Transcript Document

Radio Astronomy Receivers
Roger D. Norrod
NRAO-Green Bank
Receiver Front-End
LNA
FEED
OMT/
Polarizer
MIXER
IF
LO
FEEDS
Reflector angle strongly influences feed design
M
The function of the feed is to interface between EM fields at the reflector focal
point, and a guided transmission line, waveguide or coaxial.
Corrugated Horns
Corrugated horns are most common:
Figure from Microwave Horns and Feeds, A.D. Olver, et.al.
Orthomode Transducer (OMT)
The OMT separates orthogonal linear polarizations in a square or circular
waveguide port to two independent channels.
OMT
Circular Polarization
Addition of a phase shifter which delays one polarization by 90Eforms a
Polarizer which receives (or transmits) circular polarization. Difficulity in
building a broadband phase shifter often limits the bandwidth of modern
receivers.
Phase
Shifter
OMT
Thermal Noise Voltage
æ aö
=4kTRòç a ÷df
f1èe-1ø
f2
Filter
R at T
2
rms
v
B = f2 - f1
hf
a =
kT
If a << 1, vrms = 4RkBT
2
Available Thermal Noise Power
Filter
R at T
B = f2 - f1
R
P n = kB T
Equivalent Noise Source Temperature
Unknown source
with thermal and/or
non-thermal noise
Filter
B = f2 - f1
Measure Pn, calculate:
Pn
Ts =
kB
Amplifer Noise Temperature
Noise
Source
G
Po (bandlimited to B)
Ts
Po = GkBTs + K
Define K = GkBTe
Then, Po = GkB(Ts + Te)
Te is the amplifier Equivalent Input Noise Temperature
Noise of HFET Amps from a Recent Project
Data courtesy M. Pospieszalski of NRAO Central Development Laboratory
Amplifier Cascade Noise
G 1 , T1
Noise
Source
A1
G 2 , T2
A2
Po
Ts
Po = G1G2kBTs + G1G2kBT1 + G2kBT2
or,
Po = G1G2kB ( Ts + (T1 + T2/G1))
So, Amplifier Cascade has equivalent noise T1 + T2/G1
Input Losses
G1 < 1
Noise
Source
Loss
G 2 , T2
Po
Ts
Let L = 1/G1, then for ohmic loss at physical temperature To,
the effective noise temperature of the loss is (L-1)T o .
Effective noise temperature of the loss - amplifier cascade
is: (L-1)To + LT2 .
Frequency Conversion
LNA
MIXER
IF
Fif = |mFrf ± nFlo|
Frf
Flo
Flo
Fif
Fif
Why?
Frf
Frf
P Tunability
P Cost
P Performance
Flo
Tche bys cheff filte r re s pons e in dB:
Tcheb y( n     )
n
1 0 l og 1
  co s( n  aco s(  ) )
1 0 l og 1
  co sh( n  aco sh(  ) )
2
if  1
2
if   1
N umber of Resonat ors
5
0 .1
1
3
10
10
10
1
10

0 .1 0 .15 3
1
0.1dB Ripple  2
5 Poles
10
1
1dB Ripple
3
10
1
3 dB R ipple
Tcheb ysch eff fo r 0 .1, 1 , 3 dB Ri pp l e
50
40
Tch eb y( n    1 )
30
Tch eb y( n    2 )
Tch eb y( n    3 )
20
10
0
0
0 .5
1
1 .5

2
2 .5
3
0.1dB Ripple
Tcheb ysch eff Resp on se fo r 3 ,5,7 ,9 P ol es
80
70
60
50
Tch eb y( 3    1 )
Tch eb y( 5    1 )
Tch eb y( 7    1 )
40
Tch eb y( 9    1 )
30
20
10
0
0
0 .5
1
1 .5

2
2 .5
3
Lowpass Filter Circuit Prototype
Low pas s to Bandpas s M apping
 l pf(    o  B)
1 
B o
1
2

1 9.8
o

o
2 0.2
 1 2
B
2
1
o
1 8 1 8.0 5 2 2
Lo wp ass Resp on se Map ped to Ban dp ass
80
70
60
50
Tcheby( 5   lpf(    o  B )  1 )
40
30
20
10
0
18
19
20

21
22
Lowpass to Bandpass Mapping
Microwave Bandpass Filters
QL N
Loss = 4.34
 gi
QU i =1
QL =
QU 
fo
BW
X
R
Filter Q
Resonator or Circuit Element Q
Receiver Stability
G
t =
B
1
RC
Vo  G Tant
Tant
Vo
Vdc
Vac
Vac, rms
1
=
Vdc
Bt
Example: B = 100 MHz, J= 0.1 s, then
Vac, rms
= 003%
.
Vdc
Receiver Linearity
Pout
Pin
Linear
Saturated
Output contains not just pure sinusoids, but clipped sinusoids
and product terms. In the frequency domain, these translate to
harmonics and sum/difference terms.
Third-order Intermodulation
The most troublesome are usually 2f1-f2 products.
Po
F1
2F 1-F2
F2
Third-order
2F 2-F1
3:1
1:1
Fundamental
Pi
Summary - Centimeter Receivers
P Most common topology is dual-polarization superheterodyne with cooled HEMT low-noise amplifiers.
P Feed, OMT/Polarizer, and first amplifiers are the most
critical components.
P Gain stability must be considered in order to achieve
expected sensitivity.
P Linearity of active mixers, amplifiers, and other active
components determines performance in presence of
interference.