Integrated and simplified design

The system design of wind profiler laser digital intermediate-frequency receivers has troubled product development personnel because of the high complexity and long design period. Fortunately, advanced data acquisition digitizers provide a highly integrated hardware and software platform for system development to speed up the development progress of wind profiler laser digital intermediate-frequency receivers.

As an important part of laser systems, conventional weather laser receivers mainly control dynamic range expansion through instantaneous automatic gain, realize analog modulation for analog type I/Q modulator signals, depend a great deal on receiver hardware, and have low signal adaptive capacity; however, the emergence of software-defined radio technology has brought about a renovation of radio receivers. With booming device levels, digital receivers are becoming increasingly mature as an important part of software-defined radio technology and have already been extensively applied in radar equipment, electronic warfare, and communication receivers.

The ultimate goal of software-defined radio technology is to connect analog/digital converters (ADCs) with radio antennas firmly, realize sampling on the signal radio frequencies directly, convert analog signals into digital signals and achieve all the other processing functions under radio frequencies through software modules. Currently digital receivers are extensively deployed in weather radar equipment and have shown remarkable operational effectiveness. They have the main advantages of small size, lower costs and less complicated systems. Notably they also have greatly improved sensitivity and dynamic range performance.

Basically, these digital receivers adopt the design block diagram in Fig. 1. Each of the digital receivers consists essentially of three parts, i.e. a high-performance A/D converter (sampling device) and a super-large programmable logic device for digital frequency conversion and data transmission. The disadvantage of this kind of design is that each link needs meticulous design, so the whole design period is too long and the system is too complicated. Therefore, finding a highly integrated data acquisition platform to simplify the design has become an urgent problem.

Fig. 1 Block Diagram of a Universal Digital Receiver

ADLINK PCI-9846H high-speed digitizers can provide highly accurate, low-noise and high-dynamic range performance with high density and high accuracy. Is it possible to design a kind of wind profiler laser digital intermediate-frequency receiver based on the abovementioned characteristics of ADLINK PCI-9846H high-speed digitizers to simplify the long, complicated and error-prone design and development work of digital receivers?

Opportunity for system simplification

According to the parameter characteristics of the wind profiler laser systems of a certain model (intermediate-frequency point: 50MHz; band width: 5MHz), the digital intermediate-frequency receivers adopt a polyphase filtering-based, digital, orthogonal transformation method. This method needs no orthogonal local oscillator and has a lower subsequent digital filter order; therefore, it can be easily realized. Direct down-conversion is realized for the ADC data. After sampling of the ADC, the digital signal is first separated into two circuits, then goes through half-band filtering, speed reduction and digital filtering and is finally output as two circuits of orthogonal laser base data.

In order to verify the feasibility of the system according to the result of an analysis of the resource and structural characteristics of PCI-9846H high-speed digitizers, the establishment of a system simulation platform as shown in Fig. 2 of the design is planned.

Fig. 2 Verification Platform for Digital Intermediate-frequency Receivers

For convenient description of the operation process, the steps below will be followed:

1. Insert a PCI-9846 digitizer into the PCI slot of the PC. Start the PC, insert the technical data disk provided by ADLINK Technology INC. and install the drive and related development tools necessary for system establishment.
2. Set the external signal source to be two circuits of full phase parameter outputs and simulate the intermediate-frequency and sampling clock provided by the laser. Set one circuit to 40MHz, use it as an external clock of the PCI-9846 digitizer and connect it with the "CLK IN" end of the PCI-9846 digitizer; set the other circuit to 50MHz, use it as an intermediate-frequency input and connect it with the CH0 from the four input channels of the PCI-9846 digitizer.
3. ADLINK Technology INC. makes great efforts to support its software. ADLink provides conventional WD-DASK drivers and development link libraries and task-oriented drivers and development link libraries as software solutions for PCI-9816/9826/9846. Run the LabVIEW software to use the ADLink DAQpilot control.
4. Set the ADLink DAQpilot, use channel 0, make the amplitude range ±1V and choose the external clock.
5. Execute down-conversion for output of the ADC with the method in Fig. 2. Fig. 6 shows the I/Q separation section of the down-conversion.
6. For convenient data analysis, save the data of each level in LabVIEW and use MATLAB to perform deep analysis according to the design habits of the designer.

Fig. 3 I/Q Separation

After the simple and speedy establishment of the system simulation model, system simulation and verification can be done.

1. Analyze the ADC data of channel 1 of the PCI-9846H digitizer. According to the result of the verification analysis, when operating with a 40MHz external clock, the PCI-9846H digitizer has normal data acquisition; however, it will suffer a certain signal-to-noise ratio loss because of the undersampling (bandpass sampling) adopted.
2. By means of fast establishment of the digital under conversion system in a LabVIEW environment, the 50.3MHz intermediate-frequency input signal is converted into I/Q base data with a 4MHz bandwidth of the laser after frequency conversion and speed reduction. According to the result, the system realizes the function of digital down-conversion. The experiment in this section proves that ADLINK PCI-9846H digitizers can realize the design needs of a laser digital intermediate-frequency system of a certain model. The application of ADLINK PCI-9846H digitizers and related software tools significantly simplifies the complexity of the system design and shortens the product development period.

Entering the laser system

It can be seen from the modeling, simulation and verification introduced in the previous section that the system can realize applications of PCI-9846H digitizers in laser systems. See Fig. 4 for the block diagram of the system architecture. A PCI-9846H digitizer is directly inserted into a laser terminal, and the intermediate-frequency signal of the laser, the coherent clock signal from the frequency synthesizer, and other trigger signals are accessed by the related interfaces. The system simplifies previous digital intermediate-frequency receivers, eliminates the need for an external power supply, external space and other data connections. By means of PCI access, the system directly realizes frequency conversion and product display for the data from PCI9846H digitizers.

In order to verify if the laser system is operating normally, a signal is input from the RF front end of the laser system after the laser system is assembled in the way outlined in Fig. 4. Hereunder is the measurement information obtained when the laser system is in operation.

1. Final I/Q time domain data output when the intermediate-frequency output and power of the laser system are 50.05MHz and +5dBm, respectively:
   
   This work is mainly done to check the power stability and phase stability of the output I/Q signal. Both the actual incidents of power miscounts and incidents of phase miscounts are zero; therefore, both amplitude consistency and phase stability of the digital down-conversion in the design of PCI9846H digitizers are basically in line with the requirements.
2. The intermediate-frequency output of the laser system is a 50MHz+70Hz frequency offset, and the intermediate-frequency input power is -40dBm. The power spectrum of each beam of the wind profile is displayed. The operating frequency of the laser is 1290MHz, so the target measured value should be 8.13m/s if a 70Hz frequency offset is input. According to Fig. 5, the measured wind speed is 8.0m/s, which complies with the value obtained from the above formula. The difference is caused by the time domain coherence integration and spectrum average time error of the wind profiler laser. Therefore, the design requirements are met.

ADLINK PCI-9846H digitizers integrate abundant hardware and other development resources and can complete program design and applications quickly via the interactive interfaces provided by DAQPilot, regardless of the types of development platforms used (such as Visual Studio, Borl C++ Builder, Delphi or LabVIEW). ADLINK PCI9846H digitizers can realize earlier proposal verifications quickly and conveniently, and speed up the development of the wind profiler laser digital intermediate-frequency receivers of a certain model. It is proved by actual system implementations that ADLINK PCI9846H digitizers can meet the design needs of digital intermediate-frequency receivers.

*LabVIEW™ is a registered trademark of National Instruments.

Related ADLINK Links:

• More about ADLINK Digitizers
• More about PCI-9816/9826/9846