Signal Level of Transistor-Transistor/Low-power Schottky

Consider the signal-to-noise ratio results from the difference between the transmitting and the receiving level. Using the binary information representation, the signal-to-noise ratio can be adjusted to the environmental conditions within broad limits .

Signal level of a TTL-LS circuit
LOW level
Guaranteed transmitter level	max. 0.5 volt
Guaranteed receiver level	max. 0.8 volt
Static signal-to-noise ratio	0.3 volt
HIGH level
Guaranteed transmitter level	min. 2.7 volt
Guaranteed receiver level	min. 2.0 volt
Static signal-to-noise ratio	0.7 volt

 

Complex Waves

Complex waves include:

• Analog modulated, digitally modulated, pulse-width
• modulated, and quadrature modulated signals
• Digital patterns and formats
• Pseudo-random bit and word streams

Quadrature Modulation

In Signal Modulation waves the amplitude, phase and/or frequency variations embed lower-frequency information into a carrier signal of higher frequency. It gives signals in the form of either speech, data or video. In Analog Modulation the signal varies the carrier’s amplitude and/or frequency. At the receiving end, demodulating circuits interpret the amplitude and/or frequency variations, and extract the content from the carrier. Phase modulation modulates the phase rather than the frequency of the carrier waveform to embed the content. Digital modulation is based on two states which allow the signal to express binary data. In amplitude-shift keying (ASK), the digital modulating signal causes the output frequency to switch between two amplitudes; in frequency-shift keying (FSK), the carrier switches between two frequencies (its center frequency and an offset frequency); and in phase-shift keying (PSK), the carrier switches between two phase settings. In PSK, a “0” is imparted by sending a signal of the same phase as the previous signal, while a “1” bit is represented by sending a signal of the opposite phase. Pulse-width modulation (PWM) is another common digital format; it is often used in digital audio systems. It is applicable to pulse waveforms only. With PWM, the modulating signal causes the active pulse width (duty cycle, explained earlier) of the pulse to vary. Quadrature (IQ) modulation technology is used for building digital wireless communications networks. An in-phase (I) waveform and a quadrature-phase (Q) waveform that is delayed by exactly 90 degrees relative to the “I” waveform are modulated to produce four states of information. An in-phase (I) waveform and a quadrature-phase (Q) waveform are combined and transmitted over one channel, then separated and demodulated at the receiving end. The IQ format delivers far more information than other forms of analog and digital modulation  because it increases the effective bandwidth of the system. A digital pattern consists of multiple synchronized pulse streams. It makes up words of 8, 12, 16, or more bits wide data. The digital pattern generator, specializes in delivering words of data to digital buses and processors via parallel outputs. Digital computers have the inability to produce truly random numbers, therefore Pseudo-random bit streams (PRBS) and pseudo-random word streams (PRWS) are used. Digital video signals can have jagged lines on surfaces that should be smooth. Controlled amount of noise is added to hide these jagged lines from the eye without losing the original information. Serializers or multiplexers are tested using PRWS.

Vidicon Tubes

The key component in the vidicon tube is the photo conductive target. The lens focuses the the scene to be viewed on the target, and the target stores the scene in the form of an electrical charge. The electrical conductivity of the target varies with the amount of light absorbed. Where there is minimum light, the material has a high electrical resistance. The points on the target with considerable light have a lower resistance.

The remainder of the tube consists of an electron gun with surrounding magnetic coils. The cathode of the electron gun generates a stream of electron focused into a narrow beam to strike the inside of the target. Other magnetic coils surround the vidicon tube to produce horizontal and vertical deflection of the electron beam. Signals applied to these coils cause the electron beam to scan from left to right and top to bottom across the inside of the target. This causes a current to flow between the target and the transparent signal electrode. This current is proportional to the light amplitude. This is the video output signal.

For scanning the target, many methods are used, but the most widely used method is the standard NTSC pattern of 525 total scanning lines. When the electron beam scans the target, an electrical signal is produced. They varying amplitude in the signal represents brightness variations along one of the scan lines.

As soon as one line is scanned, the vertical deflection signals move the electron beam a small distance farther down, and another horizontal scan line is developed. Special synchronization pulses designate the beginning and end of each horizontally scanned line. The horizontal sync pulse is approximately 5 microseconds wide and occurs at a frequency of 15,750 Hz.

The total number of horizontal scanning lines is 525, but the way in which the target is scanned is unusual. The scanning takes place in two separate fields of 262 1/2 lines, each occurring every 1/60th of a second. The scan lines of the two fields are interlaced to produce a completely scanned image every 1/30th of a second. Of the 525 total lines, only about 480 of them are actually designated to the scene itself. The remaining 22 1/2 lines per field occur but they are not seen. This provides time for the electron beam to re-trace from bottom to top. Special sync pulses included in this interval keep the lines properly synchronized and meshed.

The electrical output signal has an approximate voltage range of .7 volts to 1.5 volts. The analog video signal between the sync pulses, represents the light intensity variations on one scan line. It is this voltage that will be fed to the analog-to-digital converter for translation into binary numbers.