VOLTAGE/FREQUENCY CONVERTER

Abstract: A voltage/frequency converter is formed of a V/F conver­ ter circuit (10, 12) which converts an input potential (ei) into a frequency (fc) corresponding to the instantaneous value of the input potential (ei), and a difference detection circuit (14 to 26) which generates an output (ej) corresponding to the differ­ ence between the output frequency (fc) of the V/F Converter circuit (12) and a predetermined reference frequency (fo). The V/F converter circuit (10, 12) is controlled according to the level of the output (ej) from the difference detection circuit (26), thereby reducing the deviation of the output frequency (fc) of said V/F converter circuit (12) from the reference fre­ quency (fo).

Description: The present invention generally relates to a voltage/frequency converter (hereinafter referred to as V/F converter) for converting the instantaneous value of an input potential into frequency information. In a frequency modulation system, the frequency of a carrier wave is modulated by the instantaneous value of an input potential and the modulated carrier wave is transmitted to a remote place. In such a modulation system, the carrier frequency is increased with increase of the present value of the input potential and reduced with reduction thereof. Although various type of frequency modulation systems have been known, either of these systems basically has an input/output characteristic as shown in Fig. 4. As will be seen from Fig. 4, when the input potential (input level e10) is at zero, the output frequency (fc) is fo (Hz). Output frequencies fH (Hz) and fL (Hz) are obtained at positive and negative peaks emax and -emax of the input potential, respectively. Thus, the output frequency (fc) can be deviated by ± Δf (Hz) from the center frequency of fo (Hz) ( Δf = fH - fo = fo - fL ). A V/F converter circuit used in such a frequency modulation system is conventionally made of a linear circuit (IC), and a drift of the operating point of the circuit may happen. The drift is liable to be increased not only according to ambient conditions, such as temperature, but also in long use. Such a drift causes a frequency shift in the V/F converter output. Namely, even if the input potential of the V/F converter is at zero, a frequency shift (fd) corresponding to the magnitude of a drift appears in the converter output frequency. From this, when the input is varied from -emax to emax in this situation, the output frequency is varied from fL + fd (Hz) to fH + fd (Hz). When such a drift appears, an output frequency, exactly corresponding to the input potential, can no longer be obtained, resulting in posing problems in data transmission. Suppose a case where data in a form of a series of signals having a predetermined frequency is transmitted with a certain drift, for example. Then, the data is transmitted with an erroneously shifted transmission frequency, so that the transmitted data is inaccurately confirmed at the receiver side. Similar problems may arise when a drift is involved in the input potential per se. The V/F converter output has a form of a signal series with instantaneously varied frequencies. If it is intended to effect a drift compensation through data processing of the signal series, an enormous drift compensation circuit is required. Consequently, it has been desired to develop a compensation circuit, which can achieve the drift compensation without complicating the circuit configuration. Prior art document US-A-3 753 141 discloses a controllable broad-band frequency generator comprising a voltage-controlled oscillator incorporated in a frequency controlled loop and an external controllable voltage source connected thereto. The control loop furthermore incorporates a digital crystal stable frequency discriminator and a combination device in which the direct voltage value derived from the voltage source is deducted from the output voltage provided by the frequency discriminator to obtain a direct control voltage for frequency controlling the voltage-controlled oscillator. Further, prior art document US-A-3 824 483 describes a phase locked loop voltage controlled digital synthesizer in which digital pulses produced from a voltage controlled oscillator (VCO) and a first frequency divider and a reference oscillator and a second frequency divider are applied to a comparator which produces an output signal when the compared pulses have repetition frequencies which are not equal. The comparator output signal results in a voltage which is applied to the VCO to change the frequency thereof in steps until repetition frequency equality is achieved. To shorten the time needed to cause the repetition frequency of the pulses at the output of the first frequency divider to equal the repetition frequency of the pulses at the output of the second frequency divider, a reversible counter is connected to the comparator output, and the output signal of the reversible counter is connected to a digital-to-analog converter to produce the VCO control voltage. It is an object of the present invention to provide a V/F converter which can supply, through compensation for a frequency shift due to a drift involved in a V/F