BACKGROUND ART At present, various systems are used for converting frequency of an electric signal to voltage.

The known system, described by National Semiconductor (LM2907/LM2917 Frequency to Voltage Converter, Circuit Description, February 1995), includes an input comparator, a circuit for linear charging and discharging of a timing capacitor, a timing capacitor, a current mirror, an outlet resistor and a filtering capacitor. This system is functioning in such a way that after having changed the input voltage through the reference voltage of the comparator, the charging or discharging of the timing capacitor takes place.

Said charging or discharging is linear, and voltage across the capacitor passes from one voltage level to the other, where the difference of said voltage levels is equal to one half of the supply voltage VCC. Within a half-period of the input signal, the change of the charge of the timing capacitor C is as follows: DQ = (VCC/2) x C (where D = delta) The average current value ilAVGI, by which the timing capacitor is charged or discharged, is as follows: ilAVG/=VCCxfinxC where fin represents frequency of the input signal.

Said average current is led through the current mirror to a resistor load R which is connected to the grounding potential. Current pulses are then filtered by the filtering capacitor CF. For output voltage Vo, there is approximately valid the following equation: Vo = VCC xfin x C x R Disadvantages of said system reside in the dependence of the output voltage upon changes of the supply voltage, dependence upon changes of the capacity of the capacitor by the effect of thermal and time influences, and in its low selectivity.

As to another system described by Motorola (Communications Device Data, DL 136/D, 1995, Application Notes AN 535, p. No. 4-8), its electric periodical input signal is connected to a phase detector, after which a frequency low-pass filter is connected, and after that, a voltage-controlled oscillator follows. The output of the oscillator is connected backwards through a frequency divider to a phase detector. The conversion of the electric signal frequency to voltage is realized in such a way that the phase of the input signal is compared with the signal phase of the oscillator divided by the frequency divider with the detector. The resulting error signal passes through the frequency low-pass filter and tunes the oscillator automatically until the phase and frequency of the divided oscillator frequency are identical with the input signal. The control voltage across the oscillator is directly proportional to the frequency of the input signal. The disadvantage of such a solution is that it requires application both of a voltage-controlled oscillator having linear properties and a large-range tuning ability and of a phase detector. Both of these are relatively costly.

As to another system described by National Semiconductor (LM131A/LM131, LM231A/LM231, LM331 NLM331 Precision Voltage-to- Frequency Converters, Circuit Description, December 1994), its periodical input signal is connected to a derivative element. After it, there is connected a constant pulse duration generator. Said generator is connected to a switch which, in the ON position, switches the constant current source to the capacitor and to a parallelly connected resistor, to which the output of the apparatus is then connected. The conversion of the electric signal frequency to voltage is realized in such a way that the input signal is first formed by means of the derivative element; then said signal starts the pulse generator. The pulse switches the switch which charges, within the period of the switched-on state, the capacitor from the constant current source. After the end of the pulse, the switch is switched off and the capacitor is discharged into a connected resistor.

Disadvantages of this solution are in its low selectivity, its limited frequency range, its need for application of precise heat stable components and in its need to design the constant current source in such a way that it may be independent of the supply voltage.

In US patent No. 4.222.095, an apparatus is described wherein the input electric signal is connected to the pulse generator, after which there is a switch being connected to a primary current source which is connected between a supply voltage source and a capacitor. One pole of the capacitor is connected between the switch and the primary current source. As the pulse generator, there is applied a circuit for generating a pulse with a variable duration. That is why the pulse duration is variable and proportional to the input frequency. Said circuit is not able to secure a frequency-to-voltage conversion; it serves only for determining the reference voltage which is variable. For the frequency-to- voltage conversion, it is necessary to connect an additional circuit which is described in said US patent in the form of a rather complicated circuit that includes a D-type flip-flop and a circuit for modifying the time constant. The disadvantage of said method resides in its considerable costs, in its limited frequency range and in its low selectivity.

