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NATIONAL RADIO ASTRONOMY OBSERVATORYGreen Bank, West Virginia
Electronics Division Internal Report No, 113
3-ELEMENT INTERFEROMETER3,7, 11, AND 21 CM
RECEIVERS
James R. Coe
NOVEMBER 1971
NUMBER OF COPIES: 150
3 -ELEMENT INTERFEROMETER3.7 11 D21 CM RECEIVERS
James R. Coe
This report describes the interferometer receivers and local oscillator systems
used for 3.7 cm, 11 cm, and 21 cm observations.
3. 7 and 11 cm Electronics
The 3-element interferometer was instrumented for 3. 7 and 11 cm measurements
in May 1970. It is capable of receiving and processing both right and left hand circular-
polarized double-sideband signals centered at 8085 or 2695 MHz, or one polarization at
each frequency.
Front-End Boxes
The front-end boxes on each of the three antennas contain a dual frequency-dual
polarization feed, four parametric amplifiers, four mixer preamps, and four IF ampli-
fiers. It also contains a pump and local oscillator source package, a phase-lock-loop
package, a receiver control box and calibration and monitoring components as shown in
Figure L The feed is constructed with the 307 cm horn centered in the 11 cm horn. Di-
electric polarizers are used to convert the circularly polarized waves to orthogonal,
linearly-polarized waves. These waves are then coupled to the output ports of the dual
mode transducers. The paramps are degenerate with the pump frequency twice the local
oscillator frequency. The receivers amplify and down convert both the upper and lower
sidebands. Phase shifters are provided to phase the pump and local oscillator signals to
optimize the paramp-mixer gain. The system temperatures are approximately 100 °IC at
11 cm and 120 'K at 3„ 7 cm.
The pump and local oscillator pump pack . e generates synchronous pump and LO
signals for the 3. 7 cm and 11 cm receivers as shown in Figure 10 The package con-
tains a voltage controlled crystal oscillator. The frequency of this oscillator is con-
trolled by the error signal from the phase-lock-loop box. A phase-locked oscillator is
locked to the crystal oscillator and generates 1347. 5 MHz. The 1347. 5 MHz signal is
amplified to 9 watts. This signal is split three ways through couplers. The 100 milli-
watt 1347. 5 MHz is sent to the phase-lock-loop box for phase comparison. The 800
milliwatt 1347. 5 MHz signal drives a X4 frequency multiplier to generate the 5390 MHz
pump signals for the S-band receivers. The 2695 MHz signal is multiplied by 3 to gene-
rate 8085 MHz for the X-band local oscillator. The 8085 MHz signal also drives a X2
frequency multiplier to generate the 16, 170 MHz X-band pa.ramp pump signals.
The receiver control box monitors the paramp and mixer current and controls the
pump power to keep the pa,ramp bias current constant.
The calibration components permit injection of broadband noise at a 50 Hz rate into
the para.mps to provide a continuous monitor of system noise temperatures, X-band and
S-band sweep signal generators are provided to measure paramp gain,
The model numbers, manufacturers, and specifications of the major components
are listed in Table 1.
Oscillatcj
The interferometer local oscillator system was installed in May 1967. The origi-
nal voltage controlled triode oscillators have been replaced with solid state units.
Block diagrams of the local oscillator systems are shown in Figures 2 and 3. The
local oscillator system minimizes the phase changes due to cable temperature variations
and maintains a fixed phase relationship between the oscillators in each front-end box.
A 1317. 5 MHz signal and 30 MHz signal are sent to the telescope from the interferometer
control building. The 1347. 5 MHz oscillator in each front-end box is locked to the sum
of these two frequencies. Part of the 1347. 5 MHz signal is sent back to the control
building and its phase is compared with the local 131705 and 30 MHz signals. As shown
in Figure 3, a line stretcher is used to maintain the round trip phase constant which re-30 MHz 1duces the phase errors in the loop by a factor of - With the phase1317, 5 + 1347. 5 90'
shifters operating the daily phase variation due to c Me temperature changes is less than
0. 5 degrees at 1347. 5 MHz.