converter circuit or due to a drift contained in a converter input per se, a frequency conversion output being free of the drift. To achieve the above object the present invention provides a voltage/frequency converter as specified in the claim. A V/F converter circuit converts an input potential into an output frequency which represents the instantaneous value of the input potential, and a difference detection circuit generates an output representing the difference between the output frequency of the V/F converter circuit and a predetermined reference frequency. The V/F converter circuit is controlled according to the output level of the difference detection circuit so as to reduce the amount of the output frequency shift of the V/F converter circuit with respect to the reference frequency. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a block diagram showing a V/F converter; Fig. 1A is a block diagram showing a modification of the V/F converter of Fig. 1; Figs. 2A to 2I show a timing chart explaining the operation of the V/F converter of Fig. 1; Fig. 3 illustrates an example of the count of counter 24 in Fig. 1; Fig. 4 is a graph showing an input/output characteristic of V/F converter unit 12 in Fig. 1; Fig. 5A is a circuit diagram showing a specific circuit configuration of analog adder 10 in Fig. 1; Fig. 5B is a circuit diagram showing a specific circuit configuration of analog subtracter 10S in Fig. 6; Fig. 6 is a block diagram showing another V/F converter; Fig. 6A is a block diagram showing a modification of the V/F converter of Fig. 6; Figs. 7A to 7D show a timing chart explaining the operation of the V/F converter of Fig. 6; Fig. 8 is a block diagram showing a further V/F converter; Figs. 9A to 9H show a timing chart explaining the operation of the V/F converter of Fig. 8; Fig. 10 is a block diagram showing a still further V/F converter; Figs. 11A to 11G show a timing chart explaining the operation of the V/F converter of Fig. 10; Fig. 12 is a block diagram showing a yet another V/F converter; Fig. 13A is a flow chart explaining the operation of microcomputer 240 shown in Fig. 12; Fig. 13B is a flow chart explaining another operation of microcomputer 240 shown in Fig. 12; Fig. 14 is a schematic representation of a modification of the V/F converter of Fig. 1, in which a single linear operational amplifier is applied to analog adder 10 and D/A converter 26; Fig. 15 is a block diagram showing an embodiment of the V/F converter according to the invention; and Figs. 16A to 16H show a timing chart explaining the operation the V/F converter of Fig. 15. In the following description, the same or similar reference symbols are used to indicate the same or similar elements, thereby avoiding redundant explanations. Fig. 1 shows a V/F converter in which a modulation input signal ei is led to analog adder 10. In adder 10, a drift compensation value (ej) to be described later is combined with ei. Output e10 from adder 10 is applied, as a V/F conversion input, to V/F converter unit 12. By unit 12, output e10 is converted into frequency information (fc). V/F converter unit 12 provides two separate outputs eo and e12, each representing the frequency information. Output eo may be either a sine wave signal or a triangular wave signal. Output e12 may be a rectangular wave pulse having the same frequency as eo. Reference clock oscillator 14 generates clock pulse e14 having frequency fo. Pulse e14 is frequency-divided through frequency divider 16. Frequency-divided output e16 from divider 16 is applied to inverter 18 and to NAND gate 22. Gate 22 also receives output e12 from V/F converter unit 12. Gate 22 supplies logical NAND output e22 to down-count terminal D of N-bit up/down counter 24. Inverted output e18 from inverter 18 and clock pulse e14 from oscillator 14 are fed to NAND gate 20. NAND gate 20 supplies logical NAND output e20 to up-count terminal U of counter 24. N-bit up/down counter 24 up-counts the number of clock pulses e20 at up-count terminal U when the logical level of e22 at down-count terminal D is "1". Also, counter 24 down-counts the number of clock pulses e22 at terminal D when the logical level of e20 at terminal U is "1". Several upper bits of count output d24 from counter 24 are coupled to digital/analog converter (hereinafter referred to as D/A converter) 26 with M-bit input. D/A converter 26 generates analog compensation signal ej representing the content of count d24. Signal ej is supplied to analog adder 10 so that it is added to input signal ei. Incidentally, the number (integer) N of the output bits of counter 24 is independent of the number (integer) M of the input bits of D/A converter 26, and integer M is selected to be equal to or less than the integer N. Output frequency fc (Hz) of V/F converter unit 12, obtained when input signal ei is at "0" level, is set at a value substantially equal to oscillation frequency fo (Hz) of reference clock oscillator 14. On the other hand, frequency fc may be set at a frequency-multiplied or frequency-divided value (fo*) of oscillation frequency fo ( fo* ≠ fo ). The operation of the V/F converter of Fig. 1 will now be described with reference to the timing chart of Figs. 2A to 2I. For