A STRAY PULSE ELIMINATING DEVICE FOR A DIGITAL-TO-ANALOG CONVERTER

Abstract: The invention relates to a device for eliminating stray pulses occuring on the output of a digital-to-analogue converter DA. The pulses may be caused by, for example, insufficient synchronism in the input data bits and may be predicted in time. According to the invention an inductor L is arranged between the output of the digital-to-analogue converter and a following circuit F, for example a deflecting amplifier for random scan displays, and a switch S2 is arranged between the said output and ground which switch when a pulse is predicted to occur shortcircuits the output of the digital-to-analogue converter to ground. Hereby an eliminating device of simple and cheap construction is obtained operating at high frequencies.

Description: The present invention relates to a stray pulse eliminating device and in particular to a device for eliminating stray pulses occurring at the output of a digital-to-analog converter (D/A converter), the device having an input connectable to the output of the D/A converter, an output and comprising switching means, storage means and a prediction circuit adapted to calculate the instants on which stray pulses are expected to occur and dependent thereupon to activate the switching means, a possible stray pulse at the output of the D/A converter by means of the switching means being prevented from reaching the output of the device for a time period during which the pulse occurs, the storage means being coupled to the switching means and to the output of the device for storing the signal present at the output of the D/A converter during the time that the switching means are not activated. Such a device is known from United States Patent US-A-4,132,908. In this device the switching means are coupled serially between the output of the D/A converter and the output of the device. The storage means comprise a capacitor that is arranged between the output of the device and a reference potential. At an instant of expected occurance of a stray pulse, the switching means are activated (opened) by the prediction circuit and the stray pulse is prevented from reaching the output of the device and therefore shall not reach a following circuit. The capacitor stores the prevailing signal across the capacitor just before the moment of opening the switching means, and this signal is present at the output of the device during the time the switching means are open. When the switching means are closed, the capacitor is rapidly charged to a new signal value, provided by the D/A converter. The known device has several disadvantages. Most of the available D/A converters for high frequencies have a current output. The known device therefore has to be completed by an amplifier at the input converting the current output of the D/A converter to a voltage output suitable for the stray pulse eliminating device having the above described construction. Consequently, the amplifier, having a limited bandwidth, introduces an undesired prolongation of the duration of the stray pulses. An object of the invention is to obtain a device of the kind set forth in the opening paragraph which allows the output of the D/A converter to be directly coupled to the input of the eliminating device without any intermediate amplifier distorting the shape of the stray pulses. A further object of the invention is to obtain a simple and cheap eliminating device operable at high frequencies. The objects of the invention are obtained by means of a device as described in the preamble, characterized in that the D/A converter has a current output and in that the storage means comprise an inductor arranged between the output of the D/A converter and the output of the device, the switching means being coupled between the output of the D/A converter and a reference potential for short circuiting the output of the D/A converter to the reference potential during the time period that the pulse occurs. Just before a stray pulse occurs, the switching means are activated (closed) and maintained closed for the time period during which the pulse is present, and the charge of the stray pulse is shortcuited to ground. The current from the output of the D/A converter remains stored in the inductor, the voltage across this inductor being practically zero. Accordingly, there is no significant current change during the period when the switching means are closed. When the switching means are reopened, the current at the output of the D/A converter is supplied to the inductor which rapidly changes its current to the new value. The invention is based on the insight that, in the event of a D/A converter having a current output, the amplifier at the input of the device, which amplifier converts the current output of the D/A converter to a voltage output suitable to be stored by the capacitor, can be omitted by using a device comprising the inductor and switching means for shortcircuiting the input of the inductor as long as these switching means are activated by the prediction circuit. Embodiments of the invention will now be described in more detail by way of example with reference to the accompanying drawings, in which Fig. 1 shows schematically a previously known stray pulse eliminating device, Fig. 2 shows schematically a stray pulse eliminating device according to the invention, Fig. 3 shows in more detail a further stray pulse eliminating device according to the invention and Fig. 4a-d show time diagrams for a device according to the invention. According to the known device shown in Fig. 1 an amplifier F1 is connected to the output of a D/A converter DA having current output. The output of the amplifier F1 is connected to the input of a second amplifier F2 via a switch S1. Furthermore, the input of the second amplifier F2 is connected to a reference level, in this embodiment ground, via a capacitor C. The switch S1 is controlled by a prediction circuit P which predicts the time of the occurance of a stray pulse. When a stray pulse is expected to occur the prediction circuit P ensures that the switch S1 disconnects the connection between the amplifiers F1, F2. The signal across the capacitor C at the time of disconnection then remains stored across the capacitor. When the expected stray pulse is assumed to have ceased the switch S1 is reclosed and the capacitor rapidly is charged to a signal value which now is present on the output of the first amplifier F1. A device according to the invention comprises an inductor L, such as a choke coil, connected between the output of the D/A converter and the input of an operational amplifier F and a switch S2 connected between the output of the D/A converter DA and ground. The switch S2 is controlled by a prediction circuit P which predicts the time of the occurance of a stray pulse. Just before the pulse is expected to occur the prediction circuit P ensures that the switch S2 is closed and then is maintained closed for a period containing the instant of the occurance of the expected stray pulse. The current from the output of the D/A converter DA then remains stored in the inductor L and the charge of the stray pulse is shortcuited to ground. Because the inductor L is connected between the switch S2 and the summing point of the operational amplifier F the voltage across the inductor L is practically zero. Accordingly, this does not give rise to any significant current change during the period when the switch S2 is closed. When the switch S2 is reopened the current at the output of the D/A converter DA is supplied to the inductor L which rapidly changes its current to the new value. In the embodiment shown in Fig. 3 the switch is constituted by a diode and resistor coupling consisting of four diodes and four resistors. The diodes which ought to be of a rapid type (for example hot carrier diodes) are arranged in a bridge coupling consisting of two parallel coupling paths with two unidirected diodes D1, D2 and D3, D4, respectively , in each of the coupling paths. The junction 1 between the diodes D1, D2 in one of the coupling paths is connected to the output of the D/A converter DA and the junction 2 between the diodes D3, D4 in the other coupling path is connected to ground. One of the common ends 3 of the coupling paths is connected to one terminal of the output of a high-frequency transformer TR via a resistor R1 and the other common end 4 of the coupling paths is connected to the other terminal of the output of the highfrequency transformer TR via a resistor R2. Furthermore, a resistor R3 is connected between a terminal of the transformer TR and ground and a further resistor R4 is connected between the other terminal of the transformer TR and ground. The switch S1 in the form of a diode and resistor bridge is closed by means of a pulse signal at the output of a driver, for example a TTL gate, being supplied to the resistor and the diode coupling via the high-frequency transformer TR. The signal pulls equal currents through the two coupling paths so that the junctions 1, 2 between the diodes D1, D2 and D3, D4, respectively, in the coupling paths assume substantially the same potential, that is the potential of ground. In Fig. 3 a lowpass filter LP is shown at the output of the amplifier F. The filter prevents high frequency signals from passing so that the output of the filter becomes a constant direct current. Normally, the lowpass filter function is obtained by a system coupled to the output of the amplifier, the system being in the form of, for example, a deflection system having per se a limited band width and hence an upper limiting frequency. In Fig. 4a-d time diagrams of the device according to the invention are shown. The digital input signal of the D/A converter is assumed having been generated by a BCD counter clocked with a constant frequency so that the ideal output current of the D/A converter thus should have an evenly ascending stair-case shape. Fig. 4a shows the clock pulses of the counter. Fig. 4b shows the D/A converter output current which as shown may have both positive and negative stray pulses of varying amplitudes. Fig. 4c indicates the time intervals during which stray pulses are expected to occur, that is the time intervals during which, according to the invention, the switch S2 is maintained closed. Fig. 4d shows the actual current through the inductor L at the output of the D/A converter, the upper front edge of a stairstep being shown in an enlarged scale. The region of the enlarging having oblique strokes has a substantial constant charge content for each stairstep. Accordingly, the current curve deviates somewhat from the ideal evenly ascending staircase due to transients occurring when the switch S2 is closed, but these disturbances have as has already been pointed out a substantially constant charge content.

