By Interactive Circuits and Systems Ltd.
The advent of Software Radio has promised great hope for radio communications by offering ease of design and flexibility that was not previously possible. The push is to place the ADC as close as possible to the antenna, thereby performing a variety of receive functionalities in the digital domain. This ensures ease of design and flexibility. Traditional implementation of any receiver includes a front end analog downconverter stage, which translates the input signal to baseband or near baseband frequencies. With the introduction of the high speed and high performance ADCs, bandlimited VHF signals may be directly digitized utilizing undersampling principle, thereby simplifying the design of the RF front end. The RF front end will consist of bandpass filters, AGC, but no RF downconverters. A comparison is made for designing a VHF receiver (135-175 MHz) by conventional means and by Software Radio techniques. Results are presented for a VHF FM receiver implemented based on undersampling of the VHF band (135-175 MHz).
Conventional Nyquist sampling requires that the sampling frequency FS must be more than double than the highest frequency of the signal to be digitized. However, for bandlimited signals at higher frequencies this leads to using very high sample rate. For example, a 30 MHz (including guard band) band limited signal centred at 175 MHz would require a sample rate of 380 MHz (2*190). However, using the principle of undersampling, the same bandlimited signal may be sampled using a sample frequency of 80 MHz (160/2). Images are formed as part of any sampling process and frequencies higher than the FS/2 are folded back in the 0 to FS/2 region. For proper undersampling (without aliasing), the signal must be bandlimited prior to sampling. The sample frequency FS must satisfy:
where, TBW is the total bandwidth (including guard band), FL is the low end of the band, FH is the high end of the band and k is any non zero positive integer....
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