the sake of simplicity, a specific case in which the potential of input signal ei is zero is considered. (At this time, potential of e10 is also zero.) When no drift appears in V/F converter unit 12 and if the level of input e10 for unit 12 is zero, then, as will be seen from the V/F conversion characteristic of Fig. 4, V/F converter unit 12 generates signals eo and e12 with fixed frequency fc = fo (Hz) (Fig. 2A). Output e14 (Fig. 2B) of reference clock oscillator 14 is frequency-divided by frequency divider 16. NAND gate 22 receives the fixed level (logic "1" or "0") of output e16 from divider 16 and output e12 from unit 12, and supplies logical NAND output e22 (Fig. 2E) to down-count terminal D of counter 24. Meanwhile, inverter 18 inverts the phase of frequency-divided output e16. NAND gate 20 receives the fixed level (logic "0" or "1") of output e18 (Fig. 2D) from inverter 18 and output e14 from reference clock oscillator 14, and supplies logical NAND output e20 (Fig. 2F) to up-count terminal U of counter 24. Counter 24 down-counts every time a clock pulse of e22 is applied to down-count terminal D while e20 is "1", as noted before. Namely, a period from time t0 to t1 in Figs. 2G to 2I is a down-count period. During a period from t1 to t2, on the other hand, output e20 of NAND gate 20 is applied to up-count terminal U of counter 24 while e22 is "1". Namely, this period is an up-count period. Thus, counter output d24 at instant t2 indicates the frequency difference (fo - fc), obtained at t2, between V/F converter output e12 and reference clock output e14. The above operation is repeated until time tn, whereby a count indicating the frequency difference (fo - fc) between the two clocks (e20, e22), noted above, is obtained as output d24 of counter 24. Fig. 2G shows output d24 of counter 24 in a case where the frequency of V/F converter output e12 matches the frequency of reference clock output e14 ( fc = fo ). Fig. 2H shows a case where the frequency of e12 is higher than that of e14 (fc > fo). Fig. 2I shows a case where the frequency of e12 is lower than that of e14 (fc < fo). D/A converter 26, which is located at the subsequent stage of counter 24, receives only several upper bits (M bits) of the output (N bits) of counter 24, and it converts the M-bit digital count value (d24) into a corresponding analog value (ej). The resultant analog output ej is fed to analog adder 10. The table of Fig. 3 shows the count of counter 24 when counter 24 is an 8-bit counter and D/A converter 26 is a 4-bit D/A converter. Since D/A converter 26 receives the upper 4 bits of 8-bit counter 24, the value of converter output ej retains zero for a slight frequency difference (fo - fc) which is contained in the range of the lower 4 bits. Thus, in a case where the frequency difference (fo - fc) is slight, since the application of a compensation signal component (ej) to V/F converter unit 12 is stopped, flicker (or jitter) of output eo due to the change of the frequency difference can be avoided. Referring to Fig. 3, when count d24 of counter 24 is at region B or C, the potential (or signal level) of analog output ej from D/A converter 26 retains zero. On the other hand, when count d24 enters region A or D, the potential of output ej changes in correspondence with each count d24 having a value form +1 to +7 or from -1 to -7. Such output ej is fed to analog adder 10. Fig. 2I exemplifies a case in which frequency fc of V/F converter output e12 is lower than frequency fc of reference clock e14. This means, as will be seen from the V/F conversion characteristic of Fig. 4, that a negative drift (negative DC level shift) appears in the preceding circuit stages and/or in V/F converter unit 12. When counter 24 is preset such that its counting starts from zero and if fc < fo is assumed in the case of Fig. 2I, the count of counter 24 deviates toward the positive side (+1 to +7). Positive analog output ej thus obtained from D/A converter 26 is added in adder 10 to input signal ei. Then, even if input signal ei contains a negative drift as noted above, since this negative drift is compensated (or cancelled) by positive output ej, the output of adder 10, or input e10 of V/F converter unit 12, can be free of the influence of the drift. Further, even if a DC level shift of e10 are caused by analog adder 10, or even if V/F converter unit 12 per se involves a drift, outputs eo and e12 from unit 12 can also be free of the drift because of the compensation function of ej. Thus, the V/F converter is free of the frequency shift due to any drift. In the above explanation regarding the drift, the potential or level (or magnitude) of input signal ei is assumed to be zero. However, similar drift compensation can be achieved even where input ei is an AC signal with a certain amplitude. This will now be described in a detail below. When input signal ei is a sine wave, for instance, output e12 of V/F converter unit 12 has a coarse-and-dense waveform with a frequency varying in accordance with the level of ei. However, the number of pulses of e12 contained in one cycle of ei becomes a fixed value irrespective of whether the amplitude of ei is zero or not. For instance, assume a case where