Larcan Inc T5000U Manuale Utente
Unit 2 – IF Modulator
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1. Audio and Video Modulator:
1.1. Function:
This module function is to generate two carriers: one in 45.75 MHz and the other in 41.25 MHz.
The first is modulated in AM-VSB by a video signal and the other one is modulated in frequency by an
audio signal.
audio signal.
1.2. Block Diagram:
1.3. Technical Description:
1.3.1. Video Modulator:
The transistor Q1 oscillates in the crystal frequency (45.75 MHz - IF video), and the transistor
Q2, with high input impedance, guarantees the isolation to the oscillator Q1 output. The frequency
accuracy is adjusted in the variable inductor L1. The input video signal, with adjustable level in R27
(front panel), is applied to the high impedance input push-pull amplifier formed by transistors Q6 and
Q9. The amplifier output signal modulates in amplitude, in the mixer M1, the video carrier.
accuracy is adjusted in the variable inductor L1. The input video signal, with adjustable level in R27
(front panel), is applied to the high impedance input push-pull amplifier formed by transistors Q6 and
Q9. The amplifier output signal modulates in amplitude, in the mixer M1, the video carrier.
The integrated circuits IC18 to IC21 form a group delay predistortion circuit. The syntonized
cells, at the video bandwidth, cause an advancement of 170 ns at 3.58MHz. This video pre-distortion is
inserted in the circuit through jump’s JP5 and JP6. The adjustable attenuator R146 allows equalizing
the video level when the pre-corrector circuit is turned off.
inserted in the circuit through jump’s JP5 and JP6. The adjustable attenuator R146 allows equalizing
the video level when the pre-corrector circuit is turned off.
The transistor Q12 receives the video signal, invertes its phase and applies it to the IC7
amplifier. This amplifier causes a non-linear enlargement to the video signal emphasizing the
synchronizing pulses. This signal is clamped in 0.7V by the D5 and is compared to 0.3V (due to diode
D4) in the comparator circuit IC6. In this manner, at the IC6 output, there will be only synchronizing
pulses. These pulses are applied to the IC5 (mono-stable) in the negative transition to generate
clamping pulses. The clamping pulses commute the transistor Q10 that switches the video signal
clamping it to the blanking level through R45. This adjustment allows choosing the point of modulation
in the mixer for a better linearity. If there is lack of clamping pulses, the Q11 transistor commutes,
allowing adjusting through R44 the level of the video carrier without modulation.
synchronizing pulses. This signal is clamped in 0.7V by the D5 and is compared to 0.3V (due to diode
D4) in the comparator circuit IC6. In this manner, at the IC6 output, there will be only synchronizing
pulses. These pulses are applied to the IC5 (mono-stable) in the negative transition to generate
clamping pulses. The clamping pulses commute the transistor Q10 that switches the video signal
clamping it to the blanking level through R45. This adjustment allows choosing the point of modulation
in the mixer for a better linearity. If there is lack of clamping pulses, the Q11 transistor commutes,
allowing adjusting through R44 the level of the video carrier without modulation.
The integrated circuit IC9B receives the clamping signal detected through D7, changes its state
and commutes the transistor Q13. It allows switching the external circuits connected to the Automatic
output. The led in the panel shows this operation, when it is green it indicates video on. This circuit
presents about 60 seconds delay time approximately.
output. The led in the panel shows this operation, when it is green it indicates video on. This circuit
presents about 60 seconds delay time approximately.