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Oct 19 2020

AD603 and MC34063 Based AGC Controller Design

I. Description

Automatic gain technology (AGC) is widely used in the field of industrial automation closed-loop control. In industrial control, time-varying gain amplifiers are often needed to meet production needs, or made it has a certain regularity to ensure the stability of the control output amplitude , thereby reducing the interference of the input interference noise signal. For the system to adjust quikly, this paper designs an AGC controller based on the combination of AGC chip AD603 and switching power supply chip MC34063, cleverly using MC34063's stable reference voltage and dynamic voltage adjustment output to access AD603 gain control terminal to control the amplification gain, therefore  achieve the goal of constant system output amplitude.

AD603

AD603

Catalog

I. Description

II. Working Principle of the System

III. AD603

IV. MC34063

V. System Hardware Circuit Diagram

5.1 Input Buffer Attenuation Circuit

5.2 AD603 Automatic Gain Amplifier

5.3 Output Amplitude Detector

5.4 MC34063 Feedback Circuit

VI. System Operation Results

VII. Conclusion

FAQ

 

II. Working Principle of the System


The system uses AD603 as the core control device, supplemented by the switching power supply chip MC34063 to collect the output of the controller, the output voltage is transferred to the voltage control terminal of AD603 through MC34063 to change the amplification gain. The system working principle block diagram is shown as in Fig. 1.

Figure 1 System Block diagram

Figure 1 System Block diagram

 

In this closed-loop control system, the MC34063 circuit is used as its feedback link to dynamically collect the amplitude of the output signal of the system, and control the amplification gain of AD603 by adjusting the duty cycle output voltage of the internal signal. The feedback link in the figure can be replaced with a microprocessor. The microprocessor collects the output voltage amplitude through A/D, transfers it to the microprocessor chip for signal processing, and then feeds back to the input of the entire system through D/A output control voltage . However, this method is too complicated, because the rise and fall of the digital chip take a long time to set up, which affects the response speed of the entire system, and requires relatively high signal processing algorithms. The switching power supply chip widely used in power supply technology is dynamically adjusted to improve its operating speed. In addition, its development cost is low, which is conducive to the promotion of the industrial control field.

III. AD603

 

AD603 is a chip with programmable gain, low noise, it has 3 working modes, corresponding to different gain ranges. In order to make the control more extensive, the maximum bandwidth mode is selected as 90MHz. The gain is expressed in decibels, the amplification gain is controlled by the control voltage to a linear relationship of 25mV/dB, and the slew rate is 275V/μs. The gain control voltage needs to be input during normal operation. The gain formula is:

 

In the formula: G is the gain, dB; G0 is the starting point of the gain, and the size of G0 is determined by the pin connection.

 

The circuit designed in this paper short-circuits VOUT and FDBK, G0=10dB is the wideband mode (90MHz wideband), the gain range G of AD603 is -11.09~+31.05dB, and VG is in the linear range when the range is -500~500mV. The gain control voltage VG is controlled by the MC34063 output. AD603 input signal amplitude UINP≤1.4V, the actual industrial control field often input plus interference sum is greater than 1.4V, if this signal is directly added to the system, the distortion is large and long-time work will damage the AD603, so you must add an input buffer and attenuation circuit.

 

IV. MC34063

 

MC34063 is a monolithic bipolar integrated circuit used in the field of DC-DC converter control. It is cheap and widely used in the field of switching power supplies. It can use a minimum of external components to achieve switching boost and buck. Its operating frequency is 0.1-100kHz.

 

The traditional AGC controller constitutes a closed-loop control system, which generally needs to perform A/D sampling on the output of the system, and then transfer the data to the single-chip or computer for algorithm data processing, and judge the execution signal D/A output to make the actuator execute. In this feedback process, sampling, algorithm processing and execution obviously consume too much time, and for some complex control signals, algorithm data processing requirements are high, and special DSP chips are required, which is costly. Therefore, the use of a single analog electronic circuit to achieve a closed-loop control system has higher efficiency and lower cost.

 

Inspired by the working mode of the MC34063 step-down circuit, it is a new design idea to realize the change of the AD603 gain control voltage by using the characteristics of the MC34063 to dynamically adjust the output voltage. The experimental verification is feasible and it is simpler and faster than the program control method. Figure 2 shows the MC34063 step-down circuit.

Figure 2 MC34063 step-down circuit

Figure 2 MC34063 step-down circuit

 

As shown in Figure 2, the input is +12V, the output is +5V, the reference voltage of pin 5 to ground is +1.25V, the resistance of pin 5 to ground is R1=1.2kΩ, and the output and pin 5 are connected to R2=3.6kΩ, According to the resistance divider ratio, the output is clamped at +5V, thus achieving a regulated output. Applied in the field of AGC control, you can connect the output of MC34063 to the controller gain control terminal, and the input to the output terminal of the controller. According to its working principle, MC34063 collects the output of the AGC controller and transmits it to pin 5. Its internal dynamically adjusts the PWM duty cycle, dynamically changes the AD603 gain control voltage, and can avoid the interference of the system, and realize the function similar to the PID algorithm. It replaces the algorithmic data processing mechanism, which is simple and effective, and has certain reference significance to the field of industrial automation control.

 

V. System Hardware Circuit Diagram

 

Figure 3 is the system hardware circuit diagram. The system is mainly divided into input buffer attenuation circuit, AD603 automatic gain amplifier, output amplitude detector and MC34063 feedback circuit.

Figure 3 System Hardware circuit diagram

Figure 3 System Hardware circuit diagram

 

5.1 Input Buffer Attenuation Circuit

 

Because the AD603 input signal amplitude VINP is less than or equal to 1.4V, four diode clamps are used. According to the unidirectional conductivity of the diode and the forward conduction voltage drop of silicon, the input characteristics are limited to meet the requirements of AD603. The input voltage requirements, the follower plays the role of isolating the chip. As shown in Figure 3, part ①.

