ASK signal generator

1. Overall design

1.  This circuit realizes the generation of ASK amplitude shift keying signal with a frequency of 10kHz and an amplitude of 3V.

2.  Adoption plan: sine wave and square wave signals are generated independently. The square wave signal is used as a switching signal to control the analog switch. The analog switch controls the on and off of the sine signal, thereby realizing the generation of the ASK signal. The sinusoidal signal question requirement is 10kHz, which belongs to the low frequency range, so it is generated using an RC Wien bridge oscillation circuit. The square wave signal requirement is 1kHZ, which belongs to the low frequency range. It is composed of a hysteresis comparator and an RC circuit. At the same time, the duty cycle is adjustable by using a potentiometer to control the charging and discharging time of the capacitor. An inverse proportional amplifier circuit is used to control the voltage output amplitude, and a voltage follower circuit is used to improve the load capacity of the output signal.

Figure 1-1   System block diagram

2Circuit design

2.1 Sine wave generation circuit module design

( 1 ) Functions and parameters. To generate a 10kHz low-frequency sinusoidal signal, there is no need to introduce an inductor, so an RC Wien bridge oscillator circuit is used here . The principle is that after the op amp output voltage is divided by the positive feedback network, the feedback voltage is taken as the input signal of the non-phase proportional circuit. The RC series-parallel sine wave oscillation circuit composed of operational amplifiers achieves amplitude stabilization by introducing negative feedback from the outside . According to the RC frequency selection network, the oscillation frequency can be determined. Parameters: This module mainly controls the frequency of sinusoidal signals.

 ( 2) Calculation of parameters.

1. Frequency selection. According to the frequency selection formula , taking C as 0.1μF, the solution is R=159Ω. At this time, the theoretical frequency is 10014.8Hz, and the theoretical error is (10014.8-10000)/10000=0.148%

2. Vibration starting conditions. According to the starting condition |AF|>1, so the current is small at the beginning and the diodes D3 and D4 are in the cut-off state. The resistance of the feedback part resistor is R8 with a resistance of 30k. At this time, the amplification factor A=(1+ )=4>3, so the circuit starts to oscillate. After the oscillation, the current increases, and the diodes D3 and D4 are turned on, which is equivalent to the incorporation of R12 Feedback resistor, at this time A=(1+ ) =2.83≈3, in order to achieve the purpose of amplitude stabilization.

Figure 2-1 Sine generation circuit diagram

Explanation: The output amplitude is also related to the parameters of the RC . In order to ensure the accuracy of the frequency, this circuit does not consider the output amplitude, so a reverse amplification circuit is added to the subsequent circuit to ensure the accuracy of the output amplitude.

2. 2 Rectangular wave generation circuit module design

(1) Functions and parameters. To generate a 1kHz rectangular wave signal, and because the subsequent circuit uses the analog switch CD4066 , the condition for turning on the positive and negative signals is that the low level of the switch signal is negative, so the 555 multivibrator cannot be selected, and a hysteresis device is used here. Together with the RC oscillator circuit, it forms a rectangular wave generator.

Parameters: This module mainly realizes the control of square wave signal frequency.

(2) Calculation of parameters.

1. Frequency selection. According to the frequency selection formula , take R3=R4=10kΩ, C1 is 0.01μF, and the solution is R1=15.9kΩ. At this time, the theoretical frequency is 1001.5Hz, and the theoretical error is (1001.5-1000)/1000=0.15%

Figure 2-2 Rectangle  generating circuit diagram

2. 3 Inverse proportional amplification circuit module design

(1) Functions and parameters. The input signal of this module is the output signal of the sine wave generation signal. Based on the simulation results, the amplification factor is determined and the signal output amplitude is controlled to achieve the purpose of the sine wave signal being 3V .

Parameters: This module mainly controls the amplitude of the sine wave.

(2) Parameter calculation, according to simulation, when the operational amplifier LM324 is powered by ± 12V dual power supply, the output amplitude of the sine wave signal is about 4.4V . According to the inverting proportional amplifier circuit formula , take R6=680 Ω, R7=1k , that is, when the magnification is 0.68 times, Vout=0.68*Vin ≈ 3V

Figure 2-3 Inverse  proportional amplification circuit diagram

2. 4 Voltage following circuit module design

( 1) Function.

1. Increase the input impedance to ensure stability during load testing

2. As a buffer stage, it solves the crosstalk problem to a certain extent

Figure 2-4 Voltage  following circuit diagram

2. 5 Analog switch circuit module design

( 1 ) Function. As required by the question, to pass positive and negative signals that are symmetrical about 0 , a bidirectional analog switch should be selected. Therefore, CD4066 meets the requirements and should be powered by ± 12V dual power supplies. The rectangular wave signal is used as the switching signal (the high and low levels of the rectangular wave are symmetrical about 0 ). , the sinusoidal signal is used as the input signal, and the output signal is the required ASK signal.