the output frequency of V/F converter unit 12 is set at 1,900 Hz ( fc = fo ) when the amplitude of input ei is zero, and assume that the frequency of ei is 50 Hz. (In this case, one cycle period of ei is 20 msec.) Then, 1,900 × 0.02 = 38 pulses are contained in 20 msec period. When input ei is not zero but is a 50 Hz sine wave, output e12 of V/F converter unit 12 has a coarse-and-dense waveform with an unfixed frequency ( fc = fo ± 50 Hz ) centered on 1,900 Hz (fo). Also in this case, output e12 of V/F converter unit 12 contains 38 pulses in one cycle period (20 msec) of the 50 Hz sine wave. Thus, the number of pulses contained in one cycle of e12 is fixed (38 pulses) whether the amplitude ei is zero or not. This is because the integral of the waveform of sine wave input ei over one cycle period is zero. If input signal ei involves a DC drift but this DC drift is not compensated by ej, the number of pulses contained in one cycle of output e12 varies from 38 pulses and has a value corresponding to the degree or the magnitude of the drift. It is to be noted the fact that the number of pulses contained in one cycle of sine wave output e12 is independent of the amplitude (magnitude) of e12 may be applicable to a case where the number of pulses is considered for more than one one cycle of e12. Further, when a long period of time is considered in the order of seconds or more without any particular attention to the cycle of e12, there is no problem in assuming that the number of pulses contained in such a long time period is constant on an average. Incidentally, even if no drift is involved in the embodiment of Fig. 1, frequency fc varies in accordance with the level or amplitude of signal ei. However, when such a varied frequency (fc) is averaged for a period more than one cycle of ei, the averaged frequency becomes equal to the frequency (fc) that obtained at ei = 0. Thus, the number of oscillations of the averaged frequency (fc) for a fixed period of time is equal to that of frequency fc for the same fixed period of time. As has been described above, in the V/F converter of Fig. 1, count d24 corresponding to the difference between output frequency fc of V/F converter unit 12 and reference clock frequency fo is detected to provide compensation value ej in the form of an analog quantity, and compensation value ej is used to cancel a DC drift involved in the circuit of V/F converter unit 12. Therefore, it is possible to make V/F converter outputs eo and e12 free of a frequency shift due to a drift. The V/F converter thus can solve a problem of inaccurate data confirmation of data transmission which is occurred in the receiver side due to a drift involved in the transmitter side utilizing frequency modulation. Further, when a case in which signals with a given constant frequency are to be transmitted and received to detect the defect of a transmission system based on an extraordinary change in the signal frequency is considered, according to a prior art device, a drift involved in the transmitter side causes an erroneous confirmation, at the receiver side, that the transmission system, inclusive of a local carrier station, becomes defective. Such a problem can also be solved. More specifically, when the V/F converter is applied to a case in which signals with given fixed frequencies are transmitted and received, even if a drift appears at the transmitter side, such a drift is not reproduced at a demodulator section in the receiver side. Consequently, it is possible to avoid an erroneous confirmation of the defectiveness of the transmission system. Fig. 6 shows another V/F converter. In the V/F converter of Fig. 1, counter 24 is switched to perform up- and down-counting operations for every fixed period (i.e, one half cycle of e16 or e18), with a signal of logic level "1" fed to up-count terminal U and a signal of logic level "1" fed to down-count terminal D. In the V/F converter of Fig. 6, counter 24 operates in a manner different from the operation of counter 24 in Fig. 1. More specifically, the up- and down-counting operations are freely activated alternatively, with output e12 of V/F converter unit 12 fed to down-count terminal D and with output e14 of reference clock oscillator 14 fed to up-count terminal U. Figs. 7A to 7D are a timing chart explaining the operation of the circuit shown in Fig. 6. Assume that no frequency shift due to any drift exists and frequency fc of output e12 from V/F converter unit 12 matches frequency fo of reference clock e14 (i.e., fc = fo ). Then, immediately after counter 24 up-counts one clock of e12 (t10 in Figs. 7B and 7C), it performs down-counting of e14 (t11 in Figs. 7A and 7C). Thus, count output d24 of counter 24 is either "0" or "1", and compensation value ej (fc - fo) can be considered substantially zero. If frequency fc of e12 becomes higher than frequency fo of e14 (fc > fo subsequent to t21 in Figs. 7A and 7B), counter 24 up-counts more pulses than down-counted pulses. Count d24 thus gradually increases (t21 to t30 in Fig. 7C). When the lower 4 bits of d24 from counter 24 are filled and a carry-out is produced for the fifth bit (corresponding to the least significant digit LSD of D/A converter 