INTRODUCTION
The design and building of a complete radiofrequency transceiver is used for wireless data communications in the VHF ISM bands. We can choose one of two different approaches that meet the same specification. The radios down convert the radio band of interest (27-70Mhz)to a low frequency intermediate frequency(IF)signal of about 500Khz.at such low frequency the signal can be digitized and sent to a computer where demodulation and decoding can occur.
There are two choices of design.
First Approach (Figure 1), involves the construction of a low cost first principles device whose receiver produces an analogue IF and transmit chain up converts an analogue IF.
Second Approach (Figure 2), two dedicated chips handle up and down conversion .The AD9874 digital IF receiver and the AD9854 direct digital synthesizer. The IF produced by the AD9874 is a 24 bit digital serial stream produced direct from the antenna received signal. The AD9854 produces an analogue radiofrequency modulated carrier direct from data. Since the data streams can represent just about any information, the second project is ideal for the development of a versatile consumer radio. As a result, a separate ADC and DAC are not required for the AD9874 and AD9854.
From the practical and the legal point of view, there are a number of LIPD (Low Interference Potential Device) bands available in the range 27-70 MHz. The band 26.957-27.283 MHz has an emission level of 1 Watt EIRP (Equivalent isotropic radiated power). There are several restrictions in its usage: in particular the emission scheme employed must use a bandwidth of at most 10 kHz. Care also has to be exercised to avoid interference with the 27 MHz CB bands. Perhaps the easiest band to use is that from 40.66 - 41 MHz and emissions in this band also allow 1 Watt EIRP.
In addition, there are bands on 29.7-29.72, 30-30.0625, 30.3125-31, 36.6-37, 39-39.7625 and 40.25-40.66 MHz each with an allowable power of 100 mW. The frequency range 50 - 54 MHz is part of the amateur radio band. Emissions with bandwidth 100 kHz and powers up to 150 W EIRP are permissible for holders of an advanced amateur radio license. The band from 45 - 52 MHz however belongs to TV channel 0 and must be avoided until I get a license for its use. Finally…………….So on ..........(download any of the following links to get complete paper presentation in word document)
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