Period-to-voltage converter.

Baluta, Gheorghe ; Coteata, Margareta

1. INTRODUCTION

The voltage-to-frequency and the frequency-to-voltage converters belong to a larger class of converters, which realize the conversion through the intermediary of time. This class also includes the converters voltage-duration, voltage-duty cycle, voltage-phase difference, as well as the ones, which realize their inverse functions (Hendriks, 1994).

There are known voltage-to-frequency or frequency-to-voltage converters realized by integrated circuits means, but these ones are sold at high prices and their procurement is somewhat difficult (McNamara, 2004), (Stitt, 1992).

Taking into consideration the facts presented above, in this paper the authors present a period-to-voltage converter for recording the frequency functions supplied by directly digital transducers.

2. CONVERTER'S DESCRIPTION

The period-to-voltage converters (TVC) are devices, which permit the transformation of a period applied to the converter's input into a voltage obtained at its output.

The block diagram which emphasizes the converter's operation principle is presented in Fig. 1. There has been made the following notations:

CF-pulse shaper circuit;

[M.sub.1] and [M.sub.2]-monostable circuits;

CD-discharging circuit;

GCC-constant current generator;

S/H-sample and hold circuit.

The input periodic signal is applied to the pulse shaper circuit CF, at its output being obtained rectangular pulses whose fronts point out the [u.sub.i](t) voltage zero-crossings (Fig. 2).

The [M.sub.1] monostable circuit provides at its output very short-time pulses, which indicates the input [u.sub.i](t) voltage zero-crossings through positive values. These pulses are utilized, on one hand, for discharging the [C.sub.1] capacitor of a variable linear voltage generator which becomes thus sawtooth pulses generator with the same frequency as the input voltage, and on the other hand for commanding a S/H sample and hold circuit.

The sawtooth pulses generator is made up from a constant current I generator GCC which charges the [C.sub.1] capacitor.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

The test pulses from the [M.sub.1] monostable circuit output are also applied to the [M.sub.2] monostable circuit input, at its output being obtained very short-time pulses which, through the intermediary of the CD initializing circuit, put to zero the capacitor [C.sub.1] thus fixing the null initial conditions for a new conversion period.

If the input voltage is given by a relation of the shape:

[u.sub.i](t) = [U.sub.m] x sin[omega]t, (1)

then the linear variable voltage from the emitter-repeater output, realized using the operational amplifier [AO.sub.1], follows the relation:

[u.sub.AO1](t) = I/[C.sub.1] x t (2)

If the charging constant current I[[mu]A] is chosen equal to the capacity [C.sub.1][nF], then for t=T the instantaneous value, in volts, of this voltage, becomes equal to the input signal period T[ms]. Thus, by adding a sample and hold circuit, it is obtained a period-to-voltage converter:

[u.sub.e](t)[V] = T[ms] (3)

If the voltage representing the period T in milliseconds, is applied to the divider circuit, at whose output is obtained a voltage, proportional to the frequency in kHz, of the input voltage:

[u.sub.e](t)[V] = 1/T[ms] = f[kHz] (4)

3. EXPERIMENTAL RESULTS

The experimental researches have been made in the Electrical Drives and Power Electronics Laboratory of the Electrical Engineering Faculty, Technical University "Gheorghe Asachi" of Iasi. The electrical schematic of the period-to-voltage converter is shown in Fig. 3.

[FIGURE 3 OMITTED]

The shaper circuit is realized with the voltage comparator I1 of type LM 339 whereas the monostable circuits are made in the circuits [I.sub.2] and [I.sub.3] of 74LS121 type. The emitter-repeater circuits are realized with the precision operational amplifier LM 108.

The sample and hold circuits are fabricated in a large range of integrated circuits hybrid or monolithically. Usually, the hold capacitor is connected in the exterior. The I5 utilized circuit, of LF 398 type, has a low price and presents the following characteristics:

--less than 10[mu]s acquisition time;

--TTL, CMOS compatible logic input;

--low input offset;

--low output noise;

--0.002% gain accuracy;

--wide bandwidth.

This converter has been integrated in a high-performance trial stand dedicated to the stepper motors driving systems control, using an IBM PC486 computer (Baluta, 2003).

The interface was achieved on a so-named "prototyping board" (with a connector that equips the computer mother board) and contains the necessary elements for both the openloop control and the closed-loop one. The voltage conversion of the pulse period offered by a numerical speed transducer made it possible the analogue measuring of the stepper motor instantaneous angular speed. As experimental results there are presented the instantaneous angular velocity oscillograms corresponding to:

--a linear profile (Fig. 4);

--a exponential profile (Fig. 5).

The real exponential profile recorded at the low-pass filter output is shown in Fig. 6.

[FIGURE 4 OMITTED]

4. CONCLUSIONS

The period-to-voltage converter presented in this paper may be used both in the measuring electronic devices field and in the automation one.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

It has the following advantages:

--it may be utilized for the frequency-to-voltage conversion as well;

--the period-to-voltage converter answer time is of maximum 2 periods whereas to the one of the frequency-to-voltage converter must be added the divider circuit's settling time;

--the period-to-voltage converter operates with high precision in the range of 100 Hz / 10kHz.

5. REFERENCES

Baluta, Gh. (2003). Electrical Drives with Stepper Motors (in Romanian), Gh. Asachi Publishing House, ISBN 973-621034-0, Iasi, Romania

Hendriks, P. (1994). F/V Converter Has High Accuracy, Available from: http://www.edn.com/archives/1994/ 051294/10di11.htm

McNamara, N. (2004). DDS IC Plus Frequency-to-Voltage Converter Make Low-Cost DAC, Available from: http://www.edn.com/ contents/images/20504di.pdf

Stitt, R.M. & Rod, B. (1992). Frequency-To-Voltage Conversion (AB-039.pdf), Burr-Brown Application