Filter circuit for generating a VCO control voltage responsive to the output signals from a frequency/phase discriminator
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a filter circuit for a digital phase-locked loop (PLL) circuit having a digital frequency/phase discriminator which provides two digital signals, and, in particular, to a filter circuit which receives the two digital signals from the digital frequency/phase discriminator and provides a smoothed voltage output for controlling a voltage-controlled oscillator. 
     2. Description of the Related Art 
     With regard to the stated practical application, a simple CMOS filter circuit for a phase-locked loop circuit is explained in Deog-Kyoon Jeong, et al., &#34;Design of PLL-Based Clock Generation Circuits,&#34; IEEE Journal of Solid-State Circuits, Vol. SC-22, No. 2, April 1987, pp. 255-261. Particular reference is made to FIG. 3b on page 256 of the article, which shows a circuit comprising the series arrangement of the controlled current paths of a p-conducting transistor and an n-conducting transistor. The series arrangement is disposed between the two poles of a source of supply voltage. A square-wave (i.e., digital pulse) signal from a frequency/phase discriminator is fed to one or the other of the two gates serving as the control terminals of the two transistors. The common connecting point between the two transistors is connected via a series RC circuit to one pole of the source of supply voltage. A smooth voltage is taken off this connecting point, across a series resistor. The smoothed voltage is then fed to the control input of the voltage-controlled oscillator (VCO) of the PLL circuit. The two digital signals that control the two transistors are the UP signal and the DOWN signal generated by the frequency/phase discriminator in response to whether the phase of the output of the VCO leads or lags the phase of a reference signal. 
     The filter circuit as described hereinbefore has, among other disadvantages, the disadvantage that the digital signals are transmitted via parasitic capacitances between the gate and the source/drain of the two transistors to the output. Also, the output voltage (ua) is restricted to the range lying between the magnitude of the control terminal threshold voltage (u tn ) of the n-conducting transistor and the supply voltage (u) as reduced by the magnitude of the control terminal threshold voltage (u tp ): 
     
         ua=u-|u.sub.tp |-|u.sub.tn |. 
    
     SUMMARY OF THE INVENTION 
     The above described disadvantages associated with the previous invention are remedied by the invention as described herein, in that the coupling of the two digital input signal voltages to the output voltage of the filter circuit is substantially avoided, and further in that the output voltage range is enlarged. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention will now be explained in greater detail with reference to the accompanying single drawing figure which shows the circuit diagram of a preferred embodiment of the filter circuit of the present invention, as realized in CMOS technology. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the drawing, a source of supply voltage u having a plus pole + and a minus pole - is connected to the filter circuit of the present invention. The minus pole - of the source of supply voltage u is connected to a ground reference, as indicated. The filter circuit comprises the series arrangement of the controlled current paths of a first n-conducting transistor n, a second n-conducting transistor nd, a first p-conducting transistor p, and a second p-conducting transistor pd. The current paths of the four transistors are connected between the plus pole + and the minus pole - of the supply voltage u with the current path of the of the first n-conducting transistor n connected to the plus pole +, followed in order by the current path of the second n-conducting transistor nd, the current path of the second p-conducting transistor pd and the current path of the first p-conducting transistor p, with the latter current path being connected to the minus pole - of the supply voltage u. 
     The second n-conducting transistor nd is connected as a diode in that the control terminal (i.e., gate) of the second n-conducting transistor nd is connected to the point connecting the current path of the second n-conducting transistor nd to the current path of the first n-conducting transistor n. Similarly, the second p-conducting transistor pd is connected as a diode in that the control terminal (i.e., gate) of the second p-conducting transistor pd is connected to the point connecting the current path of the second p-conducting transistor pd to the current path of the first p-conducting transistor p. Thus, the second n-conducting transistor nd and the second p-conducting transistor pd represent resistance-affected diodes. 
     The control terminal (i.e., gate) of the second n-conducting transistor nd (which is connected as a diode) is supplied with a first square-wave (i.e., digital pulse) signal s1 via a first capacitor c1. The control terminal (i.e., gate) of the second p-conducting transistor pd (which is connected as a diode) is supplied with a second square-wave (i.e., digital pulse) signal s2 via both an inverter i and a second capacitor c2. The first digital signal s1 and the second digital signal s2 are the UP and DOWN signal outputs of the frequency/phase discriminator (not shown), such as is described in the Jeong, et al., article discussed above. 
     The point connecting the current paths of the second n-conducting transistor nd and the second p-conducting transistor pd is the output for a smoothed output voltage ua. One terminal of a series RC circuit, comprising a resistor r and a capacitor c, is connected to this point. A second terminal of the series RC circuit is connected to the minus pole - of the source of supply voltage (i.e., connected to the ground reference). A common point connecting the resistor r to the capacitor c of the RC circuit is connected to the control terminals (i.e., the gates) of the first n-conducting transistor n and the first p-conducting transistor p. The voltage at this common point is designated as uc. 
     The voltage at the gate of the second n-conducting transistor nd has a magnitude of uc-u tn , where uc is the voltage at the common point of the resistor r and the capacitor c in the series RC circuit, and where u tn  is the control terminal threshold voltage of the first n-conducting transistor n. The voltage at the gate of the second p-conducting transistor pd has a magnitude of uc-|u tp  |, where u tp  is the control terminal threshold voltage of the first p-conducting transistor p. The diode-connected transistors nd and pd are equivalent to diodes in series with the ON resistance of the transistors. 
     If, thereafter, for example, the first digital signal s1 assumes an H-level (i.e., high logic level), which is transmitted via the capacitor cl to the second n-conducting transistor nd, then the capacitor c1 discharges via the second n-conducting transistor nd toward the output voltage ua and further, across the resistor r, to the capacitor c. Inversely, the second digital signal s2 acts through the inverter i and the capacitor c2 via the second p-conducting transistor pd upon the RC circuit and, consequently, upon the output ua, in the same way as has already been explained in the above-cited prior art publication with regard to the simple circuit shown and described therein. 
     In the circuit according to the present invention, the range of modulation is greater than with the prior art circuit described hereinbefore, because, for example, the capacitor c1 can be pre-charged by the first n-conducting transistor n up to the voltage +u. Thus, the output voltage ua theoretically reaches a maximum value u+(u-2u tn ). In a comparable way, the output voltage theoretically reaches a minimum voltage value -(u-2|u tp  |). The just mentioned maximum and minimum values relating to the output voltage ua represent the respective theoretical upper and lower limits which, however, are not reached in practice. Thus, for instance, with the exemplary embodiment, as realized in CMOS technology, the substrate control effect of the employed MOS transistors, the ohmic voltage drop at the diode-connected transistors nd, pd, and the opening of the source-drain substrate diode of the transistors pd, nd are responsible for the fact that the maximum possible range of variation of the output voltage ua is not reached. However, the range is still substantially greater than with the prior art circuit, as described hereinbefore. 
     It is within the scope of the invention to employ complementary bipolar transistors instead of the CMOS transistors used in the exemplary embodiment shown in the accompanying drawing. Likewise, the diode-connected transistors nd, pd may be replaced by the series connection of a resistor and a diode.