Pattern generator

  
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A pattern generator is a very useful instrument for the correct alignment of the timing circuits of a television set. The circuit we propose you to build, is a «bar generator» that will produce horizontal and vertical stripes (bars) on the TV screen, that will help you align the vertical and horizontal scanning synchronisation circuits of the receiver.
Pattern generator - schematic

Technical Specifications - Characteristics 
Working voltage: ................... 9 VDC
Current drawn: ...................... 2 mA
Operating Frequency: ........... 170 - 250 MHz (VHF)
Horizontal scan frequency: ... 16525 Hz
Vertical scan frequency: ....... 50 Hz
Output impedance: ............... 75 ohm


How it Works
The circuit can be divided  in five different stages. Four astable multivibrators and the
output stage which is built around the VHF oscillator.
The first astable consists of two NOR gates in U2 and the components R9, R15, D4, C8,
and C10. The two NOR gates are used as inverters and  together they form an astable
multivibrator which oscillates at a frequency of 16525 Hz, which is the horizontal scan
frequency of the TV, and can be fine-tuned by means of R15. The capacitor C10 is there
to provide the necessary feedback to maintain the oscillations. The signal from the output
of the oscillator is taken through C8 and R6 to the output stage and modulates it.
The other multivibrator built around the other two NOR gates in U2, together with R12,
R14, D6 and C12 produces the necessary pulses for the vertical scanning frequency which
is 50 Hz. This frequency is fed to the input of one of the AND gates in U1 (pin 1).
The multivibrator...




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