26), potential V1 is provided as ej. This potential V1 corresponds in magnitude to d24 (or to data K0) (t30 in Figs. 7C and 7D). At this instant (t30), the relation fc > fo still holds, with the number of up-counted pulses exceeding the number of down-counted pulses. Therefore, count output d24 is further increased (corresponding to Ko + 1), and the potential of ej is increased to V2 with the increase of d24 (t40 in Figs. 7C and 7D). Correction of fc is thus done according to ej which represents the difference between fc and fo. When fc become equal to fo, count d24 of counter 24 is stabilized between Ko and Ko + 1. In a case where fc < fo holds, count d24 of counter 24 is increased toward the negative side with the passage of time, and the potential of ej is reduced in the negative direction (while the absolute value of ej is increased). As to the rest, the operation is the same as in the case of Figs. 7A to 7D, and fc is compensated or corrected to fc = fo . It is to be noted that elements 10S, 12, 24 and 26 in the V/F converter of Fig. 6 form a negative feedback loop, through which frequency fc of output eo or e12 is made to follow reference frequency fo. In the V/F converter of Fig. 1, the form of change of output d24 from counter 24 has positive and negative slopes alternatively, as shown in Figs. 2G to 2I. In contrast, according to the V/F converter of Fig. 6, the form of change of output d24 from counter 24 differs from that of Fig. 1, but it always indicates the difference between the two frequencies respectively applied to the up- and down-count terminals of counter 24. Thus, the V/F converter of Fig. 6 achieves a drift compensation, without supplying to D/A converter 26 useless information from counter 24, during the period of counting. This is an advantageous feature of the V/F converter of Fig. 6 over the V/F converter of Fig 1. In the V/F converter of Fig. 1, the up-counting is effected by e20 corresponding to e14 (fo), while the down-counting is effected by e22 corresponding to e12 (fc). Thus, ej has a DC level corresponding to fo - fc. For this reason, ej is added to ei. An example of analog adder 10 which is used in this case is shown in Fig. 5A. In Fig. 5A, the symbol A10 denotes an operational amplifier; R10-R14, resisters; VR10, an off-set balancer; and Vs, a bias potential. In the V/F converter of Fig. 6, on the other hand, the up-counting is effected by e12 (fc), while the down-counting is effected by e14 (fo), so that the DC level of ej corresponds to fc - fo. That is, ej in the case of Fig. 6 has the opposite polarity to ej in the case of Fig. 1. From this, in the V/F converter of Fig. 6, drift compensation value ej is subtracted from input ei by analog subtracter 105. It should be understood that analog adder 10 of Fig. 1 be replaced with analog subtracter 10S, if e22 and e20 are applied to up- and down-count terminals U and D of counter 24, respectively. An example of such analog subtracter 10S is shown in Fig. 5B. Incidentally, when subtracter 10S of Fig. 6 is to be replaced with adder 10, a circuit configuration as shown in Fig. 6A may be employed. Fig. 1A shows a modification of the V/F converter of Fig. 1. In this V/F converter, outputs e22 and e20 are fed to up- and down-count terminals U and D of counter 24, respectively. Output e26 of D/A converter 26 is phase-inverted through phase inverting amplifier (inverter) 100 before it is fed to analog adder 10. In a V/F converter of Fig 8, latch 25 is provided at the output side of counter 24 so that output d24 of counter 24 is latched for every cycle of frequency divider output e16 (Fig. 2C). Latched output d25 is applied to D/A converter 26. Figs. 9A to 9H form a timing chart explaining the operation of the V/F converter of Fig. 8. Counter output d24 is latched at the rising edge of output e16 (Fig. 9G) from frequency divider 16, which appears at the end (t100, t101, t102 and t103) of the up-counting of counter 24. Output d25 of latch 25 is thus fixed (d24₀, d24₁, d24₂ in Fig. 9H) during each period in which no counter output is latched. With this arrangement, D/A converter 26 can be free of output d24 from counter 24 which quickly changes during counting. Instead, latched output d25, which is fixed during counting, is fed to D/A converter 26. It is thus possible to obtain compensation output ej with smooth level change. This smoothly-changed ej provides an effect of reducing slight variations in the frequency of eo (jitter), and it is an advantageous feature of the V/F converter of Fig. 8. While the V/F converter of Fig. 8 uses for counter 24 an up-down counter capable of both up- and down-counting operations, it is also possible to use a presettable counter for only up- or down-counting operation. Fig. 10 shows a V/F converter which uses such a presettable counter. In the V/F converter of Fig. 10, the upper limit of the degree of the drift compensation is prefixed. It is assumed that the frequency corresponding to this limit is denoted by fj. Preset value Ko, which represents a count corresponding to the difference (fo - fj) between reference frequency fo and drift upper limit frequency fj, is supplied to a preset value input terminal of counter 24. Output e14 of