Figure 2-5 Analog  switch circuit diagram

2. 6 Switch and power input/signal output circuit module design

( 1 ) Function. Control the on and off of the entire circuit, the input of the power supply and the output of each signal.

Figure 2-6 Switch and power input / signal output circuit diagram

3 Circuit Simulation

3.1 Sinusoidal signal frequency amplitude simulation

Figure 3-1   Sinusoidal signal frequency amplitude simulation diagram

Simulation purpose: Verify whether the frequency and amplitude of the sinusoidal signal meet the requirements of the question.

Simulation process: Connect the output signal of the sine wave generating circuit to an oscilloscope to observe the frequency and amplitude.

Result analysis: According to the figure, the amplitude of the oscilloscope at this time is 4.417V , the period (T2-T1) = 100.855us, the question requires 10kHZ to be converted into a frequency of 100us , the calculation error = ( 100.855-100 ) /100=0.8%

3.2 Sinusoidal signal amplitude simulation

Figure 3-2 Sinusoidal   signal amplitude simulation diagram

Simulation purpose: to verify whether the amplitude of the sinusoidal signal meets the requirements of the question.

Simulation process: Connect the output signal of the sine wave generating circuit through inverting amplification and voltage follower to an oscilloscope to observe the frequency and amplitude.

Result analysis: As shown in the figure, the period of the oscilloscope has not changed significantly. At this time, the amplitude is 3.014V , and the calculation error = ( 3.014-3 ) /3 ≈ 0.467%

3.3 Square wave signal frequency simulation

Figure 3-3 Square  wave signal frequency simulation diagram

Simulation purpose: to verify whether the square wave signal frequency meets the question requirements.

Simulation process: Connect the output signal of the square wave generating circuit to an oscilloscope to observe the frequency.

Result analysis: According to the figure, the period of the oscilloscope at this time (T2-T1) = 1.032ms. The question requires that 1kHz be converted into a frequency of 1ms . The calculation error = ( 1.032-1 ) /1=3.2%

3.4 ASK signal output simulation

Figure 3-4 ASK  signal simulation diagram

Simulation purpose: to verify the correctness of the ASK signal and compare it with the pulse source to observe whether it meets the question requirements.

Simulation process: Connect the output signal to the oscilloscope, and connect the sine pulse source to the oscilloscope to observe the waveform.

Result analysis: The blue waveform in the figure is the original sinusoidal input signal, the red waveform is the output ASK signal, and the green waveform is the rectangular wave switching signal. It can be seen from the figure that the ASK signal is output normally with the high and low level transformation of the rectangular signal, and the frequency and amplitude are uniform. No changes have occurred.

The simulation is now complete.

4 circuit test

4.1 Sinusoidal signal frequency test

The output waveform of the sine wave generator is shown in Figure 4-1:

Figure 4-1 Measured sine wave frequency output waveform diagram

Use an oscilloscope to connect the output of the sine wave generating circuit to obtain the waveform as shown in the figure. The amplitude is 4.40V and the frequency is 10.0017kHz. The purpose of this test is to test the frequency error. The error is (10.0017-10.0000)/10.0000=0.017% error. Basically can be ignored.

4.2 Sinusoidal signal amplitude test

The output waveform of the sine wave generator output after passing through the amplifier and voltage follower is shown in Figure 4-2 :

Figure 4-2 Measured sine wave amplitude output waveform diagram

Use an oscilloscope to connect the circuit output after the input amplifier and voltage follower after the sine wave is generated, and obtain the waveform as shown in the figure. From the figure, it can be seen that the amplitude is 3.04V and the frequency is 10.0769kHz. The purpose of this test is to test the amplitude error, and the error is ( 3.04-3.00)/3.00=1.33%.

4.3 Rectangular signal frequency test

The output waveform of the rectangular wave generator is shown in Figure 4-3:

Figure 4-3 Measured rectangular  wave amplitude output waveform diagram

Use an oscilloscope to connect the output of the rectangular wave generating circuit to obtain the waveform as shown in the figure. The amplitude is 10.0V and the frequency is 1.00118kHz. The purpose of this test is to test the frequency error. The error is (1.00118-1.0000)/1.0000=0.18%. And the duty cycle is 50%.

4.4 Overall circuit test

The overall output waveform of the circuit is shown in Figure 4-4:

Figure 4-4 ASK  signal output waveform diagram

Use an oscilloscope to connect the overall circuit output and get the waveform as shown in the figure. The red waveform in the above left picture is the ASK signal. Compared with the input sinusoidal signal, it can be observed that the two waveforms overlap, no phase shift occurs, and the amplitude is 3.04v. The error is (3.04-3.00)/3.00=1.3%.

At this point, the design and testing are completed.