Upgraded Baseband System

Upgraded Baseband System
National Centre for Radio Astrophysics
Tata Institute of Fundamental Research,
Pune University Campus, Pune INDIA
Internal Technical Report
Upgraded Baseband System
For 32 MHz GSB
Prakash Hande / Ajithkumar B.
Giant Metrewave Radio Telescope, Narayangaon, Khodad, 410504
Prakash@gmrt.ncra.tifr.res.in
Ajith@gmrt.ncra.tifr.res.in
Objective:
In receiver room, re arrange the baseband system into a new location and making
a space for installing a GMRT Analog Backend system in place of exiting baseband
system. So both the system can be use for observation.
Upgraded Baseband System- For 32 MHz GSB
Page 1
Introduction:
At the GMRT, the dual polarized voltage signals from each antenna are processed
through super heterodyne receivers and finally brought to a central location for
further processing.
The intermediate frequency signals from each antenna are then down-converted to
baseband
signals and fed to the digital signal processing backend.
A baseband system is
used for converting the IF signal to baseband signal to get a 32 MHz bandwidth
from each polarization. This baseband system is currently installed in 8 racks and
one rack for 4th Local Oscillator, which is at present located in Receiver room.
Analog backend receiver of GMRT is being upgraded as part of the eleventh plan.
These major upgrade in specification related to the analog section including
complete processing of the RF signals at the central station. For installing new
GMRT Analog backend System in receiver room, there is need for some space. So it
has been decided that this GAB system can be installed in place of current
baseband system and make a new arrangement for exiting baseband system in
receiver room.
Upgraded Baseband System- For 32 MHz GSB
Page 2
Receiver room rack configuration:
i)
Before modification:
Baseband system was installed in 8 baseband Racks (BB1 to BB8 racks), where
Channel 130 for all antennas were installed in BB1 to BB4 racks and channel 175
for all antennas were installed in BB5 to BB8 rack. All input cables were coming
from OF rack to all this rack and output cables were going from BB rack to windows
1 &2. IV th LO system and control PC for IV th LO was installed in BBLO & CON PC
racks. This is shown below.
Upgraded Baseband System- For 32 MHz GSB
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ii)
After modification:
Now the new Rack rearrangement made in such a way that the baseband system
installed in 4 racks ( BB1,BB2,BB3, BB4 ) and made a space for installing 8 racks
(GAB1 to GAB8 racks) to new GAB system. IV th LO rack is been shifted near FTs
rack. New Receiver rack arrangement after modification is shown below.
Upgraded Baseband System- For 32 MHz GSB
Page 4
Baseband system new configuration:
The baseband system contains two units, the J41 unit and the new IF conversion
unit. Both these units are housed in a metallic box. The J41 and the new IF
conversion units together make up the baseband system. The units are available
for a channel of 175 MHz as well as 130 MHz. These units play the key role in the
down conversion of the RF signal.
Block Diagram of J41 UNIT
Upgraded Baseband System- For 32 MHz GSB
Page 5
NEW IF CONVERSION UNIT:
The J41 and New IF conversion basically work on the super heterodyne principle.
The RF and LO signals given to the J41 as inputs. The mixer in the J41 converts
these two signals ,of frequency frf and flo , into the two new heterodyne
frequencies frf + flo and frf - flo. The mixer may inadvertently produce additional
frequencies such as third- and higher-order intermediation products. Ideally, the IF
bandpass filter removes all but the desired IF signal at fIF. The frequency of the
local oscillator fLO is set so the desired reception radio frequency fRF mixes to fIF.
There are two choices for the local oscillator frequency because the dominant mixer
products are at fRF ± fLO. If the local oscillator frequency is less than the desired
reception frequency, it is called low side injection (fIF = fRF - fLO); if the local
oscillator is higher, then it is called high-side injection (fIF = fLO - fRF). The IF
stage includes a filter and / or multiple tuned circuits in order to achieve the
Upgraded Baseband System- For 32 MHz GSB
Page 6
desired selectivity. This filtering must therefore have a band pass equal to or less
than the frequency spacing between adjacent broadcast channels. Ideally a filter
would have a high attenuation to adjacent channels, but maintain a flat response
across the desired signal spectrum in order to retain the quality of the received
signal. Therefore a selected frequency of 70 MHz is filtered out and amplified and
passed on to the new IF conversion unit. A central frequency 16 MHz signal in a 32
MHz bandwidth is the final output of the system.
Connection between them is as per below.
