Function generator design required program selection
Function generator design required program selection
The function generator is a multi-waveform source. It can produce sine waves, square waves, triangle waves, sawtooth waves, and even arbitrary waveforms. Some function generators also have a modulation function that can perform amplitude modulation, frequency modulation, phase modulation, pulse width modulation, and VCO control.
How to design
Designing a function generator makes it possible to generate square waves, triangular waves, and sine waves.
Frequency range 10Hz~100Hz, 100Hz~1000Hz, 1kHz~10kHz
Frequency control mode by changing the RC time constant manual control signal frequency
Voltage control frequency VCF achieved by changing control voltage Uc
Output voltage sine wave Upp≈3 V amplitude continuously adjustable;
Triangular wave Upp≈5 V amplitude continuously adjustable;
Square wave Upp≈ 14 V amplitude is continuously adjustable.
Waveform characteristic square wave rise time is less than 2s;
Triangle wave nonlinear distortion is less than 1%;
Sine wave harmonic distortion is less than 3%.
Design requirements
(1) According to the requirements of technical indicators and optional conditions for laboratory conditions, the principle circuit diagram is designed, the working principle is analyzed, and the parameters of the components are calculated.
(2) List all elements and device lists for laboratory spare parts.
(3) Install and debug the designed circuit to meet the design requirements.
(4) Record the experimental results.
1, the composition of the function generator
A function generator generally refers to a circuit or instrument that can automatically generate a voltage waveform of a sine wave, a square wave, or a triangle wave. The circuit form can be made up of op amps and discrete components; monolithic integrated function generators can also be used. Depending on the application, there are function generators that produce three or more waveforms. This topic describes methods for square wave, triangle wave, and sine wave function generators.
The function generator is a multi-waveform source. It can produce sine waves, square waves, triangle waves, sawtooth waves, and even arbitrary waveforms. Some function generators also have a modulation function that can perform amplitude modulation, frequency modulation, phase modulation, pulse width modulation, and VCO control.
Scheme selection
(I) Scenario 1: Triangle wave transform to sine wave
The op amp single and discrete components, square wave - triangular wave - sine wave function generator circuit shown in Figure 1, due to technical difficulties in the triangular wave to sine wave conversion, so the following will be detailed introduction of the triangular wave to the sine wave The transformation.
1. Using Difference Amplifying Circuit to Realize the Transformation of Triangle Wave-Sine Wave
The principle of waveform transformation is to use the nonlinearity of the transmission characteristic curve of the differential amplifier. The waveform transformation process is shown in FIG. 2 . It can be seen from the figure that the more symmetrical the transmission characteristic curve is, the narrower the linear region is, the better; the amplitude Uim of the triangular wave should be exactly close to the saturation region or the ending region. Solution one: Use a differential amplifier circuit to implement a function generator consisting of a triangular wave to a sine wave and an integrated op amp circuit
2. Using Diode Polyline Approximate Circuit to Implement Triangle Wave-Sine Wave Transform
Diode fold line approximate circuit 3
According to the diode fold line approximation circuit to achieve the triangle wave - sine wave conversion of the principle diagram, available input and output characteristics as shown in Figure 3.
When the frequency adjustment part is designed, three capacitance values ​​can be given in three frequency bands: 1000pF, 0.01Μf, 0.1μF and then calculate the size of R. Hand-controlled and voltage-controlled part of the line requires easy replacement. In order to meet the requirements of the front-to-rear edge time of the opposite wave and the maximum operating frequency (10kHz) of the sine wave, an op amp with a large Sr value (such as LF353) should be used in the integrator, comparator, sine wave converter and output stage. . In order to ensure that the sine wave has a small degree of distortion, the resistance parameters of the diode network should be correctly calculated and the amplitude and symmetry of the output triangle wave should be adjusted. The input waveform cannot contain DC components.
(II) Option 2: Implement a function generator by using a circuit composed of a diode fold line approximation circuit and an integrated operational amplifier
The figure is a precision voltage-controlled oscillator consisting of μA741 and 5G8038. When pin 8 is connected to a continuously adjustable DC voltage, the output frequency is continuously adjustable. When this voltage is the minimum value (approximately 0). The output frequency is the lowest. When the voltage is the maximum, the output frequency is the highest; the effective range of the 5G8038 control voltage is 0-3V. Since the linearity of the 5G8038 itself is only 0.2% at a scanning frequency range of 10:1, the linearity deteriorates in a larger range (eg, 1000:1), so the control voltage is sent to the 8 pin of the 5G8038 after the μA741. Will effectively improve the pressure control linearity (better than 1%). If the external resistance of the 4th and 5th pins is equal and R, then the output frequency can be determined by the following equation:
f=0.3/RC4
Let the function generator's maximum operating frequency be 2kHz. The timing capacitor C4 can be obtained from the above formula.
The RP3 in the circuit is used to adjust the symmetry of the high-frequency waveform, and RP2 is used to adjust the symmetry of the low-frequency waveform. Adjusting RP3 and RP2 can improve the sine wave distortion. The regulator VDz is designed to prevent the 5G8038 from malfunctioning due to the excessive negative pressure on the 8-pin.
(III) Scenario 3: Using Monolithic Integrated Function Generator 5G8038
Feasibility Analysis:
Among the above three solutions, the first and second principles of the triangle wave and sine wave are not the same, but they all have the common place that they must artificially build the circuit diagram of the waveform transformation. The use of an integrated chip in Scheme 3 greatly simplifies the circuit. However, due to limitations in laboratory conditions and costs, the first solution we abandoned was because it is a sacrifice of cost for convenience. The second is the comparison of Option One and Option Two. In Option One, the original electrical components such as capacitors and resistors op amps and transistors are used. In Option Two, the original components such as diodes, resistors, transistors, and op amps are used, so they are simple and convenient. The precondition for the purchase We choose option one for our final design.
Parameter determination
From the circuit design process, the circuit is divided into three parts: 1 sine wave part 2 square wave part 3 triangle wave part
Sine wave section
Because we choose the differential amplifier circuit for the triangle wave - sine wave
To perform the transformation, the first work to be done is to select the transistor, and I
We now choose the KSP2222A type triode with a static graph
Like the right picture.
According to KSP2222A static characteristic curve, select static
Work area center
The DC path is:
20 k
Because the quiescent operating point has been determined, the quiescent current becomes known. According to the KVL equation, the magnitude of each resistance in the mirror current source can be calculated:
Available
Welding debugging
(a) The circuit board is soldered according to the circuit diagram of scheme 1.
(b) Before commissioning, connect the circuit board to ±12 volts and connect the ground wire to the public ground of the power supply.
Frequency Range
To facilitate measurement, connect the square wave signal on the circuit board to the oscilloscope, close the switch with C1=10μF, turn off the switch with C2=1uF, and then adjust RP2, and measure the range of frequency variation of the square wave signal at this time;
The above frequency does not fully reach the required target range. The reason for the analysis is:
By comparison, it was found that the frequency range was entirely shifted down. There may be two reasons for this. The first is that there is wear on the feedback channel, so that the resistance value does not reach the calculated value. The second is the same problem with the resistance at the opposite end of the triangle wave op amp.
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