Wave translating circuit

#The invention relates to a wave translating circuit for supressing transient DC level disturbances of burst data signals generated by a transmitter.

#In a transmission system DC level correction of data signals is often conducted, one such example being an adjustment of the operating point of an immediately following amplifier stage.

#Several types of wave translating circuits are known, the most widely used of which a high-pass filter circuit comprising a capacitor and a resistor. Although the high-pass filter circuit is simple in construction, the DC components of data signals at the start of transmission and those of burst data signals cannot be removed sufficiently for a certain period due to the transient response of the capacitor. The unremoved portion of the DC components causes overmodulation at the transmitter end of a transmission system and a faulty decision in code regeneration at the receiver end of the transmission system.

#The present invention and its advantages will now be described in detail with reference to the accompanying drawings:

#FIG. 1 shows a conventional wave translating circuit (high-pass filter);

#FIGS. 2A and 2B show waveforms at the input (point a) and at the output (point b), respectively, of the wave translating circuit of FIG. 1;

#The conventional wave translating circuit (high-pass filter) of FIG. 1 has time constant C.sub.1 R.sub.1 formed by capacitor 1 (with capacitance value C.sub.1) and load resistor 2 (with resistance value R.sub.1). The time constant C.sub.1 R.sub.1 must satisfy C.sub.1 R.sub.1 >1/2 .pi.fp, where fp is the bit rate of pulse waveforms. In the case of continuously generated pulse waveforms no problem arises. However, for burst signal waveforms as shown in FIG. 2A, the transient response of capacitor 1 having time constant C.sub.1 R.sub.1 causes DC component disturbances in output data signals, during the period (T) starting at the instant (t.sub.1) of pulse waveform generation as shown in FIG. 2B.

#The wave translating circuit of FIG. 3 has a time constant C.sub.2 R.sub.2 R.sub.3 /(R.sub.2 +R.sub.3) formed by capacitor 11 (with capacitance value C.sub.2) and resistors 12 and 13 (with resistance value R.sub.2 and R.sub.3, respectively). This time constant must satisfy C.sub.2 R.sub.2 R.sub.3 /(R.sub.2 +R.sub.3)>1/2 .pi.fp, where fp is the bit rate of pulse waveforms. In the case of continuously generated pulse waveforms, the DC components of the output voltage in the circuit of FIG. 3 is VR.sub.3 /(R.sub.2 +R.sub.3) constant, where V is a voltage applied to one end of resistor 12. If, however, burst signal waveforms as shown in FIG. 4A are applied to point C in FIG. 3, the output pulse waveforms shown in FIG. 4B are obtained at point d in FIG. 3. As will be obvious from FIG. 4B, the DC components are disturbed for a certain period starting at the instant of pulse waveform generation, due to the transient response of capacitor 11. If data signals having the DC component disturbances as shown in FIGS. 2B and 4B are modulated with a modulator, overmodulation will result with the consequent faulty decision in a code regeneration at the receiver end.

#FIG. 5 shows a wave translating circuit, according to the present invention, immediately following waveform shaping inverters 21 and 22. If pulse waveforms in FIG. 6A are applied to the output (point e) of inverter 22, which is in turn the input of the wave translating circuit, the voltage at point e rises by V/2 (U.sub.1 in FIG. 6A) and hence capacitor 23 is charged in the positive direction. However, because limiter diode 24 conducts, the voltage rise at point f of FIG. 5 is limited to the forward direction voltage V.sub.D of diode 24, as shown in FIG. 6B. On the other hand, if the inverter 22 output voltage at point e falls by V/2 (D.sub.1 in FIG. 6A), the voltage at point f also falls from level V.sub.D by V/2 (D.sub.2 in FIG. 6B). Even if burst data signals are applied in succession to point e, as shown in FIG. 6A, the voltage at point f has almost no DC component transient response, as shown in FIG. 6B. If the voltage at point f is added to the output voltage of inverter 22 by means of variable resistor 25, pulse waveforms are obtained having positive and negative polarities, which can be translated by adjusting their amplitude by means of resistor 26 into positive pulse waveforms, with the DC component removed (FIG. 6C). The input data signal in this case is a positive pulse; however, a negative pulse can provide a similar result if the polarity of diode 24 is reversed. It is apparent that variable resistor 25 can be replaced by an adder circuit, such as an operational amplifier.