Microbit RRC-1258 MkII(s) Bedienungsanleitung PDF herunterladen (Seite 16)

Software Defined Radio

entire spectrum from VLF through to 30MHz or more! Most of these

employ some digitally switched band-pass filtering close to the

antenna, followed by some modest amplification, before sending the

entire spectrum to the ADC.

From my earlier explanation you will know that the sample rate

would need to be twice the highest frequency, i.e. at least 60MHz, with

66.666MHz being a popular choice. When combined with 16-bit sam-

ples, that gives a data rate from the ADC of 16 (bits) x 66,666,000

(sample rate) = 1.067Gb/s. Now that’s a lot of data to process and is

way too fast for most home computers to handle. To tame this data rate

a process known as decimation is employed. This is the digital equiva-

lent of a mixer and local oscillator in a conventional receiver. The deci-

mator reduces the bit rate and extracts a segment of the sampled

30MHz spectrum that can then be passed to the PC for processing (see

Fig 7.2). In most practical designs the size of the extracted segment can

be controlled by

the software

running on the

PC and ranges

from a few tens

of kHz to 10MHz

or more. Rather

than passing a

single data

stream to the PC

for processing,

two data streams are presented, one that’s in-phase and the other that’s

delayed by 90 degrees and is known as quadrature. These two streams

are the In-phase (I) and Quadrature (Q) data mentioned earlier. The

hardware used for this decimation or down conversion has to be ex-

tremely fast and many of the current designs use a Field Programmable

Gate Array (FPGA). This is a large array of high-speed gates and logic

devices on a single chip that can be programmed and interconnected

by software to create highly customised functions.

IQ Data

So, if sampling a signal at the Nyquist sample rate produces a good

representation of the signal, why do we need to bother with In-phase (I)

and Quadrature (Q) signals? Put very crudely, you can imagine IQ-data

as stereo for digital signals. In the same way as stereo audio allows your

ear/brain combination to determine the location of instruments and

provides a sense of space and increased detail, so IQ data facilitates the

extraction of more information from our digitised radio signal. Let’s just

look at IQ data in a little more detail to see how that works.

Fig 7.2: Block

diagram of a

digital HF

receiver.

1 2 ... 11 12 13 14 15 16 17 18 19 20 21 ... 29 30

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