromagnetic waves. Some of
these waves will be the signals
we are interested and some will
be noise at the same frequency of
the received signal. So filters
could not be used to remove such
noise.
Antenna noise comes from the
environment into which the
antenna is looking. The noise
power at the output of the
antenna is equal to KTaB. Ta is
the antenna temperature. The
physical temperature of the
antenna does not influence the
value of Ta.
The noise temperature of the antenna can be
reduced by repositioning it with respect to
sources of external noise
Assumptions
■ Antenna has no earth-looking sidelobes or a backtobe (zero ground
noise)
■ Antenna is lossless
■ h is antenna elevation angle (degrees)
■ Sun not considered ■ Cool. temperate-zone troposphere
Equivalent Noise Temperature and Noise Figure
Noise Figure (F)
Si + N i
Two-port
Network
F = (S/N)i/(S/N)o
Ni = Noise power from a matched load at To
=290 K;
Ni = KTo B.
F is usually expressed in dB
F(dB)=10 log F.
So + N o
Equivalent Noise Temperature (Te)
If an arbitrary noise source is white, so that its power spectral
density is not a function of frequency, it can be modeled as
equivalent thermal noise source and characterized by Te.
white
noise
source
No
R
Te
R
No
R
Te = No/KB, B is generally the bandwidth of the component or
system
Te = To( F – 1)
To is the actual temperature at the input port, usually 290 K
Examples:
(1) the noise power of a bipolar transistor at 3 GHz is
0.001 pW for a 1-MHz bandwidth. What is the noise
temperature?
Solution WN = KTB,
T = WN/KB = 72.5 K
F of the transistor is 0.97 dB
(2) the noise power of a mixer at 20 GHz is 0.01 pW for
a I MHz bandwidth. what is the noise temperature ?
Solution WN = KTB,
T = WN/KB = 725K
F = 5.44 dB
Noise Figure of Cascaded Components
F1
G1
FT = F1 +
F2
G2
F2 – 1
G1
+
FN-1
GN-1
F3 – 1
FN
GN
+ …… +
G 1 G2
Te = To (F - 1)
Ts = Ta + Te
Pn = KTsBG,
where, G is the overall gain of the system
Fn – 1
G1 G2 ….. Gn-1
Noise Figure of Passive and Active Circuits
Passive Components:
For Matching component:
F = L (L Insertion Loss)
Te = To (L-1)
F Increases if the component is mismatched.
Active Devices:
It is generally easier and more accurate to find the noise
characteristics by direct measurement
Conversion Noise
Noise Free signal and Local Oscillator:
-10 dBm
IL=7.5 dB
F=7.5 dB
RF
-17.5 dBm
IF
-130 dBm
-130 dBm
17 dBm
LO
KTB = -130 dBm
Noise Figure = conversion loss
-130 dBm
Noisy received signal:
-10 dBm
IL=7.5 dB
F=7.5 dB
RF
IF
-17.5 dBm
-97.5 dBm
-90 dBm
17 dBm
LO
-130 dBm
-130 dBm
Noisy Local Oscillator:
IL=7.5 dB
F=7.5 dB
-10 dBm
RF
-17.5 dBm
IF
-97.5 dBm
-130 dBm
-130 dBm
LO
17 dBm
-63 dBm
Noise Figure = 40 dB
Example:
FT? Ts ? No ?
L = 3 dB
F = 4 dB Mixer
Given IF bandwidth = 10 MH
Noise
BPF
LNA
L = 1 dB
G = 10 dB
F = 2 dB
Si , Ni
So , No
LO
Ta = 15 K
1) dB to numerical values
LNA
G = 10 dB (10)
BPF: G = -1 dB (0.79) Mixer: G = -3 dB (0.5)
F = 2 dB (1.58)
F = 1 dB (1.26)
F = 4 dB (2.51)
2) FT = [ 1.58 + 0.26/10 + 1.51 /7.9] = 1.8 (2.55 dB)
3) Te = To(F-1) = 290 (1.8 – 1) = 232 K
4) Ts = Ta + Te = 247 K
5) No = KTsBG,
G is the overall Gain = G1×G2×G3×….=10 × 0.79 × 0.5 = 3.95 (~6dB)
No = -98.7 dBm
Dynamic Range, and 1-dB Compression Point
Output
1 dB
power
1 dB compression
point
Input power
Noise floor
Dynamic range
Minimum Detectable Signal (MDS)
MDS is dependent of the type of modulation used in receiving
systems as well as the noise characteristics of the antenna and
receiver. For a given system noise power, the MDS determines the
minimum signal to noise ratio (SNR) at the demodulator of the
receiver. The usable SNR depends on the application, with some
typical values below
System
SNR (dB)
Analog telephone
25-30
Analog television
45-55
AMPS cellular
18
QPSK (Pe = 10-5)
10
To demodulator
Receiver
Ci & Ta
(Co/No)min
Noise
Te
F
G
Ci/Ta Can be measured immediately following the receiver
Detector: Removes the signal from the carrier
S/N Can be determine
Example: FM modulated signal
SNR = C/No - 10 log B + 20 log (fu/fmax) + q w (dB)
Where C/No = carrier to noise density (dBHz)
B = channel bandwidth (Hz)
fu = test tone deviation at 0 dBm (Hz)
fmax = maximum frequency of baseband (Hz)
qw = combined psophometric and preemphasis factors (dB)
Sensitivity: (MDS)
Receiver voltage sensitivity, usually shortened to simply the receiver
sensitivity.