Lobe Rotation
The capability of adjusting the phase of the local oscillator system to reduce the
interferometer fringe frequencies to zero was added to the interferometer LO system in
June 1971. The phase of the 30 MHz reference signals to each telescope is controlled
by the computer. The computer controls the phase difference and the frequency of the
TA
BL
E I
3. 7 a
nd 1
1 c
m D
ual
-Pola
riza
tion F
ront-
End C
om
po
nen
ts
Man
ufac
ture
r
RC
A M
issi
le a
ndR
adar
Div
isio
n
Mic
rom
ega,
Div
isio
n of
Bun
ker
Ram
o
Spe
cifi
cati
ons
Fre
quen
cy 2
645
to 2
745
MH
z80
35 t
o 81
35 M
Hz.
VS
WR
les
s th
an 1
. 1 t
o 1.
Axi
al R
atio
les
s th
an 1
. 03
aver
age.
Gai
n 17
dB
.1
dB
Ban
dw
idth
2695 4
5 M
Hz.
Noi
se T
empe
ratu
re 4
5 °K
.
Com
pone
nt
Inte
rfro
met
er F
eed
Ass
emb
lySK
JB69
0331
Par
amps
26
95 M
Hz
Deg
ener
ate
Par
a-m
etri
c A
mp
lifi
ers,
P/N
28
87
2
8085
MH
z D
egen
erat
e P
ara-
met
ric
Am
pli
fier
s, P
/N 2
84
68
Gai
n 17
dB
.1
dB B
andw
idth
808
5 ±
60
MH
z.N
oise
Tem
oer
atur
e 60
°K
a
Mix
ers
2695
Mix
er, P
/N H
O 4
24M
icro
phas
eC
orpo
rati
on
8085
Mix
er P
/N H
O 4
25
Mod
el 5
-546
9C
omde
l, I
nc.
Pum
p an
d L
O S
ourc
e M
odel
503
634
App
lied
Res
earc
h,
Inc.
Gai
n 25
dB
.IF
1 d
B B
andw
idth
2 t
o 6
0 M
Hz.
No
ise
Fig
ure
DS
B 5
. 5 d
B.
Nois
e F
i!u
re D
SB
7 d
B.
40 d
B G
ain.
1 dB
IF
Ban
dwid
th 5
-40
MH
z.1
dB C
om e
ress
ion
+22
dB
m.
Fre
quei
my P
ow
er O
ut
1347
. 5 M
Hz
100
mW
min
.26
95 M
Hz
10 m
W m
in.
5390
MH
z10
0 m
W m
in.
8085
MH
z12
mW
min
.16
, 170
MH
z12
0 m
W m
in.
4
10 kHz signals generated in the digital lobe rotator. The digital lobe rotator is described
in NRAO Electronics Division Internal Report No. 100 by R. Hallman. The 10 kHz signals
are mixed with 29.990 MHz from a crystal oscillator as shown in Figure 2. The upper
side band (30 MHz) is passed through the crystal filter, amplified and sent to the phase
comparators and then to the telescopes. As shown in Figure 2, the interferometer local
oscillator system can be locked to a rubidium standard for VLB runs. The VLB LO
synthesizer designed by R. Mauzy is utilized to generate the 13 17. 5 MHz local oscillator
RF signal from the 5 MHz VLB reference signal. The 30 MHz is generated by doubling
the 5 MHz reference signal and feeding this signal into the X3 multiplier and power ampli-
fier to generate the local oscillator 3 0 MHz IF signal.
The major components used in the local oscillator system are listed in Table 2.
11 cm and 30 7 cm IF Processing
The four IF signals from each front-end receiver are brought to the equipment
building at the base of each telescope on 1/2 inch semi-rigid cable. RG-9 jumpers are
used at the focal point and around the polar and declination axis. A block diagram showing
the IF equipment at each antenna is shown in Figure 4. A patch panel is provided in each
signal path for equalizing cable lengths. The gain of the IF amplifier is adjustable and is
used to match the amplitudes of the four IF signals. A channel selection relay selects the
two IF signals that are sent to the control building. The 3. 7 cm or X-band IF signals are
designated KR for the right-hand circularly-polarized output and XL for the left-hand
circularly-polarized outputs. The 11 cm or S-band IF signals are designated by SR and
SL. Amplitude equalizers are provided to equalize the cable attenuation with frequency
over the IF frequency band of 5 to 3 5 MHz. A total power monitor is also provided for
each of the four receiver outputs.
