Exp2 Electronic Tuner

7/29/2019 Exp2 Electronic Tuner

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SUBJECT:

Video Engineering

TITLE:

To study the Working of operating unit and Tuner Section. 1

EXPERIMENT NO. 2 DATE:

DOC. CODE: AITS/ECD/EXPT/602

REV. NO.: 1.01

Experiment No. 2Aim: - To study the Working of operating unit and Tuner Section.Apparatus: 1. Operating unit and electronic tuner section

2. Pattern Generator and D.M.M.3. Connecting wires and Power Supply

Theory: -(A) TUNER: -

The electronic tuner consists of RF amplifier local oscillator and mixer. IT receives RF inputsignal from the antenna via balun (a matching transformer with a ferromagnetic core upon whichfour tightly coupled and evenly spaced windings are connected to provide an impedancetransformation from 300 ohm balance-source to a 75 ohm unbalanced load) at the input socket.The RF amplifier: It selects signal of desired channel and provides signal amplification at a pointwhere signal strength is minimum and feds to mixer stage.

The local oscillator stage: It provides a frequency to the Mixer stage that can be heterodynedwith signals. In TV receivers the oscillator frequency is kept above the incoming RF frequency andis equal to sum of RF frequency and receiver IF frequency.The mixer stage: It receives signals from both the RF amplifier and from the local oscillatorcircuit and gives IF output which is given to VIF section.Suppose the RF tuner is tuned at channel4. The carrier frequencies for audio and video signal are 67.75Mhz and 62.25 MHz respectively. Atchannel 4 the frequency of local oscillator is 101.15 MHz. The carrier and oscillator frequenciesbeat together to produce sum and difference carrier frequencies 9 in MHz of (67.75 + 101.15),(62.25 + 101.15), (101.15 67.75) and (101.15 62.25). These difference frequencies of 33.4MHz and 38.9 MHz are called intermediate frequency (IF) signals for audio and video respectively.

Tuner AGC signal is also given to the tuner from VIF section to control its gain. AFT signalis given to tuner from VIF section for automatic fine-tuning. +12.0 V is supplied to the tuner pin 4.The desired band of the tuner is selected by giving. LB. HB and UB voltage from operating unitform the selection Band I, Band III and Band UHF respectively. Also tuning voltage (TU) is givenfrom the RR for tuning the desired channel in the selected band. C101, C102, C103, are

decoupling capacitors for +12.0 V and AGC signals respectively.

The RF input comes to RF amplifier via Antenna and Balun transformer. The RF amplifiesthis signal with maximum signal to noise ratio. Local Oscillator generates the sine wave oscillationof the frequency equal to RF channel frequency (higher side) plus 33.15 MHz. This local frequencyand RF signal from the RF amplifier is heterodyned in mixer stage to produce IF signal.E.g. for channel no.5 (174MHz to 181MHz)

local osc. Frequency is 181+33.15 = 214.15 MHz.Picture carrier frequency = 174.00 + 1.25 = 175.25 MHz.

______________________________________________________________Atmiya Institute of Technology & Science, Rajkot


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SUBJECT:

Video Engineering

TITLE:




REV. NO.: 1.01

Sound carrier frequency = 175.25 + 5.50 = 180.75 MHz.By heterodyning in mixer, output frequency of mixer are214.15 MHz 175.25 = 38.9 MHz (picture I.F.) and214.15 MHz 180.75 = 33.4 MHz (sound I.F.).The composite I.F. signal is produced at the output of tuner.

Channel No. Video CarrierMHz Audio CarrierMHz Band widthMHz2 48.25 53.75 47 to 54

3 55.25 60.75 54 to 61

4 62.25 67.75 61 to 68

5 175.25 180.75 174 to 181

6 182.25 187.75 181 to 188

7 189.25 194.75 188 to 195

8 196.25 201.75 195 to 202

9 203.25 208.75 202 to 209

10 210.25 215.75 209 to 216

11 217.25 222.75 216 to 223

12 224.25 229.75 223 to 230

(B) OPERATING UNIT (PROGRAM SELECTOR) (O.P.U.) : -The circuit diagram O.P.U. is shown in the Fig 4.1 It receives following signals :-

1. +33 V from SMPS section to give it to tuning voltage (TU) after selection.2. 12.0 V to give +B supply to the Band I, and III and UHF Tuner section in the

tuner i.e. LB, HB and UB respectively after selection by band selection unit.

First channel position is selected by one of the eight switches of switch control unit. It ismade of eight, two pole 2 way inter-connected switches. When any of the eight switches is


Twin Lead

300

AntennaInputCircuit

RFAmplifier

Filters&

TrapsMixer

Oscillator IF O/P

Block dia ram of RF Tuner


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SUBJECT:

Video Engineering

TITLE:




REV. NO.: 1.01

pressed 1st pole will connect 12.0 V supply to the Band I, Band III and UHF tuner sectiondepending upon the band selector position and LED will be ON for the corresponding switch. Itwill drop 2 to 3 volts. +12.0V supply is given to LB, HB or UB. 2nd pole will connect tuning voltagefrom band control to tuner (TU).

After selecting the channel position, SW on the band control is put at Band I, III or UHF asrequired e.g. to select channel No. 2 to 4, put band SW to Band I, channel No. 5 to 12 put bandSW to Band IIII, channel No. 21 to 68 put band SW to UHF.

This will connect +B supply to corresponding band to tuner. Then change the tuning pot ofthe corresponding channel position to select desired channel in the selected band. The tuning potwill vary the tuning voltage, which is given to the varacter diode in the tuner.

Reading: -All voltage are DC voltage measured by DMM except mentioned.

(1) Main voltages:

1. +115V Output from SMPS2. +18V Output from SMPS3. +12V Output from EHT4. +25V Output from EHT5. +150V Output from EHT6. Q714 (BF393) Collector +60V7. +330V DC at fuse F 802 with reference to bridge rectifier ground.

(2) Voltage of Tuner section: -

AFT 5.2V

AFT SW 10V when AFT SW in ON

UB 10.6V if Band UHF selected

LB 10.6 if Band I selected

TU 0 to 33 depending upon the channel selected

HB 10.6V if Band II selected

TU AGC 2.7 to 8.2

+12V 12.1

IF O/P 5.06



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SUBJECT:

Video Engineering

TITLE:




REV. NO.: 1.01

Conclusion: -

Grade Lab-In-Charge H.O.D.


Exp2 Electronic Tuner

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