1. Medichats 14 October 2008
ADC bit number and input power needed, in new radio-astronomical applications
View of the Medicina Radiotelescopes - Italy
Eng. Bianchi Germano
IRA – RadioAstronomy Institute
INAF – National Institute for AstroPhysics

2. THE SKA PROJECT (Square Kilometer Array)
5 Km
700 mt

3. THE SKA PROJECT: features
High sensitivity
High resolution
Total area = 1Km2 (sensitivity > 100 time the VLA)
Large FOV (Field Of View)
Multiuser and Multitasking
Frequency coverage = GHz
4 GHz instantaneous band
Total cost = 1.5 billion €

4. RF spectrum scenario SKA band
We have studied a procedure to estimate the required number of bit (resolution and dynamic) and the ADC input power level in radio astronomical applications.

5. (Basic Elements for SKA Training)
BEST PROJECT
(Basic Elements for SKA Training)
The experiences gained with the BEST demonstrator is very suitable for the SKA community: in the final configuration it will have about 8000 m2 of collective area, a value comparable with the area of a proposed SKA station (about m2).
A- BEST-1: One single N/S antenna
- Ag=176 m2  Aeff=125 m2
- Band: 408MHz
- 4 RX installed
BEST-2
BEST-1
B- BEST-2: 8 N/S antennas
- Ag=1410 m2  Aeff=1000 m2
- Band: 408MHz
- 32 RX installed
C- BEST-3: 14 N/S antennas + 6 focal lines on the E/W arm.
- Ag=7300 m2  Aeff=5100 m2
- Band: 408MHz
- 80 Rx installed

6. RF spectrum scenario at the Medicina site
Radio relay stations
Radio relay stations
16 MHz
Scientific stratospheric
balloon
4 MHz astronomical protected band

7. BEST PROJECT ARCHITECTURE
Front end
( MHz + Optical Tx)
Which ADC
is more suitable?
Optical RX A
DIGITAL
BACK-END A D
Fiber Optic Cable LO
Receiver room 7

8. RFI Measurement campaign
22 mt

9. Dynamic range estimation
0dBi the power level referred to an isotropic antenna (unitary gain in all directions).
The equivalent system input noise power for 16 MHz bandwidth, in a single N-S antenna, is:
The maximum dynamic range result:

10. Pd = 37.8 dB  How many bit correspond?
Number of bit
Pd = 37.8 dB  How many bit correspond?
Since an ADC converts voltage into bit and not power into bit, we need a relationship between the power and the voltage at the input of the A/D converter. If we consider the simplest possible situation, where there is only a monochromatic tone at the input of the ADC, we can easily find the relationship:
VP = voltage peak (Volt)
P = input power (dBm)
From this equation, each bit corresponds to an increment of 3dB in voltage and 6dB in power. If we divide the dynamic range Pd by 6, we can obtain the required ADC number of bits.
7 bit
The more the RF scenario is dominated by a strong signal, the more accurate the previous relationship is.

11. 10 bit 7 bit for RFIs 3 bit for the astronomical signal P AD6645
-69.4 dBm
Strongest RFIs
level
37.8 dB (7 bit)
dBm
Input noise
level (KTsysB)
3 bit
AD6645
14 bit
(ENOB = MSPS)
107.2 dBm ?
WHICH GAIN?
ADC
50 

12. G = 54.2 dB P P - 53 dBm 107.2 dBm -69.4 dBm -53 dBm -69.4 dBm
Strongest RFIs
level
- 53 dBm
-53 dBm
Input ADC
level
-69.4 dBm
Strongest RFIs
level
G = 54.2 dB
dBm
Input noise
level (KTsysB)
3 bit
107.2 dBm
ADC
AD6645
50 
ENOB = 12 bit
G = 54.2 dB
VIN-PP = 2.2 V

13. Measurement bank POWER METER SPECTRUM ANALYSER ANTENNA RECEIVERS POWER
SUPPLY
LOGIC ANALYSER AND
ADC CONFIGURATION
PROGRAM
LABVIEW
PROGRAM
ADC
14 bit, 100 MSPS

14. Results
scientific stratospheric balloon
radio relay
stations

15. Results 6 bits 3 bit for RFIs 3 bit for the astronomical signal
This measurement phase ran for few weeks to achieve data with the antenna pointed in all the directions.
From the measures we performed, only 3 bit seem to be necessary to sustain the man made radio signals, so the ADC total bit required is 6:
3 bit for RFIs
3 bit for the astronomical signal
6 bits
Following these considerations, an 8 bit A/D converter could work properly.

16. BEST Back End ibob Bee 2 4 x A/D @ 8 bit 5x Xilinx Virtex-2 Pro 70
(Serializer)
RX1
RX2
RX n-1
RXn
Dual 8 bit
Infiniband
CX4 Cables
FPGA 1
FPGA 2
FPGA-3
FPGA-4
FPGA-5
Bee 2
5x Xilinx Virtex-2 Pro 70
4 x 8 bit
(1.0 GS/sec)
ibob
Finally the signal arrives to the back end.
As back end we use the BEE2 system developed by the Berkeley University for the Allen Telescope Array, the US SKA demonstrator based on small dish with single wide band feed.

17. Estimated number of bit = 10 Number of bit measured = 6
Conclusion
Estimated number of bit = 10
Number of bit measured = 6
We attribute the difference between the estimated bit number and the measured one to the different antenna systems:
Yagi antennas pointed towards the horizon and working in the max-hold mode.
VERSUS
A half wavelength dipole focal line inside a cylindrical reflector pointed towards the sky.
We have concluded that our estimation method to valuate the number of bits is conservative, but applicable to radio astronomical scenario.
Further investigations should be performed to reduce the difference between the estimated and actual requested number of bit.