 

5.2 AD603 Automatic Gain Amplifier

 

The 3 pin of AD603 is the signal input terminal, the 2 and 4 pins are connected to the ground with R4=0, R5=0 resistance to make it work more stable. The 5 and 7 pins are connected to the output, which is the system output of the AGC controller. Pin 1 is the gain control voltage VG terminal, this control voltage is connected to the output terminal of MC34063, MC34063 generates the corresponding gain control voltage VG according to the output of the system.

 

5.3 Output Amplitude Detector

 

In the field of industrial control, the signal is only in the form of DC, and the AC signal also occupies a certain proportion. For the control of the DC signal, the system output can be directly transmitted to the MC34063 for processing, but the amplitude of the AC signal must be detected, so the design is shown in Figure 3 in part ③.

 

Common amplitude detectors, such as diode rectifier bridges, are only suitable for situations where the input voltage is far greater than the diode conduction voltage drop. In AGC control, the signal in the system is often low voltage, so it cannot be used, so it is very necessary to design an amplitude detector that can avoid diode conduction voltage drop. After RC charging, the DC voltage value with a certain relationship is obtained. In Figure 3, the voltage at the intermediate node of R13 and R14 is Uf, and the expression is:

In the formula, UINP is the input amplitude, V.

 

5.4 MC34063 Feedback Circuit

 

The intermediate node voltage Uf of R13 and R14 is properly calculated by a same-inverting amplifier and an adder, and then connected to pin 5 of MC34063. At this time, it is clamped at 5V, and Uf=1V when reversed, then the AGC controller system can be dynamically maintained stability of output voltage amplitude. When the system input is unstable or there is noise interference, MC34063 dynamically changes the output voltage value according to the amplitude detection result, so as to achieve the purpose of changing the gain control voltage VG. As shown in the lower part of Figure 3, the output voltage of pin 2 is charged and discharged through switching and specific Schottky diodes, and the attenuated partial voltage is transmitted to pin 1 of AD603, which realizes the automatic adjustment of the amplification gain and successfully realizes the switching power supply technology application in the field of automatic control gain.

VI. System Operation Results

 

The experimental setting is that if the system inputs a DC signal, the output will be a constant +1V DC; if an AC signal is input, the output will be an AC signal with a constant amplitude of +1V. In the experiment, two input methods were tested and verified, and both met the design requirements. Table 1 is part of the experimental data of the input DC signal. In the experiment, the input of the AGC controller is connected to the voltage regulator source, and the input voltage is continuously adjusted.

 

 

Table 2 is part of the experimental data of the input AC signal. In the experiment, the AGC controller input is connected to the UTG9002C signal generator, the amplitude of the input sine wave is continuously adjusted, and the output is connected to the oscilloscope to observe the waveform. Observation found that no matter the input amplitude becomes larger or smaller, the oscilloscope waveform is basically unchanged. Read the oscilloscope waveform amplitude and fill in Table 2.

 

VII. Conclusion

 

This article summarizes the design of the AGC controller based on AD603 and MC34063. Experiments have verified that the AGC controller is effective and meets the design requirements. A new application of switching power supply chips in the control field is proposed. Because the internal PWM duty cycle is faster, it can replace the traditional programmable AGC controller. Among them, MC34063 can also be replaced by other switching power supply chips. It has the advantages of universal applicability, simple design, low cost, and it has important practical value.


FAQ

  • What is AD603?

AD603 is a low-noise, voltage-controlled amplifier for radio frequency (RF) and intermediate frequency (IF) automatic gain control (AGC) systems. It provides precise pin-selectable gain, with a gain range of -11 dB to +31 dB at 90 MHz bandwidth, and a gain range of +9 dB to +51 dB at 9 MHz bandwidth. Any intermediate gain range can be obtained with an external resistor. The noise spectral density referred to the input is only 1.3 nV/√Hz, and the power consumption is 125mW when using the recommended ±5 V power supply.

  • What are the problems that need to be paid attention to when using AD603?

The voltage cannot be too high. Generally, the voltage is plus or minus 5V, and the maximum voltage cannot exceed plus or minus 7.5V. The output voltage cannot exceed 2V.

  • How to solve the self-oscillation problem of AD603?

For high-frequency operational amplifiers, the following points are the basic ways to solve self-excitation.

  1. The power supply is stable and no ripple.
  2. The electrical connection wires are as short as possible.
  3. The ad603 circuit should be far away from the power circuit, especially away from the transformer.
  4. The power transformer and the circuit board of ad603 should be shielded with a metal box and grounded if possible.
  5. One point is very important. For op amps, too large magnification can easily cause self-excitation, so reduce the magnification as much as possible and minimize the number of magnification levels (generally not greater than 4).
  6. Reverse amplification can suppress self-excitation in multi-stage amplification.
  7. If you want to connect to the power amplifier and then amplify, it is best to use two power supplies, and the circuit should be connected to the same ground.
  • What is the difference between AD603AQ and AD603AR?

Their differences are in model, Temperature, Package.

AD603AQ -40°C to +85°C 8-Lead CERDIP

AD603AR -40°C to +85°C 8-Lead SOIC_N

  • After inputting an AC signal and being amplified by AD603, why does the output contain a DC signal? How to eliminate the DC signal?

When the DC blocking capacitor is not used, the bias voltage of the input circuit needs to be adjusted for compensation.

If the DC voltage of the AC signal is not fixed, only a DC blocking capacitor can be used, or the average value can be used to eliminate it after sampling the number.

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