Vimin = (2ZoSimin)0.5
Receiver Dynamic range:
DRr = (maximum allowable signal power) / MDS
Defined by the third-order intercept point
Automatic Gain Control (AGC)
Why?
DR(at the output of the receiver) < DR(at the input)
Avoid receiver non-linearity
Receiver Gain:
should be distributed throughout the RF, IF, & Baseband to avoid nonlinearity of the RF stage and take advantage of low cost IF amplifiers
G ~ 80-100 dB.
Input and Output Receiver Dynamic Range
Pb (V)
Pr
(dBm)
Low gain
1
0
-20
-40
DRr
-60
Ci
~ 80-100 dB
-80
-100
-120
Receiver
Gain
G
DRout
~ 60 dB
0.1
0.01
0.001
High gain
Pb
IF AGC circuit
IF Amp
IF Input
Variable gain
amp/attenuator
Demodulator
AGC
detector
LPF
DC Ref
DC Amp
AGC Distributed Between RF and IF
Filter
LNA
Filter Mixer
IF
amplifier
IF
detector
LPF
Comparator
Frequency Conversion and Filtering
Selection of IF frequency:
fIF = |fRF - fLO|
fLO
fRF
For lower side band selection
fLO = fRF + fIF
Image
IF
IF
Large IF eases the cutoff requirements of the image filter
FIF > BRF/2
IF < 100 MHz
Image frequencies outside RF BW
Low cost
Transmitter
Radiate electromagnetic signal
Output:
 Desired signal power
 Harmonic
 Spurious outputs
 Wideband noise and phase noise,
Critical parameters:
 Frequency and amplitude stability
 Signal’s peak and average powers
Transmitted noise will raise the noise floor of the
receiver
Link Budgets
Baseband
signal
Baseband
output
Gr
Gt
Pr
Pt
Rx
Tx
R
Pt is the transmitted power
Gt is the transmit antenna gain
Gr is the receive antenna gain
Pr are the received power
The power density radiated by an isotropic antenna at a distance
R is given by
Savg = Pt/4pR2 W/m2
The power density radiated by the given antenna is
Savg = Pt Gt /4pR2 W/m2
The received power will be
Pr = Savg Ae Pt Gt Ae/4pR2 W
Ae = Grl2/4p m
The received power can be expressed as
Pr = Pt Gt Grl2 /(4pR)2
W
Pr / Ni = (Pt Gt) [l2 /(4pR)2] Gr / KTAB
= (Pt Gt) [l2 /(4pR)2] (Gr /TA)/KB
= EIRP Path loss Figure of merit / KB
where,
EIRP is the equivalent isotropic radiated power
TA is the antenna noise temperature
G/T is a useful figure of merit for a receive antenna
because it characterizes the total noise power delivered by
the antenna to the input of a
receiver.
The power density of the transmitted wave at the target location
is Wt
Wt = Pt Gt(q,f)/4pRt2 W/m2
RCA Radar cross section area (echo area). It depends on the
angle of incidence, on the angle of observation, on the shape of
the scatterer, on the EM properties of the matter that it is built
of, and on the wavelength.
Some Other Microwave Systems
Baseband Microwave Radio
RF Multiplexing technique
Baseband Repeater
Types of Microwave Devices
Passive Devices
Active Devices
No DC Power & No Electronic control
Uses DC Power or No Electronic control
Duplexers Diplexers
Couplers
Bridges
Dividers
Combiners
Circulators Attenuators
Adapters
Delay lines
Waveguides Resonators
Dielectrics Antennas
Opens, shorts, loads
Switches
Samplers
Transistors
Amplifiers
MMICs
VTFs
Tuners
Synthesizer
Filters
Splitters
Isolators
Cables
TL
R, L, C’s
Multiplexers
Multipliers
Oscillators
RFICs
Modulators
VCAtten’s
Converters
Mixers
Diodes
MICs
VCOs
VCAs