An analog pulse shaper

Abstract: The invention relates to the forming of a control pulse, for the transmitter in a GSM radio telephone system. A cos² pulse is formed by means of an analog circuit from a rectangular pulse which is first shaped into a triangular pulse which may be somewhat clipped at its peak. The triangular pulse is further shaped into a cos² pulse. The circuit also includes means (Cr...CN; R14...RN; Q7...QN) by using which the abruptness of the triangular pulse and thereby the abruptness of the cos² pulse can be selected.

Description: The invention relates to an electric circuit by means of which a rectangular pulse is shaped analogically into a cos² pulse the time constant of which can be varied. New devices and systems require that a control pulse should have a precisely defined form. This form is achieved by shaping an initial pulse into the desired pulse form in a shaping circuit. It is known to solve the defined problem by using the digital technique, by means of which an initial pulse can be shaped digitally with relative ease and then be converted by a D/A converter into an analog pulse of the desired form. However, a digitally shaped pulse has the disadvantage that the curtain curve of the initial pulse is step-wise, which is due to the limited number of bits of the D/A converter. The step-wise quality due to the quantization of the pulse for its part causes problems in a digital GSM system, when the power of a radio telephone is being controlled by means of a digitally shaped pulse. A pulse of this type causes a widening of the power spectrum. In addition, a digital pulse shaper requires digital control. FR-A-2454226 discloses an analog pulse shaper that has the features of the pre-characterizing portion of appended Claim 1. The task is thus to provide an electric circuit producing a pulse with a curtain curve which is not step-wise and being as simple to implement as possible. This task is solved by the present invention using the pulse shaper defined in the appended claims. Briefly summarized, the pulse shaper operates as follows. A rectangular wave supplied to the input of the circuit is converted to a triangular wave by means of resistors, a capacitor, and transistors. The leading edge and the trailing edge of the rectangular wave serve as triggers, causing the capacitor to charge and discharge with a constant current, thus forming a triangular pulse. Depending on the pulse length of the rectangular wave, the triangular pulse may be whole or clipped at its peak. Furthermore, by means of a number of transistors, the triangular pulse is shaped into a cos² pulse. Preferably, the third means comprises additional capacitors which can be switched in parallel with the above-mentioned capacitor, whereupon the time constant of the capacitor can be varied. N additional switchable capacitors make 2 N time constants possible. The circuit according to the invention is described below in detail with reference to the accompanying figures, in which: Figure 1 depicts the implementation of one circuit according to the invention, Figure 2 depicts the temporal relation of the rectangular wave arriving at the circuit and the capacitor voltage, and Figure 3 depicts as a function of time the output voltage formed by the circuit. In the circuit of Figure 1, by means of transistors Q5 and Q6 and a capacitor C1 the incoming pulse Vin is shaped into a triangular voltage effective across the capacitor, and from this triangular voltage there is further obtained a shaped output voltage Vo by means of transistors Q1-Q4 and resistors R3, R9 and R10. Capacitors C4...CN can be switched, by the control of transistors Q7...QN serving as switches, in parallel with capacitor C1. The operation of the shaping circuit is as follows: a rectangular wave Vin is formed from the supply voltage Vs by chopping it under control into inverted pulses of the desired length, a pulse being considered to be the time interval during which the voltage is 0 V. Of course, a rectangular wave can also be formed in other known ways. Transistors Q5 and Q6 function as current mirrors. At the descending edge of the incoming pulse (when the voltage