EMI m O L MFIHoDoLoGIEs - APPLICATION IN A LOW COST TV DEMOWLATOR-

1 2 3 Amiya Kumar Sarkar , S. K. Momla1 , Arun Kumar Ghosh

4 and Brig. (Dr.) G.K. Deb

Electronics Research & Development Centre, Calcutta 'Webel Bhavan', Block EPW, Sector - V, Calcutta 700 091, West Bengal, India.

Tel : 91-33-379347/47 Fax: 91 -33-379847

E-Mail: root@erdccal.ernet.in

Electromagnetic radiations from various natural and man-made sources usually affect the performance and longevity of various electronic systems provided appropriate control techniques are not applied to protect them. Severe electro- magnetic interfenence (EMI), at times, cawes permanent damage to various components/sub-sys temdsystems requiring compul- sory applications of control methodologies right along the complete course of the system design and evaluation. In this present article an indigenously built Low Cost TV Demodulator system, developed by the Heavy Electronics Ehgineering Group of ER&DC, Calcutta has been addressed with a focus to the application of various EMI control techniques for safety and secure performance of the system.

IN"RODUC!TION

Growing popularity of Doordar- shan and its country-wide deployment inmediately necessi

ates maintaining a high quality performance standard of the transmitted signals from var- ious costly high (output) power transmitters (HFTs) and Low power transmitters (LPTs) . Due to several reasons, for exam- ple, cost, maintainability, size etc., LPTs are found to be predominant in the transmitting network of Doordarshan. For high quality transmission, both HFTs and LFTs need be monitored appropriately. HPT-performance monitoring is carried out by costly imported equipments. Whereas, in LPTs, the method adopted has been purely quali- Zta5ive and is executed by subjective viewing, wherein an expert sits in front of a TV screen to evaluate the signal "quality and standard". Accord- ingly, LPT control panel is monitored for quality transmis- sion.'This process is sluggish and backdated. Necessarily, therefore, the LCCVD technology has been incepted. The L,CI'VD exploits the state-of-the-art performance monitoring. Cer- tainly, similar, possibly even better, equipment exists

128

abroad. Such equipments are too costly to be accepted for such a huge population(a few hun dreds) of LpTs over the coun try. The LCTVD is appreciated, therefore, to indigenize the technology with a motive to save huge import investment otherwise unavoidable.

Low cost TV Demodulator is a radio frequency instrument to be used for the performance monitoring of the Low Power TV Transmitters(LPTs). It is aimed to replace both the presently existing evaluation method and the futuristic possibility of resorting to costly equipments imported from abroad. Built around a perfectly solid state- of-the-art technology, this system is attributed by the properties as handy and rigid, and, low cost and 'indigenous'. Since the application is at LpTs where EM1 pollution is much higher than domestic environments but still not so severe, the EM1 protection mechanisms [ 1 ,4,5,6] adopted are not so costly and critical. The control techniques [71 which are applied can be broad- ly classified as under:

(a) Shielding - of the system, of subsystems, of cables. (b) Avoiding use of long cables . (c) Use of PCB - mountable panel accessories. (d) Power-line filtering . (e) Proper PCB-layout design. ( f) Appropriate grounding and, (g) Appropriate selection of components.

During the entire design phase, the EM1 aspects have been considered and based on the criteria, such as susceptibili ty, EM-severity and component availability, the most cost- effective yet easily viable EM1 Control techniques have been selected. Laboratory testing on some constituent sub-systems has enhanced the confidence level of the system perfor- mance

In this present article the Low Cost TV Demodulator system developed by the Heavy Elec- tronics Engineering Group of ER&DC, Calcutta has been addressed with a focus to the application of various EM1 control techniques for safety and secure performance of the system. Designed independently, this equipment is aimed at replacing the old method of llsubjective viewing and evalua- tion" of LPTs by a suitable combination of qualitative and quantitative performance moni- toring, reflecting a partial step towards import substitu- tion.

LCTVD SYSTEM -- Detailed discussion of this subject is beyond the scope of this paper. A brief is present- ed herebelow:

The simplified block diagram of the system is depicted in Fig. 1. The major constituent mod- ules are: 1. RF Attenuator

429

2. Front End Converter(FEC) 3. RF-signal preamplifier 4. SAW-Filter 5. Audio and Vision Demodulator 6. Audio Output stage including active filter. 7. Video output stage including video amplifiers.

Besides the above, there are provisions for reference pulse generation, audio and carrier level indication and channel as well as band selection.Power supplies and panel accessories are also there in the complete system.

The RF attenuator preceeding the FEC is of voltage con- trolled type and is realized by using the IC-BMC 1110. The FEC is meant for down conversion of the VHF/UHF'(RF) TV signal to IF-signal for demodulator in the audio and vision demodula- tor stage (IC-7680 of BEL, or Toshiba) . The F'EC used in this system is of 8-Pin type. Band selection for working of the FEC, is done by the band switch provided in the system front Panel. The preamplifier-SAW Filter combination acts as IF signal conditioner for enhanced audio/video SNR level at the corresponding outputs. Output signal SIW is further improved by the use of active/passive filters.