reference clock oscillator 14 is frequency divided in pulse generator 140 to obtain first latch signal e141 whose cyclic period is fixed. This signal e141 is applied to counter 24. Preset value Ko is loaded as initial value of counting in counter 24. Presettable counter 24 is a down-counter which down-counts, with loaded initial value Ko, the pulses of output e12 from V/F converter unit 12. After passing a predetermined period of time (corresponding to one cycle of e16), output d24 from counter 24 is latched at latch 25 under the control of second latch signal e142 delivered from pulse generator 140. Latch 25 then provides output d25 which contains, as parameters, information of frequency fc of V/F converter output e12, upper limit frequency fj and reference frequency fo. Output d25 is converted by D/A converter 26 into analog value e26 ( fo - fj - fc ) which is applied to analog accumulator 27. Also applied to analog accumulator 27 is bias value K1 which corresponds to fj. Then, output ej of analog accumulator 27 has a potential corresponding to the difference (fo - fc) between reference frequency fo and output frequency fc of V/F converter unit 12. This output ej is supplied as a drift compensation value to analog adder 10, so that the DC level shift of input signal ei is compensated, thereby achieving the relation fc = fo for the output (eo, e12) of V/F converter unit 12. Figs. 11A to 11G show a timing chart explaining the operation of the V/F converter of Fig. 10. When data Ko corresponding to fo - fj is preset in counter 24 under control of first latch signal e141 (t200 in Fig. 11C), counter 24 starts to down-count the pulses of output e12 (fc) from V/F converter unit 12, with Ko as an initial value. During a period in which fc = fo holds, second latch signal e142 is generated when count output d24 of counter 24 becomes a value corresponding to -fj (t202 in Figs. 11D and 11E). Since latched output d25 in this case corresponds to -fj, output e26 of D/A converter has a potential corresponding to -fj (t202 in Fig. 11F). Output e26 (-fj) thus obtained is combined with K1 = +fj in analog accumulator 27, and drift compensation output ej becomes zero (t202 in Fig. 11G). Data Ko is preset again in counter 24 by subsequent first latch signal e141 (t204 in Fig. 11C). If fc >> fo holds at this time, count d24 of counter 24 is reduced to a value corresponding to -fj - δf1 until subsequent second latch signal e142 is generated (t206 in Fig. 11E). (δf1 represents |fc - fo| when fc >> fo.) For this reason, the potential of D/A converter output e26 is lower, by an amount corresponding to δf1, than the value obtained when fc = fo holds (t206 in Fig. 11F). Compensation output ej is, accordingly, reduced by an amount corresponding to δf1 so as to lower fc (i.e., cause fc to approach fo). Data Ko is preset once again in counter 24 by subsequent first latch signal e141 (t208 in Fig. 11C). If fc > fo holds at this time, count d24 of counter 24 is lowered to a value corresponding to -fj - δf2 until subsequent second latch signal e142 is generated (t210 in Fig. 11E). (δf2 corresponds to |fc - fo| when fc > fo.) In this case, a value corresponding to - δf2 is added in accumulator 27 to e26 at t206, and the potential of e26 is further reduced (t210 in Fig. 11F) from the potential obtained at t206. Consequently, the DC level of compensation output ej is further reduced (t210 in Fig. 11G), causing fc to further approach fo. When relation fc = fo is obtained at t210, the drift in V/F converter unit 12 is compensated by the level of ej at this instant, and V/F converter output eo is fixed at frequency fo (subsequent to t210 in Figs. 11A to 11G). Incidentally, in the respective V/F converters, counter output d24 may be in the form of 1's complement, 2's complement, an off-set binary code, etc. Data in Fig. 3 are in the form of 1's complement. If the offset binary code is used, the initial value of the counter is not "0" but is set at near the one half of the full scale count value of the counter. In this case, the code format of D/A converter 26 may be selected such that compensation value ej corresponding to the central value of the full scale count is "0". Further, while the above V/F converters have been described by assuming 8 as N of N-bit counter 24 and 4 as M of M-bit D/A converter 26, N and M are not limited to these numbers but they may have any specific numbers. In the V/F converters of Figs. 1, 6, 8 and 10, the respective circuit elements can be constructed by discrete ICs. In contrast, Fig. 12 shows another type of V/F converter, in which microcomputer 240 is used for performing similar drift compensation processing with software. More specifically, output e12 of V/F converter unit 12 is fed to port P1 of microcomputer 240 so that pulses of e12 (fc) are counted for a predetermined period of time. Also, reference frequency output e14 (fo) from clock oscillator 14 is fed to port P2 so that pulses of e12 are counted for the predetermined period of time. Then, the counts (fo, fc) for e12 and e14 are compared, and their difference data d240 (fo - fc) is applied to D/A converter 26. These processes are implemented by software. Fig. 13A shows a flow chart of