Up gradation of IF system
and
New ALC

Author

Sandeep C. Chaudhari

Project Supervisor

Mr. T. L. Venkatasubramani

Mr. Shrinivas Pandharpure

What is ALC? And Need of it.

ALC is a system by means of which the overall gain of the receiver is varied automatically with the changing strength of the received signal to keep the output substantially constant.

Need of ALC :-

ALC provide the appropriate operating power level for the subsequent stage, so as to keep it in linear operating region.

ALC 'irons out' i/p signal amplitude variation & the gain control doesn't have to be readjusted every time

Role of ALC/VGA in GMRT system

ALC Mode:-

To keep the laser diode in Fiber optics system in it's linear operating region by providing appropriate power level to OFS.

VGA Mode :-

To change or adjust any gain-loss distribution of overall system.

Drawbacks of Old ALC/VGA

Total power dynamic range (ALC OFF) is very less, only 14 dB.

ALC ON dynamic range is also poor, only 18 dB.

Steps in which gain could be varied is 2 dB.

Advantages Of New ALC/VGA

ALC OFF/ VGA Mode :-

Total power dynamic range is 28 dB, compared to old ALC's 14 dB.

TOI is +27 dBm.

Output 1 dB compression point is +15 dBm, compared to -3 dBm.

Accuracy in gain adjustment is improved to +/- 0.5 dB, compared to old ALC's 2 dB.

Overall range of gain adjustment in IF system is 65 dB, compared to old ALC system's 60 dB.

Advantages Of New ALC/VGA

ALC ON Mode :-

Dynamic Range is 32 dB, compared to old ALC's 18dBm.

Output IMD :- 55 dBc.

ALC time constant :-

Trise = 220 millisecond @5 dB step size and @PBaseline = -35dBm.

T fall = 950 millisecond @5 dB step size and @PBaseline = -35dBm.

Block Diagram of New ALC/VGA

Circuit Diagram

Major Components

AD8367	:-	ALC/ VGA.

DAC 0800 :-	Useful in generating gain equivalent analog	voltage.

AD8310	:-	Senses O/p power of ALC unit.

MAR-4	:-	Provides proper operating point for ALC IC.

MSA0520	:-	Output stage for power amplification.

Characteristics of AD8367

Direction of gain of ALC can be controlled by pulling MODE pin either LO with negative slope or HI with positive slope.

To use AD8367 for both purpose i.e. ALC and VGA MODE pin is connected to ground. Hence the Gain Vs VGAIN characteristics with negative slope is selected.

Range of Gain variation -5 to 45 dB, with range of gain voltage from 1 V to 0 V.

VGA Mode control

DAC0800 is used to generate analog control voltage from digital bits with 1.28 V reference voltage given to DAC.

WHY 1.28 V REF. TO DAC?

The Range of control voltage for ALC is from 50mV to 950mV.

The linear in dB scale of AD8367 is 20mV/dB.

So for 0.5 dB resolution we require 10mV/LSB.

Also with 7 bits of control to DAC, corresponding Ref. voltage for 10mV/LSB is 1.28V.

ALC /VGA Characteristics

VGA characteristics, installed in C42/43 PIU at

G = 12 dB. (Red)

G = 4 dB. (Pink)

New ALC Characteristics, installed in C42/43 PIU (Blue).

Mat-lab Simulation and Practical Results of IF system with New ALC

New ALC Chassis Level Test Procedure

New ALC/VGA Plots

ALC Time Constant

Step size = 5 dB.

Time constant dependent upon final i/p power level.

Also dependent upon the base line.

T  1/(PFinal)

Attack Time

Release Time

Programs for New ALC

cdset6	:- A program to control the New ALC gain and other	IF system parameters, which looks in ANT.LST file	to differentiate between antennas with old ALC and	antennas with New ALC.

newauth	:- Program to read autIF values from .atn file and	generate corresponding masks for corresponding	Antennas.

pulsarset	:- Program to read corresponding IF parameters	generated from pulsar console and wrote in	antenna_setup.cur and map POST-ATN to POST-	GAIN.

IF system Monitoring

Hot-line Monitoring :-

To monitor first 1 to 4 channels, which correspondingly gives

Detector voltage CH-1(Only in VGA mode)

Feedback voltage CH-1(Only in ALC ON)

Detector voltage CH-2(Only in VGA mode mode)

Feedback voltage CH-2(Only in ALC ON)

ONLINE Monitoring :-

To monitor pre-attenuation, post-gain, Bandwidth, ALC status, different supply voltages.

Limitations of New ALC/VGA Control

Old ALC Control Bit Pattern :-

Limitations of New ALC/VGA Control

New ALC can't be switched OFF to ON and again OFF without affecting the original gain at the time of ALC OFF.

IF Setting Files

*run file for setting IF parameters.

mpa 1 10

addmcm

enamcmq

* PRE ATTENUATION CH-1 -> 14 CH-2 -> 18

ana 0B3D3x 0F3D3x 0B3D3x 0F3D3x

st64dig(10)

* Gain CH-1 12.000 dB Gain CH-2 2.000 dB CH-1: 0.667v CH-2 : 0.667v

ana 0B788x 0F788x 0BF48x 0FF48x

st64dig(10)

*BW in CH1 & CH2 are, 32 & 32 resp.

ana 0fbe4x 0fbe4x 0bbe4x 0fbe4x

st64dig(10)

dismcmq

For OLD ALC :-

For New ALC :-

*run file for setting IF parameters.

mpa 1 9

comm 29;dest 17;t3v

enamcmq

*CH1 pre & post attenuations are, 16 & 16 dB resp.

*CH2 pre & post attenuations are, 16 & 16 dB resp.

ana 0b3ccx 0f3ccx 0b7ccx 0f7ccx

st64dig(9)

*BW in CH1 & CH2 are, 32 & 32 resp.

ana 0fbe4x 0fbe4x 0bbe4x 0fbe4x

st64dig(9)

*ALCs are: CH1: OFF CH2: OFF

ana 0ffffx 0ffffx 0bfffx 0ffffx

st64dig(9)

dismcmq

Photographs
New ALC at Chassis Level

Photographs
Modified C42/C43 PIU

Photographs
IF System ALC OFF/ON

Debugging of C42/C43 PIUs at ABR

Check for all power supplies on the front panel .

Check for the gain set for C42/C43 PIU from front panel by formula :-

Gain in dB = {[90 – (decimal value of bit pattern)]/2}

Check in ALC Off mode gain with DMM from front panel gain monitoring point.

For ALC ON check for Detector voltage from monitoring point on Front Panel, it should be nearly 1.700 V.

Further Plans

Stability check of newly installed system is in progress. Data from authIF is used for study of stability of newly installed IF system.

Modification of ONLINE monitoring software for MCM-10.

Modification of all cdset x for new IF settings.

THANK YOU!

Point 1, 2, 3 are design aspects

1. Shri. TLV.

2. Shri. Sudhir Phakatkar.

3. D. K. Nanaware.

4. Navnath Shinde and Avinash Shinde.

5.Authif team of Technical Assistances:-

CPK, DKN, NDS, SSP, VBB.

6. Last but not least ... all telescope operators

SSS, MMG, DBB, SNK, SK, JP, JK,	Manisha.

2006-02-19T23:08:11