Claims: A device for eliminating stray pulses occurring at the output of a digital-to-analog converter (D/A converter), the device having an input connectable to the output of the D/A converter, an output and comprising switching means, storage means and a prediction circuit adapted to calculate the instants on which stray pulses are expected to occur and dependent thereupon to activate the switching means, a possible stray pulse at the output of the D/A converter by means of the switching means being prevented from reaching the output of the device for a time period during which the pulse occurs, the storage means being coupled to the switching means and to the output of the device for storing the signal present at the output of the D/A converter during the time that the switching means are not activated, characterized in that the D/A converter has a current output and in that the storage means comprise an inductor arranged between the output of the D/A converter and the output of the device, the switching means being coupled between the output of the D/A converter and a reference potential for short circuiting the output of the D/A converter to the reference potential during the time period that the pulse occurs. A device as claimed in claim 1, characterized in that the inductor is a choke coil. A device as claimed in any of the preceding claims, characterized in that the switching means comprise a diode coupling having two parallel coupling paths comprising in each path two unidirected diodes connected in series, the junction between the diodes in one coupling path being connected to the output of the digital-to-analog converter and the junction between the diodes in the other coupling path being connected to the reference potential, and a resistance network consisting of a first resistor connected between one input terminal of the switch and a common junction of the coupling paths, a second resistor connected between the other input terminal of the switch and a second common junction of the coupling paths, a third resistor connected between the first input terminal and the reference potential and a fourth resistor connected between the second input terminal and the reference potential. A device as claimed in claim 3, characterized in that a pulse signal applied to the first and second input terminals of the switch is coupled via a transformer.

Inventors: AKERLUND, MATS PETER, STROEMBAECK, TURE JOHANNES, Strömbäck, Ture Johannes
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN, PHILIPS NORDEN AB
Application year: 1984
Main classification: H03M1
IPC codes: H03M1