an example of the routine programmed in microcomputer 240 of Fig. 12. This routine is used to perform signal processing which is similar to that performed in the V/F converter of Fig. 1. To be concrete, frequency fc of V/F converter output e12 is counted by counter (not shown) contained in microcomputer 240 (ST10). Following to this, reference frequency fo is similarly counted (ST12). (The order of ST10 and ST12 may be exchanged.) Then, the counts of fo and fc are compared (ST14), and data d240 representing the difference (fo - fc) is produced (ST16). If the absolute value of data d240 is greater than predetermined threshold value fx which is provided for performing the decision as to whether drift compensation is to be started (YES in ST18), fc is compensated, by changing the DC level of input ei, with compensation value ej corresponding to d240 (ST20). Steps ST10 to ST20 are repeated until the absolute value of d240 becomes smaller than fx. When the absolute value of d240 becomes smaller than fx (NO in ST18), microcomputer 240 decides that fc substantially matches fo. Then, microcomputer 240 ends the processing of Fig. 13A. At this time, data d240 (i.e., the potential of ej) is locked at a certain fixed value corresponding the count difference (fo - fc). The processing of Fig. 13A is repeated periodically. Fig. 13B shows a flow chart of another example of the routine programmed in microcomputer 240 of Fig. 12. This routine is used to perform signal processing which is similar to that performed in the V/F converter of Fig. 6. To be concrete, first, either V/F converter output e12 (fc) at input port P1 or reference frequency signal e14 (fo) at input port P1 is fetched into microcomputer 240 (ST30). If the fetched signal is e12 (YES in ST32), the count (N) of a counter (not shown) in microcomputer 240 is incremented by one (ST34). If the fetched signal is not e12 (NO in ST32) but e14 (YES in ST36), the count (N) of the counter is decremented by one (ST38). If neither signal e12 nor e14 is fetched (NO in ST32 and ST36), the count value of the counter is not changed. Count N is provided as data d240 which represents the difference between fc (e12) and fo (e14) (ST40). If the magnitude (or absolute value) of data d240 is greater than given threshold value Nx representing the reference level for the start of the drift compensation (YES in ST42), the DC level of input ei is renewed by ej corresponding to d240 (ST44), so that fc is brought closer to fo. Steps ST30 to ST44 are repeated until the magnitude of d240 becomes smaller than Nx. When the magnitude of d240 becomes smaller than Nx (NO in ST42), microcomputer 240 determines that fc substantially matches fo and ends the processing of the flow of Fig. 13B. At this time, potential ej is locked at a constant value corresponding to the prevailing contents of d240. The processing of the flow of Fig. 13B is repeated periodically. In the above V/F converters, output ej of D/A converter 26 is combined with input ei for the drift compensation. However, the drift compensation may be performed by changing the operating bias point of V/F converter unit 12 with ej. More specifically, the operating bias point of unit 12, which bias always acts on the operation of unit 12, may be changed in accordance with the degree of a drift being generated. Fig. 14 shows a V/F converter, in which the drift compensation is effected by varying the operating bias point. More specifically, the DC bias applied to operational amplifier A10 is varied, by selective short-circuiting of fixed resistors R101 to R105, by switch elements SW1 to SW4 (e.g., MOS FETs) in response to the content of count d24 form counter 24. In the V/F converter of Fig. 14, resistors R100 to R105 and switches SW1 to SW4 have a D/A conversion function and, accordingly, it is possible to omit D/A converter 26 used in other embodiments. Incidentally, output eo of V/F converter unit 12 may be a triangular wave, a sawtooth wave, a sine wave, a rectangular pulse wave or any other wave, that is, the waveform is limited by no means. Further, a configuration in which output ej of D/A converter 26 is monitored by a comparator and, if the level of ej exceed a given comparison level, alarming is generated, may be adapted to a configuration in which the DC level of input ei is compensated by feeding output ej of D/A converter 26 to analog adder 10. Or, a configuration in which counter output d24 is checked through a comparator and alarming is generated when the value of d24 reaches a given comparison level, may be further employed. Fig. 15 shows an embodiment of the invention This embodiment is a modification of the V/F converter of Fig. 14 and also preferable to reduce to practice, as in the case of the V/F converter of Fig. 6. Referring to Fig. 15, latches 23A and 23B are connected to up- and down-count terminals U and D of counter 24, respectively. Latch 23A latches the signal level of V/F converter output e12 (fc), while latch 23B latches the signal level of reference frequency signal e160 (fo). Latch pulse signals e140 and e150 for respective latches 23A and 23B have the same frequency (fo*) which is far higher than fc or fo in Fig. 6, but signals e140 and e150 