Design of a high-precision dual-channel phase-sensitive amplifier (Part 2)

　　The low-pass filter is used to pass low-frequency signals and suppress or attenuate high-frequency signals. A first-order low-pass filter is used in this product. The circuit is shown in Figure 8.

　　Filter cutoff:

　　Since the cutoff frequency of the filter is much lower than the input signal frequency, the input signal of the filter can be considered as a DC component.

　　KF=Uo/U3=-Rt/R8

　　Uo=Rt/R8·U3

　　=Rt/R8·2/π·Usinφ)

　　1.3 Calculation Verification

　　From Uo=Rt/R8·2/π·Usinφ, we know that Rt is externally connected for user convenience, so only the input signal amplitude U and phase deviation φ are variables. Take Rt=160 kΩ, R8=100 kΩ, U=1.4×7 V, φ=90°, Uo=Rt/R8·2/π·Usinφ=9.98 V. From the calculation, we know that the design goal has been achieved.

　　2 Key Design of High-Precision Dual Phase-Sensitive Amplifier

　　In order to better ensure the performance of the product, according to the product's own characteristics and user requirements, the RC network of the product's low-pass filter is externally connected for easy use and adjustment. Figure 9 is the electrical principle of a high-precision dual-channel phase-sensitive amplifier.

　　2.1 Consideration and design of product channel consistency and output zero position

　　Strictly ensure channel consistency: output error is less than or equal to 100 mV, output zero position: ≤±5 mV. To this end, we not only start from the selection and performance of the device, but also introduce the tracking resistance adjustment method according to the company's process characteristics. An absorption network is added to the circuit, and the offset voltage of the operational amplifier is strictly controlled, which solves the problem of output zero position: ≤±5 mV, thereby meeting the requirements of product performance indicators.

　　2.2 Structural design

　　It adopts DIP-16 flat-bottom fully sealed metal shell base, the base is gold-plated and the shell is nickel-plated. The production process of this set of materials is very mature and easy to purchase.

　　In order to facilitate the assembly of the whole machine and withstand mechanical tests such as vibration and impact, the product uses chip components suitable for plane assembly as much as possible, which greatly simplifies the assembly process. The packaging adopts full sealing technology, which is sealed in dry and clean nitrogen. The cap and the base are sealed by energy storage welding. After packaging, the leakage rate of the detailed inspection is less than 500×10-6kPa·cm3/s, ensuring the airtightness and reliability of the product.

　　Figure 10: Internal structure of a high-precision dual-channel phase-sensitive amplifier.

2.3 Process research

　　In order to ensure the performance and reliability of the product, after a large number of process experiments, it was finally determined that the following special process measures should be adopted in the process:

　　(1) Except for chip circuits, all components are subjected to 100% screening inspection after high-temperature storage;

　　(2) Use the process of secondary overprint bonding pad to ensure the reliability of pressure welding;

　　(3) The process of welding thick film substrate and base is adopted to ensure the mechanical connection between core group and base and reduce thermal resistance;

　　(4) Optimize the materials of components used, use chip circuits from large foreign companies for key components, and use capacitors with NPO dielectrics.

　　By conscientiously implementing the process and the implementation of the above-mentioned special process measures, the production efficiency has been greatly improved, the performance and reliability of the products have been effectively guaranteed, and good results have been achieved.

　　3 Testing and User Usage

　　The test and user usage are shown in Table 1. The test circuit principle is shown in Figure 11.

　　

 

　　

 

　　4 Conclusion

　　The circuit has been verified in practice, and all parts work normally. It has been successfully used in a system with good results. This solution not only meets the requirements of dual-channel consistency and high precision, but also has the characteristics of flexible and convenient use, high reliability, small size and low cost. It is a very good circuit module.