Oscilloscope for TV. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Oscilloscope for TV. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Measuring technology

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Descriptions of oscilloscope set-top boxes for TV have already been published on the pages of the magazine ("Radio", 1959, No. 1; 1965, No. 8, etc.). However, unlike them, the proposed set-top box does not require intervention in the TV circuit (it is connected to the TV's antenna jack). Together with a sweeping frequency generator, it can be used to build IF amplifiers for radio receivers.

The prefix (Fig. 1 and 2) can be considered as a miniature television transmitter. Despite the relative simplicity of the circuit, this transmitter generates a complete television signal, which differs from the standard signal only in the absence of equalizing pulses.

Ris.1

Frame sync pulses are formed from an AC sinusoidal voltage by a limiting amplifier (T1), a differentiating circuit R8C4 and a threshold amplifier (T1). Their duration is about 1,9 ms.

Ris.2

The blocking oscillator on the Gz transistor generates horizontal sync pulses. These are not the main pulses of the blocking generator, but surges in the collector voltage that occur immediately after the main ones. An isolation diode D4 is connected between the collectors of transistors T5 and T3. At the time of generation of the main pulse, the collector of transistor T4 is closed to the chassis through an open transistor T5 and diode D3. As a result, insets appear in the vertical sync pulses, which, as required, precede the horizontal sync pulses. The windings of the transformer Tr1 of the blocking generator are wound on a toroidal core made of oxyfer (H=1000). The outer diameter of the core is 10 mm, a. thickness 2 mm. Windings I and III contain 100 turns each, ll - 30 turns of PELSHO 0,1 wire.

At the beginning of the horizontal scanning period, the voltage pulse of the blocking oscillator quickly charges the capacitor C5 through the diode D2. During the rest of the period, it slowly discharges through resistor R6. The resulting sawtooth voltage is fed to the base of the transistor T2. Here it is added to the oscilloscope voltage.

The three-stage amplifier (T2, T3, T6) due to the large gain (50000-100000) operates almost in a relay mode, characterized by a certain threshold. Since the oscilloscoped voltage is superimposed on the input sawtooth voltage of the amplifier, the switching of the output voltage coincides in time with the moment when the total voltage passes through the threshold of the amplifier. The rise time of the front of the voltage pulses at the output of the amplifier is small, again due to the high gain. The moment of occurrence of these pulses during the horizontal scanning period is determined by the instantaneous value of the oscilloscoped voltage. The set-top box parameters are chosen such that in the absence of the investigated voltage, the axial line is in the center of the screen. If necessary, the image on the screen can be shifted in one direction or another by changing the resistance of the resistor R3.

To improve the clarity of the line image on the TV screen, the amplifier (T2, T3, T6) is covered by positive feedback from the collector of transistor T3 to the base of transistor T2 through capacitor C6. This greatly increases the gain in the high frequency region and therefore increases the steepness of the output pulse edge. Visually, this is manifested in the increased sharpness of the transition from white to black.

Personnel, horizontal and video pulses are added at the input of the emitter follower (T7), which is a modulation amplifier of the VHF generator (T8). The latter is assembled according to the capacitive three-point scheme. The generation frequency should be chosen equal to the carrier frequency of the image of a free television channel. Otherwise, the set-top box may interfere with neighboring TVs. The required generation frequencies can be achieved by selecting the number of turns of the coil L1. When tuned to the second television channel (59,25 MHz), the L1 coil contains 5 turns of PEV 0,6 wire, the coil diameter is 9 mm.

The modulated RF voltage is fed to the output of the set-top box through the divider R18-R19, which lowers the voltage to 3 mV in order to avoid overloading the RF path of the TV.

The output of the set-top box is connected with a coaxial cable or twisted double wire to the antenna input of the TV.

Construction and adjustment. All parts of the set-top box, with the exception of the VHF generator, can be placed on the circuit board in any order. Parts related to the VHF generator (C11-C15, L1, T8) must have short leads, be interconnected by short conductors, and in addition, they should be grouped in one place.

No screening of the console is required. After turning it on, you need to adjust the TV as usual using the adjustment knobs (frame rate, line rate, contrast). If the frequency of the pulses of the blocking generator of the set-top box does not lie in the range of adjusting the frequency of the TV lines, it is necessary to enter it into this range by changing the resistance of the resistor R14 within a small range. It should be noted that the synchronization of the TV scans from the set-top box usually turns out to be very stable, so poor synchronization when setting up the set-top box indicates some kind of installation error. In order to achieve fine tuning of the U K V generator of the set-top box to the selected television channel, it is necessary to stretch or compress the turns of the winding of the coil L1 (that is, change the winding pitch). When set correctly, the line on the screen is sharply defined.

The settings of the set-top box are chosen so that the largest image swing on the TV screen corresponds to an input voltage of about 0,3 V. The sensitivity of the set-top box can be adjusted by changing the resistance of the resistor R2.

To check the sensitivity of the set-top box, an alternating voltage of known magnitude is applied to its input either from a power source with a voltage of 6 V, a frequency of 50 Hz through a divider, or from a sound generator.

The input impedance and sensitivity of the set-top box, if desired, can be significantly increased by connecting a conventional low-frequency amplifier with an emitter follower at the input to it.

Author: V.Krapivnikov; Publication: N. Bolshakov, rf.atnn.ru

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