Design And Research Of The Digital FM Exciter Which Is Based On DSP

FM exciter is the core component of FM transmitter, its performance and workingparameters directly affect the quality of the FM radio. The analog exciter has experienced70years of development, its frequency response, distortion, SNR(Signal to Noise Ratio) and otherimportant indications can no longer be improved significantly. And at the same time becausethe analog voice modulation radio spectrum has been saturated, more and more near frequencyand same frequency interference in the received signal,the quality of broadcast is declining. Atthis point, people began to consider using digital FM exciter. Digital exciter is not only withsmall distortion, high signal to noise, and aslo can meet the synchronous broadcasting system of"the same frequency, phase, modulation" requirement, also guarantees the consistency between thecontrol precision and equipment, and greatly improves the utilization rate of power/frequency.At present, some foreign manufacturers are engaging in the production of digital FM exciter.The digital FM exciter is much better than the analog FM exciter in some important indications,and the sound quality has reached that of "CD". But our country has not independentresearch and development of digital FM exciter, and mainly depends on imports. So, developingand researching digital FM exciter which is suited for the situation of our country is a veryrealistic and necessary task.In this paper, that has been carried on, the preliminary design and research of the forwardaudio channel sections and digital frequency synthesis for the digital FM exciter. Digital FMexciter’s forwad audio channel part is divided into analog audio and digital audio channels,respectively for the input of analog and digital audio signal processing. The design of the analogaudio channel realizes signal sampling and quantization, using TLV320AIC23B chip for audiosignal collection. It’s working and data transmission is controlled by DSP control. In the designof the digital audio channel,for the AES3data stream, CS8420chip is used for decoding andsampling rate conversion. The specific configuration of hardware’s working mode of theCS8420chip is given, and the configuration and connection between CS8420chip andTMS320VC5509A is also proformed. Then the sampling rate conversion is carried. Aftersampling of analog signals and after decoding and sampling rate conversion of digital signal,the sigal is put into the DSP chip. Interpolation processing is carried in the DSP chip.Thepurpose of interpolation processing in the DSP chip, is to improve the sampling rate, so that itcan reduce signal distortion, and improve the signal to noise ratio. To design FIR digital filter,which is used to drop out mirro image spectrum after zero interpolation. Next,using DSP torealize the synthesis of baseband signal and digital frequency modulation.The working principleof NCO(numerically controlled oscillator)is adopted. The method of searching table is used toobtain19KHz and38KHz cosine signal which is the require of the baseband signal. After thatDSP controling DDS chip AD9859is used to complish digital frequency modulation. In theend of this paper summary and analysis of the design is given.