130/175
105/200 LO
Output for GSB
4th LO input
J41 PIU
New IF Conversion
RF Cable Details
All input cables of 130 & 175 channel are taken from OF racks and routed to 130 &
175 racks. Output cables of all channels routed from BB130 racks to windows1 and
output cables of all channels routed from BB175 racks to windows 2 for GSB
system. 105 Mhz, 200 Mhz, & 4th LO cables are routed from LO rack to all BB racks.
Control PC for 4th LO is presently kept near to LO rack.
Upgraded Baseband System- For 32 MHz GSB
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RACK1 – (130CH) - CONNECTION DETAILS – RACK TOP
Input connections
Conn.
I/P1
I/P2
I/P3
I/P4
I/P5
I/P6
I/P7
I/P8
Cable
L01
L02
L03
L04
L05
L06
L07
L08
Conn.
I/P9
I/P10
I/P11
I/P12
I/P13
I/P14
I/P15
I/P16
Output Connections
Cable
L09
L10
L11
L12
L13
L14
L15
L16
Conn.
O/P1
O/P2
O/P3
O/P4
O/P5
O/P6
O/P7
O/P8
Cable
O01
O02
O03
O04
O05
O06
O07
O08
Conn.
O/P9
O/P10
O/P11
O/P12
O/P13
O/P14
O/P15
O/P16
Cable
O09
O10
O11
O12
O13
O14
***
***
Conn.
3rd LO
3rd LO
4th LO
Cable
Upgraded Baseband System- For 32 MHz GSB
Page 8
RACK2 – (130CH) - CONNECTION DETAILS – RACK TOP
Input connections
Conn.
I/P1
I/P2
I/P3
I/P4
I/P5
I/P6
I/P7
I/P8
Cable
L17
L18
L19
L20
L21
L22
L23
L24
Conn.
I/P9
I/P10
I/P11
I/P12
I/P13
I/P14
I/P15
I/P16
Output Connections
Cable
L25
L26
L27
L28
L29
L30
L31
L32
Conn.
O/P1
O/P2
O/P3
O/P4
O/P5
O/P6
O/P7
O/P8
Cable
O15
O16
O17
O18
O19
O20
O21
O22
Conn.
O/P9
O/P10
O/P11
O/P12
O/P13
O/P14
O/P15
O/P16
Cable
O23
O24
O25
O26
O27
O28
O29
O30
Conn.
3rd LO
3rd LO
4th LO
Cable
Upgraded Baseband System- For 32 MHz GSB
Page 9
RACK1 – (175CH) - CONNECTION DETAILS – RACK TOP
Input connections
Conn.
I/P1
I/P2
I/P3
I/P4
I/P5
I/P6
I/P7
I/P8
Cable
K01
K02
K03
K04
K05
K06
K07
K08
Conn.
I/P9
I/P10
I/P11
I/P12
I/P13
I/P14
I/P15
I/P16
Output Connections
Cable
K09
K10
K11
K12
K13
K14
K15
K16
Conn.
O/P1
O/P2
O/P3
O/P4
O/P5
O/P6
O/P7
O/P8
Cable
P33
P34
P35
P36
P37
P38
P39
P40
Conn.
O/P9
O/P10
O/P11
O/P12
O/P13
O/P14
O/P15
O/P16
Cable
P41
P42
P43
P44
P45
P46
***
***
Conn.
3rd LO
3rd LO
4th LO
Cable
Upgraded Baseband System- For 32 MHz GSB
Page 10
RACK2 – (175CH) - CONNECTION DETAILS – RACK TOP
Input connections
Conn.
I/P1
I/P2
I/P3
I/P4
I/P5
I/P6
I/P7
I/P8
Cable
K17
K18
K19
K20
K21
K22
K23
K24
Conn.
I/P9
I/P10
I/P11
I/P12
I/P13
I/P14
I/P15
I/P16
Output Connections
Cable
K25
K26
K27
K28
K29
K30
K31
K32
Conn.
O/P1
O/P2
O/P3
O/P4
O/P5
O/P6
O/P7
O/P8
Cable
P47
P48
P49
P50
P51
P52
P53
P54
Conn.
O/P9
O/P10
O/P11
O/P12
O/P13
O/P14
O/P15
O/P16
Cable
P55
P56
P57
P58
P59
P60
P61
P62
Conn.