The IF signals are transmitted to the Interferometer Control Building through
buried 1/2 inch semi-rigid cable. The IF processing system at the control building is
shown in Figure 5. Two IF signals from each antenna are processed. The IF signals
are amplified and the total power is monitored in the IF monitor section. The IF sig-
nal is then routed through the delay system, through the IF Level Control Section and
to the correlators ° The delay system consists of a set of 14 binary delay elements with
the smallest 1.9 nanosec and the longest 16 psec for a total delay range of 0 to 3 1.998 psece
13 1
7. 5
MH
z F
ilte
rsT
elon
ic T
BA
13
17. 5
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6551
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on
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Com
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ryst
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scil
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odel
CO
-228
Vec
tron
Lab
ora
tori
es,
Inc.
Out
put
101.
3461
54 M
Hz.
Pow
er o
utpu
t 5
mil
liw
atts
.S
tab
ilit
y 3
par
ts i
n 1
08 p
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ay.
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ower
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put
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atts
, gai
n 10
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.
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er D
ivid
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o. 1
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el A
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etro
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Spe
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ons
1304-1
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1 M
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90-1
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z 30
dB
poi
nts
Inse
rtio
n l
oss
4 dB
Cen
ter
freq
uen
cy 3
0 M
Hz
6 kH
z ba
ndw
idth
60 d
B a
t 29
. 990
MH
z.
LE
L 3
0 M
Hz
Am
plif
ier
Lim
iter
Mod
el I
F 6
904
LE
L D
ivis
ion
of
Var
ian
Mas
ter
Osc
illa
tor
Man
ufac
ture
r
TA
BL
E 2
Lo
cal
Osc
illa
tor
Sy
stem
Co
mp
on
ents
6
The delay system contains 5 quartz ultrasonic delay lines which have a center frequency
of 60 MHz. The IF signal of 5 to 35 MHz is mixed with a 40 MHz signal to convert it up
to 45 to 75 MHz for transmission through the delay elements. It is then converted back
down to 5 to 35 MHz using the same 40 MHz signal and sent to the IF level control sec-
tion. The IF level control system holds the IF level into the correlators constant at
1 milliwatte The gain of the IF amplifier is varied to keep the IF level detector output
constant.
The IF level detector output is also used to provide a noise temperature monitor.
The noise modulation injected into the paramps in each front-end is synchronously de-
tected. The change in synchronous detector output is proportional to
where AT is the change in system temperature and T is the initial system temperature.sys
The IF signal from the level control section is sent to the six-way power divider
in the correlator section. A six-way divider was utilized to provide parallel and cross-
polarization correlators for a four-element dual-polarization interferometer. At the
present time only 12 of the 24 correlators provided are utilized. The correlator consists
of a double-balanced mixer and a DC amplifier with a low-pass bandwidth of 10 Hz. The
correlator amplifier gain steps are 50, 500, and 5000 for the parallel-polarized signals
and 500 and 5000 for the cross-polarized correlators. These gains are controlled by the
interferometer on-line computer.
21 cm Electronics
The interferometer was instrumented for 21 cm line measurements in June 1971.
The additions to the existing interferometer consist of a 21 cm feed and receiver for
each antenna. IF converters, Lobe Rotators, and the Digital Correlation Receiver Model
III are required at the interferometer control building.
21 cm Front End
The 21 cm feed is mounted over the existing interferometer feed and connected
to the receiver through 7/8 inch rigid air line cable. The receiver is mounted on a plate
in the 3.7 and 11 cm front-end box. Figure 6 shows the 21 cm receiver and Table 3 lists
the major components. The paramp is non-degenerate with a pump frequency of 20. 815
GHz. The paramp has a gain of 18 dB with a 1 dB bandpass of 1370 to 1430 MHz. The
pump source is an avalanche diode oscillator followed by a tripler to generate 80 milli-
watts. The 1347. 5 MHz signal from the interferometer phase-locked oscillator in each
front-end box is used as a local oscillator signal for the mixers. The IF frequency band
is from 22. 5 to 8205 MHz. The IF signal is sent to the base of the antenna through 1/2
inch semi-rigid cable and the total power is monitored at the antenna.
The IF signal is transmitted to the interferometer control building through one of
the buried semi-rigid coax lines.
At the control building the IF signals are converted to a 30 MHz wide band
centered at 20 MHz for transmission through the delay system, IF level control, and
to the correlators.
The IF processing is shown in Figure 7. The input signal from the telescope is
amplified, part of the signal is coupled into the IF monitor detector and the remainder
converted up to a 30 MHz band centered at 120 MHz. After the IF signal is filtered in
the 105 to 135 MHz filter, it is mixed with the 140 MHz to give a signal of 5 to 35 MHz.