drops from Vs to 0 V) transistor Q6 becomes conductive and the current flows via the transistor part switched as the diode of transistor Q6. Because of the switching the same current passes via the other transistor part of transistor Q6. This current Ic charges capacitor C1. The intensity of the charging current is: Ic= Vs-0.6 v R1 The time constant T1 of the charging of capacitor C1 is: T1= C1 xVs Ic The time constant depends on the value of C1. As was stated above, capacitors C4...CN can be switched in parallel with capacitor C1 by means of control lines Csw1...N, in which case the time constant and thereby the rising rate of the capacitor charging voltage can be varied. In a corresponding manner, at the rising edge of the rectangular wave Vin (the voltage rises from 0 V to Vs), transistor Q5 becomes conductive and capacitor C1 discharges with a current the intensity of which is the same as that of the charging current, if R1=R2. Thus the gradual-edged waveform shown in Figure 2 is formed. Figure 2 shows how the voltage V1 effective across capacitor C1 rises after time constant T1 to a value close to Vs and respectively drops within the same time to 0 V, when the rising and descending edges of the rectangular wave function as triggers. If the rectangular pulse is short, a triangular pulse is obtained from the voltage V1, and the pulse being long, a pulse with a clipped peak is respectively obtained. The abruptness of the pulse is affected by means of capacitor C1. Further referring to Figure 1, it is observed that when the rising voltage effective across the capacitor reaches the base-emitter voltage 0.6 V of transistor Q1, this transistor becomes conductive and thus the emitter voltage V2 begins to rise. When this voltage has reached the base-emitter voltage 0.6 V of transistor Q2, the emitter voltage V3 begins to rise. The voltages V1, V2 and V3 are summated by means of resistors R3, R9 and R10 as the output voltage Vo of the circuit, the voltage having the shape of a cos² pulse. Vo=( V1 R3 + V2 R9 + V3 R10 )x( 1 1/R3+1/R9+1/R10 ) is valid regarding the voltage Vo. The voltage is a function of time, since V1, V2 and V3 are functions of time. Figure 3 shows the form of the output voltage Vo of the circuit as a function of time. The form of the initial voltage is easy to understand starting from a situation in which the descending edge of an inverted rectangular wave causes a linear growth of the charging voltage of capacitor C1. In this case, from angle point 1 to point 2, V2 and V3 are zero and V1 rises to 0.6 V, which it reaches at angle point 2. At angle point 2, V2=0 and V3=0 and V1=0.6 V At angle point 3, V2=0.6 V and V3=0 V and V1=1.2 V From angle point 4 onward, V3 is constant, i.e. V3= R13 R12+R13 xVs-0.6V At angle point 4 V1= R13 R12+R13 xVs+0.6V+ V3 R11xh FE xR7+ V2 R8xh FE xR h FE = direct-current amplification of transistors Q1 and Q2 From angle point 5 onward, V2 is constant V2 = R6/(R5 + R6) x Vs - 0.6 V and V1= R6 R5+R6 xVs+0.6V+ V2 R8+h FE xR4 The first half of the output pulse Vo is formed in this manner. The other half is formed as its mirror, when capacitor C1 begins to discharge at the rising edge of the incoming pulse Vin. In theory the formed cos² pulse has acute angles at the angle points, but in practice, owing to the characteristic curves of the transistors and to capacitor C3, the angle points are rounded, whereby a cos2 signal of a good quality is obtained. The described pulse-shaper circuit according to the invention has many advantages over the prior art circuits implemented by using the digital technique. There are no quantization steps, and no digital control is needed. The circuit does not produce quantization noise, which is a considerable advantage considering the use of the circuit. The circuit is of simple construction and contains few components, and it is easy and inexpensive to integrate as an IC circuit. The pulse-shaper circuit according to the invention is in particular intended for forming a control pulse for the transmitter of a mobile radio telephone or a ground station in a GSM system, although it can, of course, be used for forming a control pulse for any device.