DISCLOSURE OF INVENTION The above-mentioned drawbacks may be obviated by the solution according to the invention. There is designed an apparatus for converting frequency of an electric signal to voltage, where the input periodical electric signal is connected to a constant pulse duration generator after which there is connected a switch, being connected to a primary current source connected to a voltage source, i.e., to a supply voltage source. As to said connection, after the switch there is connected one pole of the main constant current load and a capacitor.

The main constant current load is a term generally meaning a circuit through which a constant current passes which is independent of the value of the source voltage to which the main load is connected; in the case of the invention it is to the capacitor. The value of the passing current is controlled, as to the main constant current load, by the control voltage, which means, as to the said arrangement of the circuit according to the invention, that it may be controlled by the supply voltage. From the above-mentioned facts it is evident that as the main constant current load there may be applied, e.g., a transconductance operational amplifier or a circuit consisting of an operational amplifier, of a field-effect transistor and of a resistor. So it was explained that as the main constant current load there cannot be applied, e.g., a resistor, a resistance of a channel of a field-effect transistor, an integrating element or another impedance.

One pole of the capacitor is connected after the switch; the other pole may be grounded or connected to another potential. The constant pulse duration generator may be realized as an integrated circuit or made of mutually connected elements. By realizing and connecting the circuit according to the projected solution, an apparatus is formed for converting the frequency of the input electric signal to the output voltage, where the conversion is very selective and where the influence of changes of the supply voltage is suppressed. Thermal influences are simultaneously also suppressed. At the mentioned connection of the switch and of the capacitor, the output voltage at zero frequency of the output signal has zero value. The output voltage is increasing if the input frequency is increasing. The capacitor need not be of a very high quality, which decreases costs. The apparatus does not require expensive materials.

The main constant current load is advantageously connected to the supply voltage source, so that it is controlled by the supply voltage. Also, the primary current source is connected in such a way that it is controlled by the supply voltage.

If between the capacitor and the outlet of the apparatus, into the above- mentioned circuit, additionally a frequency low-pass filter is connected, the voltage ripple across the output of the apparatus, caused by charging and discharging the capacitor, is suppressed. The apparatus may be functioning even without the frequency low-pass filter, eventually without a connection to the control of the above-mentioned elements by means of the supply voltage, but the achieved function result of the apparatus provided with the wiring, according to this and the preceding paragraph, is considerably better.

The wiring according to the preceding paragraph has two alternatives. In the circuit of the apparatus there may be included an auxiliary constant current load, connected to the outlet of the apparatus, so that it is controlled by the output voltage. Alternatively to the circuit of the apparatus there is connected the secondary constant current source which is connected to a voltage source, i.e., supply voltage source. The secondary constant current source is also connected to the apparatus output, so that it is controlled by output voltage. In this way, the slope of the electric signal frequency to voltage conversion is affected.

Both alternatives mentioned in the preceding paragraph have two possibilities of a particular wiring which must be selected from with respect to the required kind of conversion slope control.

The auxiliary constant current load is a term which generally means a circuit through which constant current passes, being independent of the value of the voltage source, to which the auxiliary load is connected. In the case of this invention it is to the capacitor. In case that one pole of the auxiliary constant current load is connected to the capacitor, the said connection is direct; in case that the said pole is connected between the switch and the primary current source, it concerns the connection to the capacitor through the switch. The value of the passing current is controlled, as to the auxiliary constant current load, by control voltage, which is represented, in the case of this invention, for all alternatives, by the output voltage. From the above- mentioned facts, it is evident, that as an auxiliary constant current load there may be applied, e.g., a transconductance operational amplifier, or a circuit consisting of an operational amplifier, of a field-effect transistor and of a resistor. In this way it is explained that as an auxiliary constant current load, there cannot be applied, e.g., a resistor, a resistance of a channel of a field effect transistor, an integrating element or another impedance.

The auxiliary load, if it concerns a connection when it is controlled by output voltage, may be alternatively connected either to the capacitor, or it may be connected before the switch to the primary current source.

The secondary constant current source, if it concerns a connection when it is controlled by output voltage, may be connected in such a way that it is connected between the capacitor and supply voltage source; or, alternatively, one of its poles may be connected to the supply voltage source and the other pole connected between the switch and the primary current source.