This signal is then sent through the delay system, the IF level control and to the correla-
tors. The correlators are connected to give a sin and cosine component for each of the
three baselines. The analog correlator arrangement is shown in Figure 7. Part of the
IF signal (5 to 35 MHz is mixed with 40 MHz to provide a 10 MHz wide signal centered
at 30 MHz for processing by the Digital Correlation Receiver Model III. Therefore, the
digital correlator processes the frequency spectrum which was centered at 10 MHz as
it passed through the delay system. This minimizes phase changes as the delays switch.
Each delay step of 1.9 nanosecs causes 608° phase shift at 10 MHz.
The portion of the RF input spectrum which is processed by the digital correla-
tor is determined by the frequency synthesizer. The frequency synthesizer is computer
controlled. The digital control is described in the NRAO Electronics Division Internal
Report No. 102 by D. Schiebel. The frequency synthesizer setting can be determined
Cen
ter
freq
uenc
y 14
05 M
Hz.
RF
to I
F G
ain
60 d
B.
1 dB
IF
Ban
d 10
to
110
MH
z.N
oise
fig
ure
9 dB
max
imum
.
3 dB
ban
clpa
ss 1
200
to 1
460
MH
z.G
ain
25 d
B.
Noi
se f
igur
e 4
dB.
App
lied
Tec
hnol
ogy,
Div
. of
Itek
Tra
nsi
stor
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p S
P-1
405/7
0P
art
No 3
1-0
23873-0
1
Mic
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er P
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MC
60-1
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55-9
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atio
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Mic
row
ave
Cor
pora
tion
Out
put f
requ
ency
20,
815
MH
z.P
ower
out
put 8
0 m
W.
.6 t
o 2
0 d
B +
8 t
o -
8 V
.
Alt
erna
te S
ourc
e P
ump
Osc
illa
tor
Par
t N
o. 1
2110
-336
33an
dD
iode
Att
enua
tor,
Par
t N
o. 3
4165
TA
BL
E 3
21 c
m R
ecei
ver
Com
pone
nts
Com
pone
nt
L-B
and
Sca
lar
Fee
d
7/8"
Coa
x C
oupl
er M
odel
SP
9266
Par
amet
ric
Am
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fier
Mod
el 2
6339
Man
ufac
ture
rS
peci
fica
tion
s
Fre
quen
cy r
ange
136
0-14
40 M
Hz.
Po
lari
zati
on l
inea
r.V
SW
R <
1.2
Con
trol
Dat
aC
orpo
rati
on
Cou
plin
g 30
dB
.V
SW
R L
04
max
imum
.In
sert
ion
loss
002
dB
.
Mau
ryM
icro
wav
eC
orpo
rati
on
1 dB
ban
dwid
th 1
320
to 1
430
MH
z.N
oise
tem
pera
ture
60
°K m
axim
um.
Gai
n 18
dB
.P
ump
pow
er <
50
mil
liw
atts
.
Mic
rom
ega
Syl
vani
a K
-Ban
d S
ourc
eSY
G-2
030
Syl
vani
aE
lect
roni
cC
ompo
nent
s
Out
put
freq
uenc
y 20
, 815
± 5
0 M
Hz.
Pow
er o
utpu
t 80
mil
liw
atts
.
by subtracting 1217. 5 MHz from the required observing frequency. For example, to
receive and process 1420 MHz the synthesizer setting would be 202. 5 MHz.
The 21 cm system is operated single sideband and IF path length changes shift
the phase of the correlator outputs. The 3. 7 and 11 cm systems are double sideband
systems and the correlator output phase is independent of IF path length changes.
CONTRIBUTORS
The development of the interferometer electronics described in this report has
required assistance from most of the NRAO Electronics Division and Machine Shop
personnel. M. Balister and S. Weinreb provided general design guidance. K. Wesseling
designed the local oscillator system. R. Mauzy designed the para,mp controls and mixer
current monitors, and monitored development of the pump and local oscillator sources.
The front-end box temperature controllers were designed by J. Payne and constructed
by R. Becker. J. Payne designed and S. Mayor constructed the noise tube power supplies.
J. Davis designed the system malfunction detector. 0. Bowyer, D. Williams, and
M. Barkley designed the dual-frequency feed and 21 cm feed mounting system. A. Miano
provided considerable assistance in designing the layout for the front-end box. R. Ervine,
J. McCormack, J. Oliver and W. Shank constructed and tested the receivers, IF proces-
sing and local oscillator systems.
TR1C
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. 7