AReS

PART16Signal Converter

Experiment 4 :V-F Converter

Theory

fig16-13

Voltage-Frequency converter(V-F converter) is composed of integration circuit. Fig.16-12 is the circuit of V-F converter, which amplifies the input voltage V_i inversely by IC1 and executes the integration by C1. This integrator executes the feedbackof the reversed current to the input so the output voltage V_O can be expressed as below.

V_o=-1/C1 ∫▒〖i_f dt〗=-1/C1 ∫▒V_i/R_i dt=-1/(R_i C1) ∫▒〖v_i dt〗

(Here, i_f is the feedback current.)
The transient response of this circuit is as below when expressed using Laplace Transform.

V_o (S)=-1/sT V_i∙1/s=-1/s^2 ∙1/T V_i

(Here, T is R1C1, and s is the differential operator(=d/dt) Therefore, if V₀(t)=L^-1[V₀(s)] is applied, it is as below.

V_o (t)=-t/T V_i

This formula means that the output voltage of the time when t=0 is V₀=0, and that of the time after t=T becomes V₀=V_i. Here, R2 is the resistance to correct the bias resistance, and it becomes R2 ≅ R1. Therefore, if the time constant of R1C1 is determined, the output waveform is expressed as integrated saw tooth wave as V01 of fig.16-14. Also, IC2 compares the output voltage of integrator and standard voltage V_ref and when V01 is lower than V_ref, the output voltage of comparator becomes (+) and the transistor TR1 is turned on, and the charge of C1 is discharged to TR1. On the contrary, when VO1 becomes V and passes through the comparing area of IC2, the output voltage is saturation voltage of VO2 (-) and TR1 is turned off.

The integration starts in the direction that V_O1 as the time constant of R1C1 becomes 0V. In this case, IC1 is the time constant of R1C1, and when normal integrator is operated TR1 has very small saturation voltage V_r so + voltage of IC2 becomes 0V potential which is same as the voltage of IC1.

Therefore, in the potential where V_O1 is over 0V, IC2 is reversed and TR₁ is turned off. The problems of this circuit are that the leakage current of collector should always be integrated when TR₁ is turned off, that the leakage current should be small, and that the welding resistance(R_r) of TR₁ should be smaller than R1. To determine the output frequency of this circuit, if the period of time at the moment of integration is T₁ and the time needed for reset is T₂, then T₁, T₂ and the oscillation frequency are as below.

V_o1=-1/R1C1 ∫_0^T1▒〖V_i dt〗

∴T_1=(V_ref/V_i )R1C1

V_o1=-1/(R4+R_r )C1 ∫_0^T2▒〖V_ref dt〗=V_ref

∴T_2=(V_ref/V_ref )(R4+R_r )C1=(R4+R_r )C1

(Here, R1 is the resistance when TR₁ is welding.)

f_0=1/(T_1+T_2 )

However, actual T₂ must have very small time constant comparing to T₁.
Therefore, the circuit constant should satisfy the two conditions below.

R4>R_r,(R4+R_r )C1≪R1C1

In the end, the output oscillation frequency to the input voltage of this circuit is influenced greatly by the time constant R1C1, so to setup the standard or oscillation frequency, R1 or C1 should be determined or the standard voltage V_ref should be adjusted. However, if V_ref is set up to high, the intersecting point with the maximum value of integrated output voltage does not exist and the output does not occur so the voltage should be set up as about half of the integrated output.
IC for V-F converter circuit is TC9400 and fig.16-15 shows its block diagram.

fig16-15

Fig. 16-15TC9400 Block Diagram

Experiment Process

1. For the experiment, use Circuit-4 of M-16.

2. Connect between 4a-4b terminals and adjust R₄ so that the output frequency becomes 0Hz.

3. Measure the output frequency when voltage value of of input signal is increased by 1V to 4a-4b as in table 16-4 and record the result in the relevant column and test the linearity of input voltage and output frequency.

tab1

Experiment 16-4.1 V/F Converter Converter Circuit (Compose as Circuit-4 of M16.)

1.Connection

1.Circuit Connection

In Circuit-4 of M16, connect between 4d-4g, between 4f-4g terminals with yellow lines.

2.Power connection is internally connected.

3.Measuring Instrument Connection

Connect BNC cable to Signal Output on the front panel, connect red lead wire to 4a of Circuit-4 and black lead wire to 4b terminal.

Voltmeter Connection(Zero ADJ Measurement)

On the Multimeter of front panel, connect between High terminal and 4h terminal of Circuit-4 with red line, and between Low terminal and 4j terminal with black line.

Voltmeter Connection(F-OUT)

Connect between 4i terminal of Circuit-4 and A+ terminal of Signal Output on the front panel with red line, and between 4j terminal and A- terminal with black line.

2.Wiring Diagram

flash

3.Measurement

1Adjust ZERO ADJ.
Choose dmm at Touch LCD, click dcv and connect between 4a-4b terminals of Circuit-4 with yellow line and adjust R5 so that the output voltage becomes 0V.
2Measure the output frequency when voltage value of of input signal is increased by 1V to 4a-4b as in table 16-4 and record the result in the relevant column and test the linearity of input voltage and output frequency.
Connect BNC terminal to Signal Output of front panel, and connect red lead wire to 4a terminal and black lead wire to 4b terminal.

Choose analog output at Touch LCD and choose Oscilloscope tab.

Click quick launch at the bottom left, choose Analog Output and click arrow right of function to make DC Source window appear.

Click arrow left arrow rightat 1v of Range to adjust as the input voltage of table 16-4 and output the frequency by the output waveform and record the result in the relevant column.

4.Calculation

1. Calculate the output frequency following the formula below and record the result in the relevant column of table 16-4.

In Circuit-4, R_IN=R1, C_REF=C1, V_REF=-5V.

F_OUT=V_IN / R_IN ×1 / ((C_REF )∙(V_REF))

Experiment Result Report

result
Title	V-F Converter	Date
Department		Id Number		Name

1. Experiment Result Table

Table 16-4

result_table
Input Voltage [㎐]		1V	2V	3V	4V	5V	6V	7V	8V
Output Frequency [mV]	Measurement
Output Frequency [mV]	Calculation