Microbit RRC-1258 MkII(s) Manual de usuario descargar pdf (Pagina 18)

Software Defined Radio

oscillator carriers at 0 and -90 degrees and then combines the result, as

shown in Fig 7.5. This makes for a very elegant modulation system that

can handle a huge range of operat-

ing modes at any carrier frequency.

When it comes to reception

with SDR systems, IQ data is the

vital ingredient, as monitoring the

change in IQ values over time will

reveal the modulating message,

regardless of whether it’s a form of

AM, FM, PM or a combination of all

three, thus providing a simple multi-

mode demodulator.

Fast Fourier Transforms

Another important process in the development of SDR and digital signal

processing in general, has been the use of Fast Fourier Transforms (FFT)

to analyse sampled data streams from the ADC. One of the great

operational benefits of practical SDRs is the presentation of band

segments in spectrum analyser style. This enables the operator to

quickly locate active stations within a band and to use the click of the

mouse to tune to that station. Converting the sampled data stream into

a spectrum analyser format is handled by the FFT, so let’s take a quick,

non-mathematical look at how this works.

The FFT is an ingenious mathematical algorithm that can examine

a stream of sampled data from an ADC and separate the stream into its

component frequencies. It does this by dividing a section of the sam-

pled data into a number of bins (containers) each of which contains a

narrow band of frequencies – see Fig 7.6. It then measures the energy in

each bin and it is this data that’s used to feed the spectrum display or

drive other facilities. The size of each bin is dependent on the sample

rate of the digital signal and the number of bins used by the FFT. When

using FFT to analyse an audio signal we might typically have a sample

rate of, say, 44kHz – so that it can accurately capture frequencies as

Fig 7.5: Transmit

up-converter for

baseband IQ

data.

Fig 7.6:

Illustration of

using FFTs to

convert data

samples to FFT

frequency bins.

1 2 ... 13 14 15 16 17 18 19 20 21 22 23 ... 29 30

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