3rd LO
3rd LO
4th LO
Cable
Upgraded Baseband System- For 32 MHz GSB
Page 11
Antenna Configuration : 130 RACK 1 & 175 RACK 1
C00
C02
C03
C01
C04
C06
C08
C05
C09
C10
C11
C12
C13
C14
Upgraded Baseband System- For 32 MHz GSB
Page 12
130 RACK 2 & 175 RACH 2
W01
W02
W05
W06
E04
E05
W03
W04
E02
E03
E06
S01
S04
S02
S06
S03
Upgraded Baseband System- For 32 MHz GSB
Page 13
4th LOCAL OSCILLATOR :
4Th Local Oscillator system
Upgraded Baseband System- For 32 MHz GSB
Page 14
LO Distribution:
130 RACK
130 RACK
Baseband
12Wa
y
Splitte
Baseband
System
Rack-1 130 CH
12Wa
y
Splitte
Baseband
System
Rack-2 130 CH
2Way
Splitte
175 RACK
Baseband
12Wa
y
Splitte
Baseband
System
Rack-1 175 CH
12Wa
y
Splitte
Baseband
System
Rack-2 175 CH
2Way
Splitte
Upgraded Baseband System- For 32 MHz GSB
Page 15
175 RACK
105MHz
12Wa
y
Baseband
System
Rack-1 175 CH
12Wa
y
Splitte
Baseband
System
Rack-2 175 CH
2Way
Splitte
r
130 RACK
200MHz
12Wa
y
Splitte
Baseband
System
Rack-1 130 CH
12Wa
y
Baseband
System
Rack-2 130 CH
2Way
Splitte
Upgraded Baseband System- For 32 MHz GSB
Page 16
Psc 4-3
power
Divider
(-6.5dB)
Psc 12-1
power
Divider
(-11.6dB)
Upgraded Baseband System- For 32 MHz GSB
Page 17
Baseband System Testing after Modification:
Test Set Up :
Sig Gen
Baseband System
Spectrum
Analyzer
Procedure:
For measuring 130 CH.
1. Connect Sig. Gen to 130Ch input of baseband system at Optic Fiber End of selected Ant.
2. Set Sig. Gen – Center Frequency: 130Mhz, Amplitude: -47dBm
3. Connect Spectrum Analyzer to output of baseband system of selected Ant. at Corr. Window end.
4. Setting of Spectrum Analyzer : Center Freq. 19Mhz Span : 10 Mhz, RBW: 300 Khz, VBW:3Khz
5. Set Baseband LO : 51MHz
6. Note Down the Peak Marker Value @19Mhz on Spectrum Analyzer.
7. Repeat the procedure for Antenna of 130Channel.
For measuring 175 CH.
1. Connect Sig. Gen to 175Ch input of baseband system at Optic Fiber End of selected Ant.
2. Set Sig. Gen – Center Frequency: 175Mhz, Amplitude: -47dBm
3. Connect Spectrum Analyzer to output of baseband system of selected Ant. at Corr. Window end.
4. Setting of Spectrum Analyzer : Center Freq. 19Mhz Span : 10 Mhz, RBW: 300 Khz, VBW:3Khz
5. Set Baseband LO : 51MHz
6. Note Down the Peak Marker Value @19Mhz on Spectrum Analyzer.
7. Repeat the procedure for Antenna of 175Channel.
Upgraded Baseband System- For 32 MHz GSB
Page 18
Input Power@OF End : -47dBm
Antenna.
C0
130 Ch. Power@BB o/p(in dBm)
-16.8
175 Ch. Power@BB o/p(in dBm)
-15.9
C1
-16.5
-14.8
C2
-18.0
-17.6
C3
-16.2
-16.3
C4
-16.1
-13.5
C5
-18.3
-20.1
C6
-18.6
-14.7
C8
-17.7
-15.4
C9
-17.0
-18.3
C10
-15.7
-16.2
C11
-18.3
-16.2
C12
-16.0
-17.2
C13
-16.3
-15.3
C14
-14.0
-15.0
W01
-17.0
-15.0
W02
-16.5
-14.0
W03
-17.3
-17.7
W04
-16.2
-16.4
W05
-17.0
-15.3
W06
-17.0
-17.9
E02
-17.2
-15.0
E03
-16.4
-16.2
E04
-15.8
-18.7
E05
-14.1
-15.3
E06
-22.4
-16.8
S01
-14.5
-15.4
S02
-11.0
-18.0
S03
-13.0
-16.0
S04
-16.7
-16.0
S06
-14.6
-16.5
Upgraded Baseband System- For 32 MHz GSB
Page 19
Baseband system Power Response
Input Power (in dBm)
-66
-64
-62
-60
-58
-56
-54
-52
-50
-48
-46
-44
-42
-40
-38
-36
-34
-32
-30
Output Power (in dBm)
-36
-34.5
-32.5
-30.4
-28.5
-26.3
-24.4
-22.5
-20.5
-18.3
-16.3
-14.2
-12.1
-10.3
-8.4
-6.3
-4.3
-2.3
-0.4
Upgraded Baseband System- For 32 MHz GSB
Page 20
Baseband system Power Response
Upgraded Baseband System- For 32 MHz GSB
Page 21
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