The system consists of the following five PCBs: 1. PCB-01 RF Circuits. 2. PCB-02 Digital Channel Display Circuits. 3 . PCB-03 Audio Level and

Carrier Deviation Indication Modules. 4. PCB-04 Front Panel Display Board for Selected TV Channel, Selected Frequency M d , Atten- &tion Control and Band Con- trol. 5. PCB-05 Front Panel Switching and Control Section.

GENERAL SPECIFICATIONS

The LCTVD system is built around .the following general specifications:

1. Frequency Range:

BAND I 47 to 68 MHz BAND I11 174 to 230MHz BAND U 470 to 860MHz

2. Standards: PAL, - B/G.

3. Band Selection: By band switch available in the front panel.

4. Channel Selection: By Chan- nel count and channel reset switch available in the front panel.

5. Vision IF: 38.9 MHz

6. Sound IF.: 33.4 MHz

7. FF Input:

Input Impedance : 50 ohms Connector : E4-C female! Input Level : -15 to +I5 dbm Input Attenuation : 0 to 20 dB

in -to dB steps.

8. Video Output: 2 NOS.

430

Gutput Level : 1.ov Output Impedance : 75 ohms Connector : BNC Female

9. Audio Output:

Output : +6dbM Output Impedance : 6W ohms Connector : XLR type

IO. Reference Pulse Output:

Output Level ,: 1 .Ov Output Impedance : 600 ohms Connector : XLR type

11. Power Requirements: 230v, 50Hz AC

12. Size:

1g1I width, 2U height, 1211 depth

13. Weight: 6 Kgs.

Besides, the above, the follow- ing important features have been added to the system for efficient and fruitful applica- bility as a LPT-performance monitor:

i) Audio Level Metering through Bar Graph Display.

ii) Carrier Deviation Indica- tion through Bar Graph Display.

iii) RF/IF Input position indication by LEDs.

iv) Reference pulse for 100% modulation generation.

v) Digital Display of selected TV channels.

vi) Band Indication by LEDs.

NOISE CONSIDERATION

Many types of noise [2,5,6] must be dealt within this system and they are as follows:

External Noise: The environmen- tal noise radiated into the system.

Power-line Noise: The noise induced into signal lines from adjacent signal lines.

Siml-current noise: Noise generated in stray impedances throughout the circuit from the flow of required signal cur- rents.

Transmission - line reflections: Noise from unterminated trans- mission lines that cause ring- ing and overshoot and finally, the current spike from gates etc.

EMI-CONTROL IN Lc"rvD SYSTEM --- Reduction of emission has been done in two ways, (i) the circuit design and trace layout are made to take proper care to minimise cross talk, impedance mismatch, conductor inductance, formation of loops and also in selecting right components, (ii) filtering, component placement, shielding and other barrier techniques have been applied to contain the emission within the box. The following most commonly used and most proven techniques [I ,5,6,71

43 1

have been used to control the EM1 problems of the LervD system.

Distribution

a. Supply & return traces are located closely. b. Decoupling capacitor has been placed at each IC power supply unit. c. R-Core transformer has been used. d. RF and Digital Power sup- plies are separated. e. Bulk charge capacitor (tanta- lum type) has been used. f. Lead-lengths of decoupling capacitors are minimised.

Minimising Component Leads

The footprints for resistors and different types of capaci- tors in the artwork are made in a way to minimise the component leads. A large lead length deteriorates the performance by forming antenna and, introduc- i.ng series inductance.

* Separating PCBs for analog (RF) and digital circuits to reduce EMI. * The unused space of PCBs has been covered by ground plane to provide "quite groundll that provides a low impedance return path to high frequency signals, resulting in reduced EM1 ef- fects, cross-talk and the undesired effect due to mutual capacitance. * Supply and ground lines have been made as thicker as possi- ble. * Signal lines are also made as

thicker as possible. * Heat-sinks used in the power devices are grounded. ** To improve ground bounce and connector impedance mismatch for EM1 reduction, ground and return have been introduced for every 3 to 6 pin in all connec- tors.

* A Hybrid of Multipoint ground system and single point tlstarll or parallel ground system has been used. The star or parallel ground system is a preferred method to help control interac- tion between circuits. This uses separate return or ground paths for each signal trace. All of these are ultimately tied together at one point. * Outermost boundary of all PCBs are guarded by ground trace to reduce interference between adjacent PCBs. * Since the system will work upto 860 MHz of frequency, precautions have been taken: (a) To reduce the trace length to as-small-as possible. (b) To Distribute ground appro- priately.

Component Selection

Judicious selection of compo- nents reduces the EM1 emission to a great extent. In the digital portion of the system the slowest speed logic (name- ly, the CMOS) has been used.