Claims: An analog pulse shaper which shapes a rectangular pulse into a cos² pulse, including: an input terminal (Vin) for receiving a rectangular pulse, a supply voltage terminal (Vs), a first means connected to the input terminal (Vin) for shaping the rectangular pulse into a triangular pulse (V1) with rising and falling sides and possibly with a clipped peak, the first means including means (C1) by which the abruptness of the sides of the triangular pulse may be varied, a second means for receiving the triangular pulse formed by the first means and for shaping the triangular pulse into a cos² pulse and an output terminal (Vo) for providing said cos² pulse, characterized in that the second means includes at least a first circuit section (Q1, Q4, R4, R5, R6, R8) receiving the triangular pulse voltage (V1), whose minimum value is zero, and providing a second voltage (V2) rising and descending with the triangular pulse voltage (V1) and being limited between zero as a lower limit and an upper limit said limits corresponding respectively to a first value greater than zero, and a second value greater than the first value but smaller than the maximum value, of the triangular pulse voltage (V1), a second circuit section (Q2, Q3, R7, R12, R13, R11), operationally similar to the first circuit section, receiving the second voltage (V2) and providing a third voltage (V3) rising and descending with the second voltage (V2) and with the triangular pulse volyage (V1) and being limited between zero as a lower limit and an upper limit said limits corresponding respectively to a third value greater than the first value but smaller than the second value, and a fourth value greater than the third value but smaller than the second value, of the triangular pulse voltage (V1), and summing means (R3, R9, R10) receiving and combining the triangular pulse voltage (V1), the second voltage (V2) and the third voltage (V3) to provide the output voltage (Vo) having angle points (2, 5, 3, 4) corresponding respectively to the first, second, third and fourth value of the triangular pulse voltage (V1) and approximating a cos² pulse shape. An analog pulse shaper accordmg to Claim 1, characterized in that the first means include a capacitor (C1), a first current mirror (Q6) and a second current mirror (Q5), the first current mirror (Q6) being connected to the input terminal (Vin), via an input resistor (R1), to the supply voltage terminal (Vs) and to the capacitor (C1), the capacitor being charged with a first constant current from the first current mirror (Q6) when the rectangular pulse voltage is low, and the second current mirror (Q5) being connected to the input terminal (Vin), via an input resistor (R2), to a ground potential and to the capacitor (C1), the capacitor being discharged by the second current mirror (Q5) with a second constant current when the rectangular pulse voltage is high, the triangular pulse being voltage (V1) across the capacitor (C1), and in that the first constant current is determined by a voltage difference between the supply voltage and the rectangular pulse, when its voltage is low, the first current mirror (Q6) and the first input resistor (R1), and the second constant current is determined by a voltage difference between the ground potential and the rectangular pulse, when its voltage is high, the second current mirror (Q5) and the second input resistor (R2). An analog pulse shaper according to Claim 1 or 2, characterized in that the abruptness of the sides of the triangular pulse is made selectable by capacitors (C4, ...,Cn) which can be switched selectably (CSW1, Q7; ... ; CSWn, Qn) in parallel with the capacitor (C1) included in the first means. An analog pulse shaper according to any of the preceding Claims, characterized in that the first and second circuit sections include respectively: a first transistor (Q1; Q2) receiving the triangular pulse voltage and respectively the second voltage (V1; V2) at the base thereof and providing the second and respectively the third voltage (V2; V3) at the emitter thereof the transistor (Q1 ; Q2) becoming conductive and respectively being cut off and the emitter voltage (V2; V3) thereof starting to rise from, and respectively descending to, zero at the first and respectively the third value of the triangular pulse voltage (V1), a second transistor (Q4; Q3) coupled in series with the first transistor (Q1; Q2) and having a voltage devider (R5, R6; R12, R13) connected to the base thereof for providing the upper limit of the emitter voltage (V2; V3) at the second and respectively the fourth value of the triangular pulse voltage (V1). An analog pulse shaper according to any of the preceding Claims, characterized in that the summing means include a first resistor (R3), a second resistor (R9) and a third resistor (R10) for receiving respectively the triangular pulse voltage (V1), the second voltage (V2) and the third voltage (V3) at the first terminal thereof the second terminals thereof being connected together to provide the output voltage (Vo) in accordance with the equation Vo=( V1 R3 + V2 R9 + V3 R10 )×( 1 1/R3+1/R9+1/R10 ). An analog pulse shaper according to any of the preceding Claims, characterized in that a capacitor (C3) is connected between the output voltage terminal (Vo) and the ground for smoothing the angle points of the slope of the output voltage (Vo) so as to provide a closer approximation of a cos² shape.

Inventors: PEKKARINEN VESA, SARASMO JUKKA, VAISANEN RISTO, PEKKARINEN, VESA, SARASMO, JUKKA, VAISANEN, RISTO
Applicants: NOKIA MOBILE PHONES LTD, NOKIA MOBILE PHONES LTD.
Application year: 1990
Main classification: H03K6
IPC codes: G06G7, H03K4, H03K6