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Oct 15 2020

AD603 Based Time-varying Gain Amplifier Design

I. Introduction

 

Ground penetrating radar (GPR) has been widely used in many fields due to its non-destructive detection characteristics. Its detection principle is: use an antenna to transmit high-frequency broadband electromagnetic waves to the ground, and then receive the reflected echo from the interface of the underground medium. Through the processing and analysis of the echo signal, infer the structure of the underground medium. The impact ground penetrating radar has received extensive attention due to its simple structure and rich echo information. The ground penetrating radar mentioned below refers to the impact ground penetrating radar.

 

Due to the sharp loss on the propagation path, the dynamic range of the echo signal received by the antenna is extremely large, generally up to 150dB. The dynamic range of the A/D converter in the radar receiving system can generally only reach 80-90dB, which is difficult to meet the requirements of the radar system. At the same time, due to the small echo amplitude of deep targets, the detection depth and resolution of the ground penetrating radar will be seriously affected if it is not processed. In order to improve the detection depth and resolution of the radar, and at the same time increase the dynamic range of the receiving system, this article uses AD603 to design a time-varying gain amplifier to process the echo signal, and give different gains to the echo signal at different times. To compensate the lack of the dynamic range of the A/D converter, and achieve the purpose of matching the dynamic range of the echo signal.

AD603

 

Catalog

I. Introduction

II. Design of Time-varying Gain Amplifier

III. Design of the Zero Adjustment Circuit of the Time-varying Gain  Amplifier

IV. Measured Results

V. Conclusion

FAQ

 

II. Design of Time-varying Gain Amplifier

 

The so-called time-varying gain amplifier simply means that the gain of the amplifier is a function of time. Since in the ground penetrating radar system, time actually corresponds to the distance between the target and the antenna, from this perspective, in the radar system, it can be called a range gain amplifier. The mechanism of action is to use attenuation or lower gain amplification for the scattered echoes of near-distance targets, and use higher gain amplification for the scattered echoes of long-distance targets, so that the echo signals entering the data acquisition circuit become relatively stable. In the end, the strong signal of the shallow target echo is attenuated or suppressed to avoid the amplifier from being saturated and overloaded or the amplifier output exceeds the input range of the A/D converter; the weak signal of the deep target echo is effectively amplified to ensure the acquisition and discernment of the target signal.

 

There are many schemes for designing time-varying gain amplifiers. In view of the requirements of ground penetrating radar system, this article uses the new voltage-controlled amplifier AD603 produced by American Analog Devices to realize the time-varying gain amplifier. The advantages of AD603 are: low noise, wide band, gain, gain range are adjustable, the gain value  changes linearly with the external control voltage, and the bandwidth does not change with the gain, etc., which can fully meet the requirements of the radar system. The schematic diagram is shown in Figure 1.

Figure 1 AD603 schematic diagram

Figure 1 AD603 schematic diagram

It can be seen from the schematic diagram that the internal structure of AD603 is divided into 3 functional areas: gain control area; precision passive input attenuation area; fixed gain operational amplifier area.

 

The control voltage in the gain control area controls the continuous attenuation of the attenuator. It is like sliding the arrow on the non-inverting end of the fixed gain op amp in the figure between 0 and -42.14dB. The gain range and bandwidth of AD603 are determined by the connection mode of VOUT and FDBK. When VOUT and FDBK are short-circuited, the gain range is 10~30dB, and the bandwidth is 90MHz; when the output terminal VOUT and the feedback terminal FDBK have an indirect 2.15kΩ resistance. When the feedback terminal FDBK is grounded through 5.6pF, the gain range is 0~40dB, frequency bandwidth is 30MHz; when VOUT and FDBK are open, and the feedback terminal FDBK is grounded through 18pF, the gain range is 10-50dB and the frequency bandwidth is 9MHz. Once the gain range is determined, the bandwidth of the entire amplifier is also determined. And within the gain range, the bandwidth does not change with the gain. This is because the gain adjustment is realized by the R-2R ladder resistance attenuation network before the fixed gain op amp, instead of changing the feedback resistance of the op amp, so the bandwidth of the entire amplifier is not affected by the gain adjustment.