Ceramic capacitors have been used frequently. At some criti- cal couplirg and biasing points, polysterene capacitors

432 .

have been used because they have extremely low series re- sistance and have very stable capacitance-frequency charac- teristics.

Inductors may be susceptible to stray magnetic field and source of undesired magnetic field [I ,3,61. The air-core or open magnetic core inductors are most likely to cause interfer- ence since the core, a low reluctance path concentrates the external magnetic field and cause more of the flux to flow through the coil. A close magnetic core is less suscepti- ble than an open one, but more susceptible than an air-core. It has been found necessary to shield inductors to confine their magnetic and electric fields within a limited space.

------ -

Looking from the system point of view, majority of circuits are designed on a single PCB to avoid EMI-effects due to other- wise would-be-necessary long- cable connections. PCB mount- able panel accessories are used for further reduction of EM1 effects. Surface Acoustic Wave (SAW) filter is used for better group delay performance and steeper skirt attenuation outside the passband of the channel of interest. Further- more, to avoid EM1 double sided PCBs are designed for this system. Care has been taken in PCB design by keeping RF compo- nents as close as possible for reduced EM1 effects. R-Core transformer has been used for better regulation and ELCOM-6AP

power-line filter has been used to combat the conducted EMI.

TYPICAL PERFORMANCE MEASURES:

The TV-demodulator needs be tested by feeding several standard test signals. The desired video performance metrics for the Demodulator under consideration, for the luminance bar lltest signal" is recorded below.

- The Measurements on Luminance Bar : - a. b.

C.

d. e. f.

g.

Video level: I .O 'J Luminance bar amplitude: 710

mV Luminance bar amplitude

Rise time: 396 ns Bar tilt: < 0.9% Luminance non-linearity:

Base line distortion: < 2.6%

error: +I .5%

13.7%

a.

b.

d. e.

C.

f. g-

h.

Differential Gain Within:

Differential Phase: <5 deg. Inter-Modulation: <I -3% SNR Un-weighted: >50.7dB Frequency Response (w.r.t. 6OOKHz): +I .7dB

+5%

Audio output level: +6dBm. Total Harmonic Distortion: (fd=50KHz, fm=lKHz) : <2.5% Simal to Noise Ratio:

-- The Other Important Measure- ments:

(fd=3OKHz, fm=lKHz) : 40 dB i. Frequency Response :+1.2 dB (40 Hz to 10 KHz)

433

The measured performances with the TV DEMODULATOR developed by ER&DC(I), Calcutta have been found to be within 2-10% range, referred to typical performance measures listed above. For example, the video SNR is 46 dB for the developed demodulator. F'urther enhancement of perform- ance is in progress.

CONC!LUSIONS

The LCI'VD system meant for LFTs is a rugged RF instrument not only for the performance moni- toring of the LpTs, but also for upkeeping their health. This instrument in the product-form will be manufac- tured by Webel Mediatronics Ltd. (WML) Calcutta for Doordar- shan. A technology incepted by All India Radio R&D, New Delhi has been made acceptable to Industry/User by ER&DC (I), Calcutta Unit, by way of sever- al enhancements and value-addi- tions through implementation ofseveral new design concepts. EMI-control techniques have been applied to enable the system withstand the hostile EMI-environment around the ILFTS.

References:

€bards., IBM J. Research & Development Centre, 33, 33- 50(1984)

3 . F.W. Grover Inductances Calculations , Dover, NY, 1946

4. G. K. Deb Vntroducing EMC design practices and uni- form EM1 test procedures for computing devices in commer- cialsectorT1 In EMC Journal,

Apr.92, pp. 12-16. Vol. 4 & 5, NO. 2 & 1, Oct. 91-

5. H.W. Ott Noise reduction techniques in Elec- tronic systems. Second Ed., John Wiley Inc.(NY), 1988.

6. G.K. Deb, Ed.,Electromag- netic Interference and Electro- magnetic Compatibility, Vol. 4: IETE Book Series, Tata Mcgraw- Hill Publishing Co., New Delhi, 1995.

7. G. K. Deb, Amiya K. Sarkqr and D. Majumdar, "EM1 con- trol in microprocessor and computer systems,11 in Electro- magnetic Interference and Electromagnetic Compatibility, Vol. 4: IETE Book Series, Tata Mcgraw-Hill Publishing Co., New Delhi, 1995, pp. 179-198.

1. C. R. Paul Introduction to Electromagnetic Compatibili- ty, John Wiley and Sons, Inc., 1992.

2. C. R. Paul Modelling Electromagnetic Interference Properties of Printed Circuit

434

BASIC BLQCK DIAGRAM

- - t

IF DEMOD.

AGC

AFC TUNING

@

ATT, FRONT * .I F FILTER4 0-20 END -

dB CONVERTER

c_

O/P AUDIO

1

LOW PASS 4

AUDIO

BUFFER FILTER

SOUND

FILTER

REJECT FILTER

Fk.4 LOW 'COST TV DEM0D:ULATOR

435