are in an opposite phase relation to each other. For this reason, the latch timing of latch 23A is deviated from the latch timing of latch 23B by one half the cycle of pulse signal e140. This latch timing deviation has an effect of preventing count pulses e23A and e23B from being simultaneously fed to terminals U and D of counter 24. In other words, in the embodiment of Fig. 15, counter 24 will never perform erroneous counting operation due to simultaneous appearance of count pulses e23A and e23B. Figs. 16A to 16H show a timing chart explaining the operation of the circuit shown in Fig. 15. When there is no frequency deviation or frequency shift due to a drift and output e12 (fc) of V/F converter unit 12 has the same frequency ( fc = fo ) as reference clock e160 (fo), immediately after counter 24 has up-counted one clock due to e23A (fc) latched under control of e140 (Fig. 16B), it turns to perform down-counting due to e23A (fc) latched under control of e150 (Fig. 16A). Thus, count output d24 of counter 24 is either "0" or "1", so that the level of ej (fc - fo) is substantially zero. When fc > fo holds (t300 to t304 in Figs. 16C and 16D), counter 24 up-counts more pulses than down-counted pulses, so that count d24 is gradually increased. (t300 to t302 in Fig. 16G). When the lower 4 bits of output d24 of counter 24 is filled, for instance, a carry-out for the fifth bit is generated and voltage V1 is provided as ej in response to the value of d24 (t302 in Fig. 16H). Since there still holds relation fc > fo at this instant (t302), that is, since the number of up-counted pulses surpasses the number of down-counted pulses, count d24 is further increased to increase the potential of ej to V2 (t304 in Figs. 16G and 16H). As a result, fc is corrected by ej corresponding to the difference between fc and fo, and count d24 is stabilized at a value corresponding to fc = fo . As has been described in the foregoing, a frequency drift generated in a circuit, inclusive of the V/F converter unit, can be compensated. Thus, it is possible to substantially eliminate any drift involved in the output from the V/F converter. Further, while the count of counter 24 corresponding to a drift is fed as an analog compensation value through D/A converter 26, since only upper several bits of counter 24 are fed to D/A converter 26, only a certain drift, which will bring about frequency deviations in excess of a predetermined extent, is compensated slowly (in the order of seconds, minutes or more). Thus, the V/F converter output (eo) can be compensated without generation of jitter which may otherwise accompany compensation for a slight drift. While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is understood that the invention is not to be limited to the disclosed embodiment.

Claims: A voltage/frequency converter comprising: voltage/frequency converter means (12) for generating a voltage/frequency conversion output (eo, e12) having a frequency (fc) corresponding to the potential of a voltage/frequency converter input (e10, e10S); signal generator means (140 to 160) for generating a reference signal (e160) having a predetermined reference frequency (fo), generating a first latch pulse (e140) having a frequency (fo*) higher than said reference frequency (fo), and generating a second latch pulse (e150) synchronized to but having a different generation timing from said first latch pulse (e140); first latch means (23A) for latching the signal level of said voltage/frequency conversion output (e12) by said first latch pulse (e140) to provide a first count signal (e23A); second latch means (23B) for latching the signal level of said reference signal (e160) by said second latch pulse (e150) to provide a second count signal (e23B); counter means (24) for providing a digital count output (d24) which corresponds to the difference between the number of oscillations of said first count signal (e23A), obtained within a predetermined period of time, and the number of oscillations of said second count signal (e23B), obtained within said predetermined period of time, said digital count output (d24) being incremented by either one of said first and second count signals (e23A, e23B) and being decremented by the other; digital/analog converter means (26) for converting said digital count output (d24) into an analog compensation signal (ej) having a potential corresponding to the content of said digital count output (d24); and signal mixer means (10, 10S) for mixing said analog compensation signal (ej) with said conveter input signal (ei) to produce said voltage/frequency conversion input (e10, e10S), characterized in that said counter means (24), digital/analog converter means (26), signal mixer means (10, 10S), first and second latch means (23A, 23B) and voltage/frequency converter means (12) constitute a negative feedback loop to cause the value of the frequency (fc) of said voltage/frequency conversion output (eo) to follow said reference frequency (fo).

Inventors: FUJISAWA, NORIO C/O PAT.DIV.KABUSHIKI KAISHA TOSH, YAMAURA, MITSURU C/O PAT.DIV.KABUSHIKI KAISHA TOSH
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1986
Main classification: H03C3
IPC codes: H03C3, H04L27, H03K7