Eng. Bianchi GermanoIRA – RadioAstronomy Institute INAF – National Institute for AstroPhysics

View of the Medicina Radiotelescopes - Italy

ADC bit number and input power ADC bit number and input power needed, in new radio-astronomical needed, in new radio-astronomical

applicationsapplications

Medichats 14 October 2008Medichats 14 October 2008

5 Km

700 mt

THE SKA PROJECTTHE SKA PROJECT(Square Kilometer Array)(Square Kilometer Array)

High sensitivityHigh resolution

- Total area = 1Km2 (sensitivity > 100 time the VLA)

- Large FOV (Field Of View)

- Multiuser and Multitasking

- Frequency coverage = 0.1-25 GHz

- 4 GHz instantaneous band

- Total cost = 1.5 billion €

THE SKA PROJECT: featuresTHE SKA PROJECT: features

RF spectrum scenarioRF 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.

C- BEST-3: 14 N/S antennas + 6 focal lines on the E/W arm. - Ag=7300 m2 Aeff=5100 m2

- Band: 16MHz @ 408MHz - 80 Rx installed

A- BEST-1: One single N/S antenna - Ag=176 m2 Aeff=125 m2

- Band: 16MHz @ 408MHz - 4 RX installed

B- BEST-2: 8 N/S antennas - Ag=1410 m2 Aeff=1000 m2

- Band: 16MHz @ 408MHz - 32 RX installed

BEST-1BEST-2

BEST PROJECTBEST PROJECT(Basic Elements for SKA Training)(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 10000 m2).

4 MHz astronomical protected band

16 MHzRadio relay stationsRadio relay stations

Scientific stratospheric balloon

RF spectrum scenario at the Medicina siteRF spectrum scenario at the Medicina site

Fiber Optic Cable

Front end(16 MHz @ 408 MHz + Optical Tx)

AD

A

Optical RX

DIGITALBACK-END

Which ADC is more suitable?

Receiver room

BEST PROJECT ARCHITECTUREBEST PROJECT ARCHITECTURE

LO

RFI Measurement campaignRFI Measurement campaign

22 mt

0 69.4dBiP dBm 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:

dBmBkT

P syssys 2.107

101log10

310

0 69.4 ( 107.2) 37.8d dBi sysP P P dB The maximum dynamic range result:

Dynamic range estimationDynamic range estimation

KMHzTSYS 2.86408@ MHzBBEST 16

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:

2

10 1030 10log 30 20log2 100P PV V

PR

VP = voltage peak (Volt)P = input power (dBm)

The more the RF scenario is dominated by a strong signal, the more accurate the previous relationship is.

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.

3.66

8.37

6 d

d

PN

Number of bitNumber of bit

Pd = 37.8 dB How many bit correspond?

7 bit

3 bit-107.2 dBmInput noiselevel (KTsysB)

P

-69.4 dBmStrongest RFIslevel 37.8 dB (7 bit)

AD664514 bit

(ENOB = 12 bit @ 100MSPS)

? ?

WHICH GAIN?

7 7 bit for RFIsbit for RFIs3 bit for the astronomical signal3 bit for the astronomical signal

10 bit10 bit

ADC

50

-107.2 dBm

ADCAD6645

- 53 dBm

G = 54.2 dB50

-69.4 dBmStrongest RFIslevel

-107.2 dBmInput noiselevel (KTsysB)

P P-69.4 dBmStrongest RFIslevel

-53 dBmInput ADClevel

G = 54.2 dB

-107.2 dBm

ENOB = 12 bitVIN-PP = 2.2 V

3 bit

VV LSBPP 5372

2.212

mVVV NPP 3.42537 3

mVV

V NPPRMS 48.0

20

2/103.4

20

2/ 3

Measurement bankMeasurement bank

POWER METER

SPECTRUMANALYSER

ANTENNARECEIVERS

ADC 14 bit, 100 MSPS

LABVIEWPROGRAM

POWERSUPPLY

LOGIC ANALYSER ANDADC CONFIGURATION

PROGRAM

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39

Frequency [MHz]

Po

wer

[d

Bm

]

scientific stratospheric balloon radio relaystations

ResultsResults

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:

Following these considerations, an 8 bit A/D converter could work properly.

3 bit for RFIs3 bit for the astronomical signal

6 bits6 bits

This measurement phase ran for few weeks to achieve data with the antenna pointed in all the directions.

ResultsResults

BEST Back EndBEST Back End

A/D1

AD2

Ibob(Serializer)

A/D1

AD2

Ibob(Serializer)

RX1

RX2

RX n-1

RXn

Dual 1GS/sec @ 8 bit

Dual 1GS/sec @ 8 bit

InfinibandCX4 Cables

FPGA 1FPGA 1 FPGA 2FPGA 2

FPGA-3FPGA-3 FPGA-4FPGA-4

FPGA-5FPGA-5

Bee 2

5x Xilinx Virtex-2 Pro 70

4 x A/D @ 8 bit(1.0 GS/sec)

ibob

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.

Estimated number of bit = 10Estimated number of bit = 10Number of bit measured = 6Number of bit measured = 6

ConclusionConclusion