 

Since the dynamic range of the ground penetrating radar echo signal is extremely large, in order to provide a larger gain to the weak echo signal in the deep layer, for subsequent data acquisition and processing. In the specific implementation, a two-stage AD603 cascade method is used to realize variable gain amplification, and the gain range of the front and back stages is set to 0-40dB. In this way, the two-stage amplifying circuit can provide a total variable gain range of 0-80dB, which can meet the needs of the ground penetrating radar to expand the dynamic range. At the same time, in order to improve the signal-to-noise ratio of the two-stage amplifier circuit as much as possible and reduce the possibility that the noise generated by the previous amplifier is amplified by the latter amplifier, the two-stage amplifier adopts a sequential control connection method. The circuit principle is shown in Figure 2.

Figure 2 Schematic diagram of variable gain amplifier circuit

Figure 2 Schematic diagram of variable gain amplifier circuit

In order to minimize the frequency band loss after the cascade of the two-stage amplifier, improve the low-frequency response characteristics of the amplifier, and avoid the loss of low-frequency components in the ground penetrating radar echo signal, the direct coupling method is selected in the design. As the gain change range of the single-stage amplifier is set to 0~40dB, its bandwidth is 30MHz. After the two-stage amplifier is cascaded, the total amplifier 3dB bandwidth will be reduced, and the bandwidth at this time is about 21MHz, but the dynamic range is improved.

 

For a ground penetrating radar with an antenna center frequency of 100MHz, the highest frequency component of its echo signal is about 150MHz. Assuming that the transmitted pulse repetition frequency is 300kHz and the sampling time interval is 0.1ns, the highest frequency component of the echo signal after equivalent sampling transformation can be obtained as:

It can be seen that the highest frequency component of the signal after sampling and transformation is much smaller than the bandwidth of the amplifier, which can ensure that the signal is amplified without frequency distortion。

 

After actual measurement, it is found that the maximum amplitude of the echo signal after the equivalent sampling transformation is about ±2.5V, and the maximum allowable input voltage of AD603 is ±1.4V. If the echo signal is directly input, the output signal will be distorted, and the AD603 will be damaged in severe cases, so the input signal must be attenuated first. Because the input impedance of AD603 is 100Q, a 100Q resistor R is connected in series between the input signal and the input of AD603 to form a 1:1 resistor divider to attenuate the input signal. The maximum amplitude of the attenuated signal is about ±1.25V, ensuring that it is within the allowable input voltage range of AD603.

 

Taking into account that in some abnormal situations, the maximum amplitude of the input signal after attenuation is still greater than ±1.4V, so here, D1D2D3D4 diodes are used in series in the same direction, and then connected in anti-parallel between the AD603 input terminal and the analog ground. Using the unidirectional conductivity of the diode and the characteristics of the PN junction forward voltage drop of about 0.7V (for silicon materials), the input signal is limited, and the maximum amplitude of the signal after limiting is exactly about ±1.4V, which satisfies the input voltage requirement of AD603. Based on the same principle, 4 diodes D, D, D, D are used at the input end of the second stage AD603. Limit the signal and limit its amplitude to within ±1.4V.

 

III. Design of the Zero Adjustment Circuit of the Time-varying Gain  Amplifier

 

Since the AD603 has an output offset voltage (DC offset voltage) of about 20-30mV, when the two-pole AD603 is directly coupled, the output offset voltage of the previous AD603 will be amplified by the next AD603. When the gain of the subsequent stage is large, the DC potential of the amplified echo signal will deviate greatly from the zero point, resulting in a part of the upper or lower half of the output signal waveform being cut off, resulting in serious nonlinear distortion. And because the front-stage receiving and sampling gate circuit will also bring a DC offset voltage, that is, the input signal of the first stage AD603 contains a DC offset component, so the DC offset after amplification by the two-stage amplifier circuit will be more serious. Based on the above reasons, a DC offset zeroing circuit must be designed to adjust the DC potential of the output signal, so that no nonlinear distortion is produced when the output signal amplitude reaches the maximum.

