IL30947A - Circuit for scanning by means of electrical signal pulses - Google Patents

Description

^ pwn πιηκ ma^ys *"y aaaafr Vaya CIRCUIT FOR SCANNING BY MEANS OF ELECTRICAL SIGNAL PULSES P.A,309^7/2 The present invention relates to a circuit arrangement for pulse controlled scanning of electrical signals by pulse controlled semiconductors.

Circuits of this kind are employed, for example( in time division multiplex systems, especially in pulse-code-modulation systems (PCM-systems). To each signal channel, in such arrangements, there is provided a switching system by means of which speech signals are scanned according to the timing of the signal scanning frequency.

Similar arrangements are also employed in radar systems. There has been known,by way of example, an arrangement for the servo control of the antenna wherein conical scanning of a target modulates an echo signal, and a sampling of said modulated echo signal is made by way of such pulse controlled scanning, whereby each sample being extracted from the peak amplitude, and the amplitude thereof is stored until the arrival of the next sample. Furthermore it is known in this field to take samples from a steady video signal, for leading to an indicator, or for transmitting in coded form over lines, respectively.

For switches there can be used semi-conductor elements i.e., diodes or transistors, presentin ordinarily unneglectible differences. The difference with respect to the conductivity of the contact-break P.A.309^7 2 distance are especially disturbing in PCM technique, the only thing of importance - in this connection -is the spread between units of those switches which belong to a channel group, because of a time scanning error, being the same for all switches can be compensated with relatively small technical effort. Those scanning errors which are referred to as carrier residual voltages have an influence on the quality of the transmission insofar as they decrease the dynamic range of the system.

Today in PCM systems the total dynamic range is subdivided Into about 2,000 quantized voltage steps. At a highest amplitude of 5 a quantization step is 2.5mV. Thus an Ideal scanning switch should be in a position to switch through voltages between 2.5mV and 5V - lmV. This would mean an accuracy of about.0.2$ relative to the highest possible voltage amplitudes. The speech signals in PCM systems and the modulation of the echo signal in radar systems are both bipolar signals, that is to say, they present amplitudes in positive as well as in negative direction with respect to a reference voltage ground level.

Switching transistors are employed usually only for processing unipolar signals, wherefore bipolar signals are - in a preceding step - converted into unipolar signals by superposition of a DC voltage. In addition, transistors are current controlled switching devices so that when switched - on, P.Α. 3Ρ9¾7/2 a control current passes there through which is apt to cause a disturbing voltage in the input circuit. This disturbing voltage produces together with the residual voltage portion over the switching region (the offset voltage), the total residual voltage of the switching arrangement* Contrary to. conventional transistors, field effect transistors are current controlled devices which do not lead to any residual voltage portion (offset voltage) over the contact-break distance.

These field effect transistors are classified in two groups, that is to say, metal-oxide-Junction field effect transistors (MOSFET), and junction contact field effect transistors (PET). The junction contact field effect transistors are bipolar switching devices which have essentially symetrical characteristic curves in relation to a zero point* The bundle of characteristic curves also passes through the zero point Itself so that no residual voltage portion over the contact-break distance will occur.

The object of the invention resides in providing a switching arrangement for pulse controlled $canning of electrical signal pulses which can be used for high frequency pulse repetition rates and which exhibits very stee switching edges, and which feeds no current from the control c i 'ircuit to the so .urce of signal . current.

This can be attained thereby that as a semiconductor switch there is used a Junction contact field effect transistor, the source electrode of P.A. 30 472 diode to a blocking voltage and via a capacitor to pu ses., further thereby that the scanned electrical signal is taken from the drain electrode and that the blocking voltage equally blocks the diode.

The invention will now be explained in connection with the annexed drawings. Figs 1 and 2 show a circuit of scanning switches of known kind, Fig. 3 shows a circuit of an improved scanning switch, Fig. shows a train of pulses for the control of the scanning switches, and finally Figs. 5» 6, and 7 show voltage diagrams at the gate electrode of the FET as compared with the employed control voltages for the scanning switches in Figs. 1, 2 and 3· Fige 1 and 2 show known examples of the use of field effect transistors as switches In analogue switching circuits. The three schemes of Figs. 1, 2 and 3 distinguishes only in the feeding line of the gate electrode T of the field effect transistor FET. Thus, it would be sufficient to explain the arrangement of the circuit in connection with Fig. 1. To a field effect transistor FET having a source electrode Q, a drain electrode S, and a gate electrode T, a signal Current is fed to the eource electrode Q, such current being derived from a signal source O. The internal resistance Rl of the signal eource G is shown in series with the signal source G. The source electrode Q is P.A. 30 ^7 2 between the drain electrode S and ground.

According to Pig* 1 there is connected to the gate electrode T a control voltage UP via a diode P and a capacitor C2 wired In parallel and being connected to Juncture UP, This voltage UP is illustrated in Pig. & with respect to time. The voltage UP is a pulse like voltage varying between a negative value S and a positive value L. Die negative value S represents the voltage when the field effect transistor PET is non-conducting while the positive value L is the voltage when the field effect transistor FET is conducting.