The proposed apparatus may be utilized anywhere in the sphere of electrical engineering for converting the electric signal frequency to voltage. It enables a selective and linear conversion of a frequency input electric signal to an output voltage and a simultaneous suppression of the influence of changes of supply voltage and/or temperature. The apparatus does not require expensive materials. It may be easily overtuned and/or applied in a wide frequency range. This makes it possible to change the slope of the frequency- to-voltage conversion.

BRIEF DESCRIPTION OF DRAWINGS The invention is explained in more detail by means of drawings where Fig. 1 to Fig. 4 show alternative embodiments of the apparatus according to the invention.

MODES FOR CARRYING OUT THE INVENTION Example I The apparatus for converting frequency of an electric signal to voltage, the circuit of which is shown in Fig. 1, is, according to our opinion, an example of the optimum embodiment according to the invention.

An input electric signal is connected to a constant pulse duration generator 1. After the constant pulse duration generator 1 there is connected a switch 2 to which a primary current source 3 is connected. Said primary current source 3 is connected to a supply voltage source 4, so that it is controlled by means of the supply voltage. After the switch 2 there is connected. a capacitor 5 and the main constant current load 6 which is connected to the supply voltage source 4, so that it is controlled by the supply voltage. Between the switch 2 and the main constant current load 6 there is connected to the capacitor 5 the auxiliary constant current load 7, connected to the output of the apparatus, so that it is controlled by the output voltage of the frequency low-pass filter 8, i.e.

by the voltage across the output of the apparatus. The frequency low-pass filter 8 is connected between the capacitor 5 and the main constant current load 6.

The apparatus works as follows: In the constant pulse duration generator 1, the input periodical electrical signal starts a pulse of a constant duration, viz., in every period and, eventually, in every half-period. Said pulse switches ON the switch 2 within its duration, and the capacitor 5 is charged by the electric charge. The electric charge is being continuously discharged from the capacitor 5. If the quantity of the electric charge which flowed into the capacitor 5 within the pulse duration is larger than the quantity of the electric charge which was discharged within the duration of the period, eventually half-period, of input signal into the main constant current load 6, the electric charge across the capacitor 5 starts increasing, and in this way voltage across the capacitor 5 is increased. The frequency low-pass filter 8 suppresses the voltage ripple caused by charging and discharging the capacitor 5. The voltage across the output of the frequency low-pass filter 8, i.e., voltage across the output of the apparatus controls the auxiliary constant current load 7 in such a way that with the increasing voltage the current passing through the auxiliary constant current source load 7 is increasing. It results in a rise of a negative feed-back which causes a decrease of the slope of the input electric signal frequency to voltage conversion.

When increasing the supply voltage from the supply voltage source 4, the current value supplied through the primary current source 3 is increased; simultaneously increased is the current quantity passing through the main constant current load 6. Both effects are eliminated in this way, and the influence of the increased supply voltage from the supply voltage source 4 to the function of the circuit is suppressed. A thermal influence which increases the current value supplied by the primary current source 3 increases, at the same time, the current quantity passing through the main constant current load 6. Both effects are eliminated in this way and the thermal influence as to the function of the circuit is suppressed. The decrease of the supply voltage from the supply voltage source 4 decreases the current value supplied by the primary current source 3, and at the same time, there is also decreased the current value passing through the main constant current load 6. Both effects are eliminated in this way and the influence of the decrease of supply voltage from the supply voltage source 4, as to the function of the circuit, is suppressed. The thermal influence that decreases the current value supplied by the primary current source 3 decreases at the same time the current value passing through the main constant current load 6. Both effects are eliminated in this way and the thermal influence, as to the circuit function, is suppressed.

At an identical connection, voltage across the output of the frequency low-pass filter 8, i.e., voltage across the output of the apparatus may affect, on the contrary, the auxiliary constant current load 7 in such a way that, with the increasing voltage, the current passing through the auxiliary constant current load 7 is decreased. It results in a rise of a positive feed-back which introduces the hysteresis and increases the slope of the input electric signal frequency-to- voltage conversion.

