SU499687A1 - TV transmitter modulator - Google Patents

Description

one

The invention relates to television technology.

A known modulator for a television transmitter, comprising a carrier frequency generator, a frequency converter with two transformers, at the mid points of which is a modulating signal from an input unit and a signal from a transistor current source of balancing, a filter and an amplifier with automatic gain control.

However, the known modulator does not maintain the modulation depth with increasing magnitude of the modulating signal above the nominal value.

In order to preserve the modulation depth

when increasing the amplitude of the modulating signal, the nominal value of the transducer detector connected in series with the threshold element in the base circuit of the transistor and the emitter follower and the threshold balanced amplifier of two transistors are introduced into the proposed modulator, the output of the threshold balanced amplifier is connected to the input of the transistor current source balancing, and a modulating signal is fed to the input of the amplitude transistor detector l

The drawing shows the scheme of the proposed modulator for a television transmitter.

The modulator for a television transmitter contains a carrier frequency generator 1, a frequency converter 2 with two transformers 3 and 4, at the midpoints of which is a modulating signal from the input unit 5 and a signal from a transistor current b of the balancing current, filter 7, an amplifier 8 with automatic adjustment gain, additional input unit 9 and connected

successively amplitude transistor detector 10 with a threshold element 11 in the base circuit of the transistor 12 and an emitter follower performed on the transistor 13, and the threshold balanced amplifier 14

direct current on two transistors 15 and 16. The output of the threshold balanced amplifier 14 of direct current is connected to the input of the transistor source 6 of the balancing current, and to the input of the amplitude transistor

detector 10 through the additional input unit 9 filed a modulating signal. The modulator operates as follows. The modulating signal, which is a video signal, through the input unit 5, in which

its constant component is amplified and restored due to the fixation of the level of clock pulses, which is fed to the input of frequency converter 2. Through the additional input unit 9 the modulating signal is fed to the input of the amplitude transistor detector 10. In the base circuit of the transistor 1.

the amplitude transistor detector 10, the peak of the sync pulse is associated with a certain constant level. Next, the modulating signal is detected at the base-emitter transition of the transistor 12, the constant voltage across the emitter follower made on the transistor 18 is fed to the input noporoBOiro of the balanced amplifier 14 direct current.

If the magnitude of the modulating signal at the input of the modulator is less than or equal to the nominal value, then the transistor 15 of the threshold balanced DC amplifier 14 is closed by the voltage coming from the emitter of the transistor 16. From the output of the threshold balanced amplifier 14 of a direct current, a constant voltage is applied to the transistor balancing current source b, which creates on the frequency converter 2 an offset at which the required modulation depth is established.

If the magnitude of the modulating signal at the modulator input exceeds the nominal value, then the DC voltage at the output of the amplitude transistor detector 10 increases, the transistor 15 of the threshold balanced amplifier 14 of the current is opened. With a further increase in the amplitude of the modulating signal, the current of the transistor 15 increases, and, consequently, the voltage across the emitter of this transistor increases, which closes the transistor 16 of the threshold balanced DC amplifier 14, which leads to a change in the bias voltage coming from the output of transistor source 6 current balancing on the frequency converter 2, which, in turn, leads to an increase in the carrier frequency at the output of the frequency converter 2 and to maintain the set depth the signal is dulled at the output of this converter as the amplitude of the modulating signal at the input of the modulator increases.

Claims (1)

Invention Formula A modulator for a television transmitter containing a carrier frequency generator, a frequency converter with two transformers, to the midpoints of which a modulating signal from the input unit and a transistor current source of balancing are supplied, a filter and an amplifier with automatic gain control, characterized in preserving the modulation depth with an increase in the amplitude of the modulating signal above the nominal value, the amplitude transistor detector with a threshold is connected in series an output element in the base transistor circuit and an emitter follower and a threshold balanced DC amplifier on two transistors, the output of the threshold balanced amplifier connected to the input of a transistor current source of balancing, and a modulating signal is fed to the input of the amplitude transistor detector.