Patent GB-970751-A (GB)
IPC: H04B14/06
Filed: 1962-06-29
Abstract: 970,751. Semi-conductor pulse circuits; delta modulators. PHILIPS ELECTRICAL INDUSTRIES Ltd. June 29, 1962 [July 3, 1961; March 27, 1962], No. 25063/62. Headings H3T and H4L. A pulse code delta modulator contains two dipole elements which exhibit negative resistance values when a critical applied voltage is exceeded, and the input and comparison signals are fed to and the output pulses are derived from a pair of opposed electrodes of the dipole elements while to the other electrodes thereof the control pulse generator is connected in antiphase and serves to shift at each pulse the operating points of the two dipole elements towards their negative resistance values. Speech signals from microphone 1, Fig. 1, are fed via filter 2 and amplifier 4 to a pulse code modulator 3 comprising tunnel diodes 9 and 10 with opposed electrodes connected together. A control pulse generator 5 is connected to the other electrodes of diodes 9 and 10 by a pushpull transformer 14. The output pulses from modulator 3 are transmitted over line 13, a pulse amplifier 15 (which suppresses the negative pulses) and modulator 15 from aerial 18, and are also fed to a negative feed-back loop containing a phase inverter 7 and R.C. integrating circuit 6, 8 giving rise to the comparison signal. The signal thus applied to modulator 3 is the difference between the signal input and comparison signals. Depending on the polarity of this difference signal one of the tunnel diodes 9, 10 is biased in the pass direction while the other is biased in the reverse direction. If, for example, tunnel diode 9 is biased at point P in the pass direction, Fig. 3, and diode 10 at point Q the reverse direction then the effect of the pulses fed in opposite phases to 9 and 10 via transformer 14 will shift the operating points P and Q to e.g. position P<SP>1</SP> and Q<SP>1</SP>. Because of their negative resistance characteristics the resistance of tunnel diode 10 is then a factor lower than that of 9. The two diodes constitute a potential divider across transformer 14 and the negative voltage pulse occurring across the half of the transformer connected to tunnel diode 10 is fed to the output conductor 13. Similarly if the polarity of the difference signal is reversed a positive pulse is fed to output conductor 13. Fig. 2b illustrates the output pulses while Fig. 2a shows the difference between input signal b and comparison signal a. An alternative circuit is described, Fig. 5 (not shown), utilizing PNPN diodes with an integrating network, comprising a resistance shunted condenser and an output transformer, connected across the same two electrodes as the control pulse generator. For use with very high pulse frequencies the phase invertion and integration network of Fig. 1 may be replaced by a series combination of a resistor and an integrating coil, Fig. 4 (not shown). If desired, one of the tunnel diodes can be operated with the electrode remote from the junction of 9 and 10 at earth potential while the other diode is shunted by a balancing impedance, Fig. 7 (not shown), in which case pulses of one polarity only are fed to the amplifier 15 so that no suppressor is needed therein.