Apr 202011
 

LOW POWER TV TRANSMITTERS

VHF Low Power TV Transmitter

(VHF range : 54 to 216 MHz)

The transmitter design is based on solid state techniques and employs modular construction.  The video and audio signals are processed in the exciter electronics and modulated at low level, at IF frequency of 38.9 MHz and 33.4 MHz respectively.  The modulated IF signals are combined and passed through IF corrector and VSB filter.  SAW filter is used for vestigial sideband shaping.  The combined signal is upconverted to desired channel frequency and amplified in linear power amplifier to obtain 100 watt (sync peak) visual power and 10 watt aural power.  RF is finally routed to antenna through channel filter and directional coupler.  A leveled block schematic of such transmitters available in the network is shown in Figs. A brief description of the exciter and power amplifier is given in the following paragraphs.

 

 

Exciter

 

Video Signal Processing and Modulation

 

The leveled block schematic of old and new generation exciter is given in figure 2 (a) and 2 (b).  The  1VPP input video signal is limited to 5 MHz in low pass filter and is compensated for group delay in delay equalizer and receiver precorrector unit.  The resulting signal is subjected to DC restoration by clamping at back porch, amplified and inverted in video processor.  The output of the video processor is fed to visual modulator where the same is amplitude modulated with negative polarity at 38.9 MHz IF and amplified.

In the new generation exciter, the electronics for base-band processing (LPF, DEq and V. processor) is integrated, resulting into a single and compact video processor unit.  The output of later has normal polarity.  The modulator used is a ring modulator instead of double diode balance modulator.  The video signal is reversed in polarity and modulates the vision IF.  The IF at 38.9 MHz is generated in the modulator unit using a transistorized crystal oscillator (TCXO).  The SAW filter in the unit shapes the modulated IF to vestigial sideband format.

The audio signal is frequency modulated at IF of 33.4 MHz in a varactor VCO modulator.  The VCO oscillates at centre frequency of 33.4 MHz.  The incoming audio is passed through a balanced to unbalanced transformer and pre-emphasised in a 50 micro second pre-emphasis network.  Signal is then amplified and applied to varactor diodes.  The information  contained in the amplitude variation of audio is converted into frequency variation in the VCO.  The VCO frequency deviates about centre frequency in proportion to audio amplitude.  The centre frequency of VCO is maintained at 33.4 MHz which is below vision IF by 5.5 MHz.  This is achieved by a phase locked loop (PLL).  The concept of the same is given in figure

 

 

 

Output from VCO (33.4 MHz FM) is mixed with vision IF of 38.9 MHz (taken from IF oscillator unit) using a transistorized mixer.  The resulting difference signal of 5.5 MHz is shaped for square pulses and using suitable dividers the frequency is reduced to a low value.  In most of the transmitter the dividers used are   5,   16   and    128.  Thus the 5.5 MHz square shaped wave is reduced to 537 Hz.  A crystal oscillator of suitable frequency (normally 1.1 MHz) is used as standard oscillator.  The output of crystal osc after getting shaped suitably is also passed through dividers, so that a frequency similar to above value is obtained.  Normally the o/p is divided by 16 & 128 so that a reference frequency of 537 Hz is obtained.  The above two outputs are fed to a comparator circuit, so that an error signal is generated whenever there is a drift of centre frequency of VCO from its nominal value of 33.4 MHz.  The frequency of the VCO is kept under control using the error signal.  The same error signal is used for providing visual indications through LED and a DC ammeter to indicate the deviation of the center frequency of the VCO.  

In some of the exciters, the audio is first modulated at an inter carrier frequency of 5.5 MHz and then converted to standard IF of 33.4 MHz by mixing with vision IF in a mixer.  New generation exciters have additional circuitry in audio modulator.  This includes a test tone for test purpose and dual sound/stereo coder.  

 

  One Response to “LOW POWER TV TRANSMITTERS”

  1. Hi there, just wanted to say, I enjoyed this post. It was practical. Keep on posting!

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