Microbit RRC-1258 MkII(s) Bedienungsanleitung PDF herunterladen (Seite 15)

Computers in Amateur Radio

we ought to take a look at these before moving-on to real-world sys-

tems. They are:

Analogue to Digital Conversion (ADC),

In-phase/Quadrature Data (IQ data), and

Fast Fourier Transforms (FFT).

Analogue to Digital Conversion

As you are no doubt aware, all computer systems talk in numbers, so

before a microprocessor can do anything useful in radio, the analogue

signals need to be digitised. The method of achieving this has remained

stable for some time and involves measuring and storing the instantane-

ous level of the analogue signal at a very high speed. You can visualise

this as taking regular measurements with a digital voltmeter (see Fig

7.1). So how frequently do you think we need to take measurements

(samples) in order to create a realistic

digital equivalent of the original signal?

The fundamental work to determine this

was first published in 1928 as the Nyquist-

Shannon sampling theorem. The theorem

states that you must sample at a rate that

is at least twice as fast as the highest

frequency component you want to digi-

tise. Therefore, to digitise an audio signal

that contains frequencies from 20Hz to

20kHz you need to take measurements at

40,000 times per second, i.e. twice 20kHz.

The other vital point to consider is the accuracy or resolution of each

sample. You can think of this in terms of how many digits you have

available on your digital voltmeter. If you were to use an 8-bit measure-

ment you would have just 256 possible values for each measurement,

which may be a bit too coarse for many purposes. As a result, it is

common practice to use 16-bits or greater for radio applications – a 16-

bit sample contains 65,536 possibilities for each measurement. When

the ADC process is complete, the result is a data stream of 16-bit

numbers that are generated at the sampling frequency. Returning to our

simple audio signal, that would produce a data stream running at 16

(bits) x 40,000 (sample rate) which is 640,000 bits per second. As you

can see, one of the problems with the ADC process is the relatively high-

speed data streams that have to be processed.

So where in the receiver chain do you put the ADC? In many

systems the ADC in the computer’s soundcard is employed and whilst

this works very well, there is so much more you can do if you can

move the digitisation closer to the antenna. At the time of writing,

there have been a number of systems introduced that digitise the

Fig 7,1:

Illustration of the

analogue to

digital

conversion

process.

1 2 ... 10 11 12 13 14 15 16 17 18 19 20 ... 29 30

Kommentare zu diesen Handbüchern

Keine Kommentare

Microbit RRC-1258 MkII(s) Bedienungsanleitung Seite 15

Kommentare zu diesen Handbüchern

Verwandte Produkte und Handbücher für Fernbedienungen Microbit RRC-1258 MkII(s)