 

Because AD603 itself does not have zero adjustment control terminal, so can only add a DC offset zero adjustment circuit before the first stage AD603. In the specific design, an inverting addition amplifier composed of operational amplifiers can be considered. At the inverting input terminal of the operational amplifier, a DC voltage is input through another input loop, and the input echo signal is added to cancel the DC voltage. The offset component will not affect the echo signal itself, so as to achieve the purpose of zeroing the DC offset. Of course, there is another reason for using an op amp to form a DC offset nulling circuit, that is, because the input impedance of AD603 is very low (about 100Q), if it is directly connected to the output of the sample-and-hold circuit of the antenna system, a driving current may appear. The problem of insufficient. The input impedance of the operational amplifier is high, and the output impedance is low. It is connected between the output of the antenna system and the input of AD603, so that the output of the pre-stage sample-and-hold circuit will not be too high, and it can also output a large enough current to drive the AD603, which plays a role of isolation and buffering. The impedance of the level circuit is matched. Figure 3 is a schematic diagram of the DC offset zeroing circuit designed and implemented. The op amp in the picture uses the ultra-low noise, ultra-low distortion operational amplifier AD797 produced by the American Analog Devices company.

Figure 3 Schematic diagram of the DC offset zeroing circuit of the variable gain amplifier

Figure 3 Schematic diagram of the DC offset zeroing circuit of the variable gain amplifier

There are two adjustment methods for the DC offset zeroing circuit: one is manual adjustment, and the other is automatic adjustment. It should be noted that the zeroing here is different from the zeroing of ordinary op amps. It is no longer for single-stage zeroing, but for multi-stage zeroing, that is, DC potential compensation is performed on the output of the entire amplifier circuit system. In the actual radar system, an automatic adjustment method is used for zero adjustment. The so-called automatic adjustment method is that in the initialization stage before the formal data collection, the computer calculates the DC offset according to the pre-collected echo signal data. Then the offset is sent to a digital-to-analog converter (DAC, digital-to-analog converter), and the converted analog voltage is the DC potential offset compensation voltage required by the DC offset zero circuit.

 

IV. Measured Results

 

Of course, the actual use of the module also includes the design of the active filter and the time-varying gain controller. Due to space reasons, these two modules are not described in detail here. The time-varying gain controller module adopts a design scheme based on DSP and FPGA. The process is as follows: First, set the time-varying gain curve on the PC, and obtain the gain value of each point in an A scan after calculation. Then transfer these gain values ​​as working parameters to the DSP in the radar host through the USB interface. After receiving these parameters, DSP forwards them to FPGA as they are. FPGA then stores these gain values. After the data acquisition starts, the FPGA, under the control of the external synchronization signal, reads out the previously stored gain value in turn, and sends it to the D/A converter for conversion to obtain a time-varying voltage signal. The shape of this signal is basically consistent with the time-varying gain curve set on the PC. Using this signal to control the variable gain amplifier, we can get the time-varying gain we need. In the actual time-varying gain control of ground penetrating radar, two threshold judgment voltages are generally set in advance, that is, set the high threshold voltage to 2V and the low threshold voltage to 1.8V, then if the peak value of the amplified echo signal voltage is greater than 2V, then It is judged that the gain is too high, and the gain is reduced; if the peak value of the amplified echo signal voltage is less than 1.8V, it is judged that the gain is too low and the gain is increased.

 

The time-varying gain amplifier designed with the above design ideas is used in the actual radar prototype. The echo signal after the time-varying gain amplified by the oscilloscope is shown in Figure 4. From the figure, it can be seen that it satisfies As required by the radar system, the amplitude of the echo signal is relatively stable, which ensures the acquisition and identification of the target signal.

Figure 4 Echo signal amplified by time-varying gain

Figure 4 Echo signal amplified by time-varying gain

V. Conclusion

 

This article summarizes the design of the time-varying gain amplifier based on AD603, and gives detailed design ideas and schematic circuit diagrams. The actual test results show that the time-varying gain amplifier designed in the article can meet the needs of radar systems and has strong 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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