The apparatus could function even without the control by means of the supply voltage source 4 of the primary current source 3 and of the main constant current load 6, but the suppression influence of changes of the supply voltage would not be achieved. The apparatus could also function without the frequency low-pass filter 8 and the auxiliary constant current load 7, but a ripple suppression of voltage across the output of the apparatus, as well as the required slope of the frequency to voltage conversion could not be ensured.

The alternative mentioned in this exemplary embodiment according to the invention may be considered as the most advantageous because of the fact that, with respect to the circuit embodiment, it is easier to realize the auxiliary constant current load 7, one pole of which being connected to the grounding potential, than to realize the secondary constant current source 9, connected to the supply voltage source 4.

Example 2 An apparatus, the circuit of which is shown in Fig. 2, is another embodiment according to the invention.

The apparatus is similar to the preceding one, but it differs in the fact that the auxiliary constant current load 7 is connected between switch 2 and the primary current source 3, and also to the apparatus output, i.e., to the output of the frequency low-pass filter 8, so that it is controlled by the output voltage.

The apparatus is functioning analogously as the apparatus according to example 1.

Example 3 An apparatus, the circuit of which is shown in Fig. 3, is another embodiment according to the invention.

The apparatus is similar to that in the first example, but it differs in that in this circuit there is not included the auxiliary constant current load 7; the secondary constant current source 9 is connected there. Said secondary constant current source 9 is connected between the supply voltage source 4 and the capacitor 5, and simultaneously, it is connected to the output of the frequency low-pass filter 8, so that it is controlled by the output voltage of the apparatus, i.e., by the output voltage across the frequency low-pass filter 8.

The apparatus works as follows: In the constant pulse duration generator I the input periodical electric signal starts a pulse of a constant duration, viz., in every period and, eventually, in every half-period. Said pulse switches ON, within its duration, the switch 2, and the capacitor 5 is charged in the same moment with an electric charge. The electric charge is being continuously discharged from the capacitor 5. If the quantity of the electric charge which flowed into the capacitor 5 from the primary current source 3 within the constant pulse duration is larger than the quantity of the electric charge which was discharged from it during the period, eventually half-period, of the input signal into the main constant current load 6, the quantity of the electric charge across the capacitor 5 starts increasing. In this way, the voltage across the capacitor 5 is also increased. The frequency low-pass filter 8 suppresses the voltage ripple across the output of the apparatus caused by charging and discharging the capacitor 5. Voltage across the output of the apparatus, i.e., across the output of the frequency low-pass filter 8, also controls the secondary constant current source 9 in such a way that, at the increasing voltage, current flowing out of the secondary constant current source 9 also increases. It results in a rise of a positive feed-back which introduces the hysteresis and causes the slope increase of the frequency input signal to voltage conversion. The influence of the increase or decrease of the supply voltage, as well as thermal influences, are suppressed the same way as in the preceding example.

At an identical connection, voltage across the output of the frequency low-pass filter 8, may, on the contrary, affect the secondary constant current source 9 in such a way that with the increasing voltage, the current flowing out of the secondary constant current source 9 is decreased. This results in a rise of a negative feed-back which causes the decrease of the slope of the frequency input signal to voltage conversion.

The apparatus could function even without control by means of a supply voltage source 4 of the primary current source 3 and of the main constant current load 6, but a suppress ion of the influence of changes of the supply voltage would not be achieved. The apparatus could also function without the frequency low-pass filter 8 and the secondary constant current source 9, but a ripple suppression of voltage across the output of the apparatus, as well as the required slope of the frequency to voltage conversion, could not be ensured.

Example 4 An apparatus, the circuit of which is shown in Fig. 4, is another embodiment according to the invention.

The apparatus is similar to the apparatus in example 3, but it differs in that one pole of the secondary constant current source 9 is connected to the supply voltage source 4 and the other pole is connected between the primary current source 3 and the switch 2, and that simultaneously, said constant current source 9 is connected to the output of the apparatus, so that it is controlled by the output voltage across the apparatus, i.e., by the output voltage across the frequency low-pass filter 8.

The apparatus is functioning in an analogous way as the apparatus according to the preceding example.