Abstract:
Self-oscillation of a prescalar circuit is avoided by including offset generators on the inputs of the prescalar circuit. This ensures that when the transistors in one differential pair in the prescalar circuit transition from ON to OFF, the other differential pair of transistors will not transition. As a result, spikes are prevented in the differential pair that does not have a transition. A quadrature signal generator constructed with such a prescalar circuit provides an accurate output despite weak or non-existent input signals.

Description:
This is a continuation of application Ser. No. 08/539,978, filed Oct. 6, 1995 and now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is directed to providing a circuit having an analog divider or radio frequency (RF) prescalar and a quadrature signal generator and, more particularly, to a non-oscillating and spike-free RF prescalar circuit and accurate quadrature signal generator suitable for use in a digital cellular telephone, or related applications. 
     2. Description of the Related Art 
     A conventional quadrature signal generator including an analog divide by 2 or prescalar circuit using emitter follower logic (EFL) is illustrated in FIG. 1. As indicated, the input signals IN and INB are each supplied to the bases of two transistors. Input signal IN is supplied to transistors Q8 and Q19 and input signal INB is supplied to transistors Q9 and Q18. When expected signal levels are received, a signal IN like that illustrated in FIG. 2A will produce the signals I and Q illustrated in FIG. 2A. Since input signal INB is simply the inverse of signal IN, signal INB is not illustrated in FIGS. 2A and 2B. 
     Unfortunately, the ideal situation illustrated in FIG. 2A will not always exist. The circuit illustrated in FIG. 1 has a natural or self-oscillation frequency. If this frequency is near the range of frequencies at which the circuit is operated or tested, a weak or non-existent input signal can produce an output signal at the self-oscillation frequency, instead of at the frequency of the weak input signal. This is a problem in applications where an input signal is not provided at all times that power is provided to the circuit. For example, prescalar circuits used in equipment which has power saving features to power down portions of the circuit that are not in use may result in circuits being powered up in unexpected modes. This can result in the quadrature circuit illustrated in FIG. 1 producing a false signal for a period of time before or after an input signal is received. 
     In addition to a totally false signal like that discussed above, a spiked signal may be produced by a circuit like that illustrated in FIG. 1. The mechanism for generation of self-oscillation and spikes is similar. When the voltages supplied to inputs IN and INB are very close to each other (e.g., under weak input signal conditions or when the input levels of IN and INB are passing through zero), the divider circuit illustrated in FIG. 1 is converted from a digital flip-flop to an analog high gain circuit with positive feedback, composed of differential pairs Q8, Q9 and Q18, Q19. It is this positive feedback that causes the unwanted spike or self-oscillation. Of course as soon as the inputs IN and INB have a few millivolts of difference, the circuit converts back to a digital circuit and behaves properly. 
     Typically, as illustrated in FIG. 2B, spikes occur in the output signal which is not undergoing a phase change at the time that the other output signal undergoes a phase change and the inputs are passing through a zero crossing. These spikes can affect the performance of circuits which use the output of the quadrature circuit illustrated in FIG. 1. The spikes may be detected as a change in phase of the signal, so that the frequency of the quadrature signal is detected as higher than desired. Furthermore, the noise on the input may produce spikes that are formed at irregular intervals, with the result that the false detection of phase changes in downstream circuits will be more erratic than in the example illustrated in FIG. 2B. 
     When the prescalar circuit is part of an integrated circuit, prior to packaging the circuit will commonly undergo wafer testing. Conventionally wafer testing uses test equipment that cannot operate at frequencies as high as the normal operating range of a high frequency circuit, due to excessive inductance in the length of the probes used in wafer testing. As a result, wafer testing is done at lower frequencies. The spikes or self-oscillation can disrupt this essential test. A solution is then needed to eliminate the unwanted spikes and self-oscillation. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a prescalar circuit having a spike-free output. 
     It is also an object of the present invention to provide a prescalar circuit that will not oscillate even if the input signal level is weak or the input signal lines are an open circuit. 
     It is another object of the present invention to provide quadrature signal outputs that do not have spikes caused by a prescalar circuit. 
     It is an additional object of the present invention to provide a quadrature signal generating circuit which does not produce false output signals during normal operation or testing. 