At steady state the voltage S (potential gradient at point UT as shown in Fig. 5) appears at the juncture UP, and consequently also at the gate electrode T. The fidd effect transistor FET is thereby blocked. A voltage Jump to the value L (Fig. ¾) at the Juncture UP changes - via the capacitor C2 - the potential at point UT to the potential L, whereby the region between the shource electrode Q and the drain electrode S becomes immediately conductive. Via the p-n Juncture, between the source electrode Q and the drain electrode S, being biased in forward direction', the capacitor C2 discharges until this p-n juncture is no longer conducting. When switching off, the negatively directed edge of the control voltage HP changes the potential at point UT, via the capacitor control voltage into the negative range* This negative potential raises slowly via the blocked diode D tothe value S, In view of the time needed for the discharge of capacitor C2, such switches can be used only for pulses trains of low frequency* In the second example. Fig, 2, the control voltage UP is fed from the junctur UP via a diode V. , The gate electrode T of the field effect transistor FET is connected via a resistor R3 to the source electrode Q, . .

For an explanation of the mode of operation, reference is had to the flow of current at the juncture UT of the gate electrode T in Fig, 6, At steady conditions a negative voltage, blocking the switch, of the value S' is at the Juncture UP, The diode D is therefore bias l . in forward direction and is of lo resistance. The current which flowing through the. diode D and the resistors R3 and Rl causes a voltage drop between the source electrode Q and the gate electrode T blocking the effect transistor FET, A positive pulse at the juncture UP of the form shown in Fig, k, causes the diode D to be reverse-biased and therefore becomes a high resistance. Therefore at the juncture UT of the gate electrode T there appears the same potential as appears at the source electrode Q, and thus the field effect transistor FET is conductive.

With a resistor RC of high resistance, the the gate electrode T and the. source electrode Q has to be discharged via this resistor R3. In the case of a lower resistance value of resistor R3, 1 a high current flows through resistors R3 and Rl causing a voltage drop at resistor Rl within th#fc>, signal source. '.This voltage, is superimposed upon the signal. As a numerical example with a value of 100k ohm forthe resistor R3» a source resistor Rl of 5¾ ohm , and a pulse amplitude of 12 volts, the noise voltage will attain a value of 570mV. In addition the swltching-on constant at the' barrier laye capacitance between gate electrode T and source electrode Q of 5pF is about 0.5· xa · This is represented in Fig. 6 by the rising edge of the switching voltage at point UT* In the arrangement according to Fig. 3 the pulse voltage from juncture UP is fed, for control via capacitor C2 to the gate electrode T, and in addition, a blocking voltage derived from a second juncture US is also fed to electrode T over a blocking diode D.

For better understanding, it is presumed that there is an n-channel field-effect transistor FET.

In: steady state condition there is at the juncture UT a negative voltage US being fed from Juncture US via a conducting diode D.

For better understanding, it is presumed that there is an n-channel field-effect transistor FET.

In steady state condition there is atthe juncture UT a negative voltage US being fed from juncture US P.A. 309^7/3 PET ie thus blocked. A positive voltage jump over the capacitor C2 between the potentials S-L in Pig. k changes the potential at the juncture UT first to a positive value L» constituting the difference (UP-US) between the voltage UP and US (Fig. 7). The fleld effect transistor PET thus becomes conductive immediately. Via the p-n juncture between the gate electrode T 7a- and thl source electrode Q being biased in forward direction the capacitor C2 is quickly discharged until the p-n junction no longer conducts. With a small capacity value of capacitor C2, as compared with that of capacitor CI, only a very small disturbing voltage is superposed onto the signal* When switching-off, the negatively. directed voltage Jump L-S, Pig. ¾ changes the potential at juncture UT, via the condenser C2, first by the amount L-S to the negative value S', Thus the field effect traneietoir FET is immediately blocked between the source electrode Q and the drain electrode S. The charge on capacitor C2 is quickly lead off via diode D being biased in forward direction this being the case until diode D is blocked. At the juncture U there is accordingly again the potential S.

A scanning switch according to Fig. 1 shows * as can be seen from Fig. 5, .satisfactory behavious concerning the ewitching-on and switching-o f edged. However, this switch, due to the time of discharge of the capacitor C2, can be used only for pulse trains of low frequency. In the contrary to this the arrangement according to Fig. 2 can be used for pulse trains of high frequency} however, the value of resistor R3 can be shoosen for a steep ewitching-on edge, resulting ah inaccurate scanning or otherwise for an accurate scanning but a flat switching-o edge.

With the arrangement according to the inventio as shown in Pig. 3 and as can be seen from the diagram of Pig. 7# is possible to attaint with pulse trains of high frequency, both, steep switching-on edges and very exact scanning* In the arrangement according to Pig. 3 with a capacitor CI of 18 np- 2.5$ a capacitor C2 of $0 pP - 5T¾, and resistors HI and R2 each of 3.3 k ohm there has been attained at controlling with a pulse train of 15V and 10MHz and a switching-on time of s, a. total residual voltage of UR=8tnV. Thus it becomes possible to build a channel group with n switches according to Pig. 3 which compensates a middle residual voltage. , Scanning errors are then approximately 0,3$. calculated on. the value of the maximum control voltage (U « 3V )· tt x max ss

Claims (2)

WHAT IS CLAIMED IS: - ;·,. ■

1. ; ' A circuit arrangement for pulse controlled scanning of electrical signals by: means of semi-conductor switches controlled^ by pulses , characterized thereby that there is provided as a semiconductor switch a junction contact field effect transistor, the source electrode (Q) of which is au plied with the electrical signal, and the gate electrode (Τ) of which is subjected via a diode (D) to a blocking voltage (US) and via a capacitor (C2) to pulse (UT),further thereby that the scanned electrical signal is tkken from the drain electrode (s) and the blocking voltage (US) equally blocks the diode (D) .

2. A circuit substantially as described and illustrated in the accompanying drawings. -10-