Design And Implementation Of Test System For Radio Frequency Front-End Of Weather Fax Receiver

As one of the main equipment for ships,the meteorological fax receiver receive the weather information from the meteorological fax transmitting station.It provide guarantee for the safe navigation of ships.The receiver is composed of the radio frequency front-end and the recording device.Therefore,it is necessary to check whether the radio frequency front-end can receive the fax signal completely and accurately.However,according to the requirements of the WMO,the area covered by each station has strict regulations.Beside,the broadcast schedule of each station are also different.The weather fax signal contains start signal,phase signal,line synchronization signal,image signal and stop signal.Conventional signal generators cannot simulate the signal.It is necessary to provide a simulator of signal source(2～24 MHz)for receiver in full band and time.The radio frequency front-end of receiver are double superheterodyne architecture,composed of the frequency band selection,two-stage mixing,beat detection,filtering and amplifying.These processing are mainly realized by discrete devices.There are many signal nodes and complex coupling relationship.It is difficult to determine the fault point.Due to the limited production scale and quantity,manual welding is used,so the manufacturing plan of segmented welding and segmented testing is generally adopted.In order to ensure quality and efficiency,it is also necessary to design a dedicated test system to test and record the parameters in the semi-finished state.By analyzing the signal formation and the structure of the RF front-end,the test system is proposed.It can simulate the fax signal in full frequency band and time period,collect the data of key nodes,display the waveform and frequency spectrum of the signal,realize the data record for failure analysis and design improvement.The test system is composed of a shortwave signal simulator,an acquisition board and an upper computer operation display panel.The simulator is used to provide the halftone weather fax signal with 16 gray levels for testing.The MCU obtains the picture data from the upper computer by USB,combines the picture data,the start signal,the phase signal,the line synchronization signal and the stop signal according to the formation of the fax signal,and modulate it into base-band signal.The base-band signal is modulated into weather fax signal by tow-stage mixing and beat frequency oscillator.The core component of beat frequency oscillator is DDS.MCU control the DDS to generate the carrier frequency.And the carrier frequency is mixed with the base-band signal.Signal filtered by frequency band selection module,amplified by controllable gain module.Then it is transmitted by the antenna.The acquisition board is composed of conditioning circuit,ADC circuit,FPGA,MCU and peripheral circuits.The conditioning circuit amplifies the measured signal,converts single-ended signals to differential signals.The processed signal is converted into a digital sequence by ADC and buffer into the on-chip dual-port RAM in FPGA.FPGA send an interrupt request to the MCU.MCU accept the request and read the data from the dual-port RAM and upload it to the upper computer by USB.At the same time,MCU realize the anti-aliasing filter parameter selection and the gain control of the amplifying circuit.The upper computer control the signal source and receive the data uploaded by the signal acquisition board by USB,and then display the waveform and frequency spectrum on the panel,record the collected data.The operator can judge the correctness of the signal chain of the front-end board according to results.This paper has completed the overall scheme design of the test system,analyzed the composition and functions of each part of the system,completed the detailed design of the system hardware and software,verified the function of the system.