     These together with other objects and advantages which will be subsequently apparent, reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic circuit diagram of a conventional quadrature signal generator with an emitter follower logic divide by 2 flip-flop; 
     FIGS. 2A and 2B are graphs of signals in the circuit illustrated in FIG. 1; 
     FIG. 3 is a schematic circuit diagram of an embodiment of a quadrature signal generator with an emitter follower logic divide by 2 analog divider according to the present invention; 
     FIG. 4 is a graph of signals in the circuit illustrated in FIG. 3; and 
     FIGS. 5A and 5B are schematic circuit diagrams of alternative methods of generating an offset voltage for a quadrature signal generator according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As illustrated in FIG. 3, a prescalar circuit according to the present invention is similar to the prescalar circuit in the conventional quadrature signal generating circuit illustrated in FIG. 1. The difference between the conventional circuit illustrated in FIG. 1 and the quadrature signal generating circuit illustrated in FIG. 3 is the addition of dual offset generators at the inputs. Instead of supplying the input signals IN, INB directly to the bases of transistors Q8, Q9, Q18 and Q19, the input signals IN, INB are supplied to the dual offset generators. The upper offset generator illustrated in FIG. 3 produces signal CLKI and its inverse CLKIB which are supplied to the bases of transistors Q8 and Q9, respectively. The lower offset generator illustrated in FIG. 3 produces outputs CLKQ and its inverse CLKQB which are supplied to the bases of transistors Q19 and Q18, respectively. 
     In the upper offset generator which produces signals CLKI, CLKIB, transistors Q29, Q30 form a differential pair acting as an amplifier. Transistors Q34, Q35 are emitter follower buffers. The current source Q27 draws current from the resistor R25 to produce an offset voltage. In the preferred embodiment the current and resistance are selected to produce an offset voltage of 10 millivolts. The offset voltage causes transistor Q29 to turn OFF before input signal INB crosses zero volts, i.e., earlier than transistor Q39 in the lower offset generator turns OFF due to the drop in voltage of input signal INB. Due to differential coupling, transistor Q30 turns ON when transistor Q29 turns OFF. As a result, signal CLKI goes up before signal CLKQ. 
     The lower offset generator which produces signals CLKQ, CLKQB is constructed in a similar, but symmetrical manner compared to the upper offset generator. Transistor Q40 and resistor R33 produce a similar offset voltage which causes transistor Q38 to turn OFF before transistor Q30 in the upper offset generator, as the input signal INB crosses zero. As a result, signal CLKQ will go from low to high before signal CLKI. 
     Referring to FIG. 4, it is clear that CLKI always changes its state before the positive slope zero crossing of IN and after the negative slope zero crossing of IN. On the other hand, CLKQ changes state after the positive slope zero crossing of IN and before the negative slope zero crossing of IN. In a state where INB equals IN, i.e., during zero crossings, Q30 is ON and Q29 is OFF. Similarly, Q39 will be ON and Q38 will be OFF. This is made possible by resistors R33, R25 and current sources Q27, Q40 which force the bases of Q29 and Q38 to be lower than the bases of Q30 and Q39, respectively. This in turn causes the differential pairs Q8, Q9 and Q18, Q19 to be unbalanced when IN equals INB. In the prior art circuit these differential pairs were balanced under these conditions causing the circuit to become a high gain linear amplifier with positive feedback. 
     Due to the addition of the offset generators in a prescalar circuit according to the present invention, transistor Q8 will be ON and transistor Q9 will be OFF during the transition of transistors Q18 and Q19 in both directions. As a result, the positive feedback condition is eliminated, thereby preventing self-oscillation and producing a spike-free output. 
     The present invention is not limited to the embodiment illustrated in FIG. 3. For example, FIG. 3 illustrates an EFL circuit, but the invention may be used in prescalar and quadrature circuits implemented using current mode logic (CML), etc. In addition, other methods of producing an offset may be used. 
     In the embodiment illustrated in FIG. 3, a fixed current flows through two of four resistors at the inputs of the offset generators to produce an offset. Two alternatives to this method of producing an offset are shown schematically in FIGS. 5A and 5B. In the alternative embodiment illustrated in FIG. 5A, the areas of the differential pair transistors are different. The area corresponding to reference character X1 is twice as large as the area corresponding to reference character X2. The difference in the areas of the two transistors which form the differential pair illustrated in FIG. 5A will cause the tail current to be split unequally (with a 2:1 ratio) when the differential input to the differential pair is zero. This will cause an offset at the two emitter follower outputs. As a result, if an identical ramp signal is supplied to the bases of the differential pair transistors illustrated in FIG. 5A, the left-hand transistor will turn ON before the right-hand transistor. 
     In the alternative embodiment illustrated in FIG. 5B, the same effect is achieved by resistor ratios where one emitter resistor is X times larger than the other emitter resistor. Again under a zero volt differential input, the left-hand transistor will have less current than the right-hand transistor in the differential pair illustrated in FIG. 5B. This will cause the two output emitter followers to have unequal output voltages. 
     The circuits shown in FIGS. 5A and 5B are functionally equivalent to the offset generator circuits added to FIG. 3. All of these circuits will produce the same offset signals which produce CLKI, CLKIB, CLKQ and CLKQB illustrated in FIG. 4. 
     The many features and advantages of the invention are apparent from the detailed specification and, thus, it is intended by the appended claims to cover all such features and advantages of the invention which fall within the true spirit and scope of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.