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Software

Radio

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Digital RDigital R

Digital R

eceiver Handbook:eceiver Handbook:

eceiver Handbook:

Basics of Software RBasics of Software R

Basics of Software R

adioadio

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entek, Inc.entek, Inc.

entek, Inc. • One Park Way, Upper Saddle River, NJ 07458 • Tel: (201) 818-5900 • Fax: (201) 818-5904 • Email: digrec@pentek.com • http://www.pentek.com

Digital Receiver Handbook:

Basics of Software Radio

Fourth Edition

entek, Inc.entek, Inc.

entek, Inc.

One Park Way, Upper Saddle River, New Jersey 07458

Tel: (201) 818-5900 • Fax: (201) 818-5904

Email: digrec@pentek.com • http://www.pentek.com

Contents of this publication may not be reproduced in any form without written permission.

Specifications are subject to change without notice.

Pentek, GateFlow and VIM are a registered trademarks of Pentek, Inc.

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ApplicationsApplications

Applications

roductsroducts

roducts

odger Hodger H

odger H

. Hosking. Hosking

. Hosking

Vice-President & Cofounder of Pentek, Inc.

Digital RDigital R

Digital R

eceiver Handbook:eceiver Handbook:

eceiver Handbook:

Basics of Software RBasics of Software R

Basics of Software R

adioadio

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entek, Inc.entek, Inc.

entek, Inc. • One Park Way, Upper Saddle River, NJ 07458 • Tel: (201) 818-5900 • Fax: (201) 818-5904 • Email: digrec@pentek.com • http://www.pentek.com

AMP

LOCAL

OSCILLATOR

DEMODULATION

(Detector)

AUDIO

AMPLIF IER

IF AMPLIFIER

(Narrowband Filter)

ANTENNA

SPEAKER

MIXER

Figure 1

The conventional heterodyne radio receiver as seen

in Figure 1 has been in use for nearly a century. Let’s

review the structure of the analog receiver so comparison

to the digital receiver becomes apparent.

First the RF signal from the antenna is amplified,

typically with a tuned RF stage which amplifies a region

of the frequency band of interest.

This amplified RF signal is then fed into a mixer

stage. The other input to the mixer comes from the local

oscillator whose frequency is controlled by the tuning

knob on the radio.

The mixer translates the desired input signal to the

IF (intermediate frequency). See Figure 2.

The IF stage is a bandpass amplifier which only lets

one signal or radio station through. Common center

frequencies for IF stages are 455 kHz and 10.7 MHz

for commercial AM and FM broadcasts.

The demodulator recovers the original modulating

signal from the IF output using one of several different

schemes.

For example, AM uses an envelope detector and FM

uses a frequency discriminator. In a typical home radio,

the demodulated output is fed to an audio amplifier and

then to a speaker.

sig

MIXER TRANSLATES

INPUT SIGNAL BAND

to IF FREQUENCY

LOCAL

OSCILLATOR

= F

sig

- F

IF BW

Figure 2

Analog RAnalog R

Analog R

eceiver Block Diagrameceiver Block Diagram

eceiver Block Diagram

Analog RAnalog R

Analog R

eceiver Mixingeceiver Mixing

eceiver Mixing

The mixer performs an analog multiplication of the

two inputs and generates a difference frequency signal.

The frequency of the local oscillator is set so that the

difference between the local oscillator frequency and

desired input signal (the radio station you want to

receive) equals the IF (intermediate frequency).

For example, if you wanted to receive an FM

station at 100.7 MHz and the IF frequency is 10.7 MHz,

you would tune the local oscillator to:

100.7 - 10.7 = 90 MHz

This is called “down conversion” or “translating”

since a signal at a high frequency is shifted down to a

lower frequency by the mixer.

The IF stage acts as a narrowband filter which only

passes a “slice” of the translated RF input. The band-

width of the IF stage is equal to the bandwidth of the

signal (or “station”) that you are trying to receive.

For commercial FM, the bandwidth is about 100

kHz and for AM it is about 5 kHz. This is consistent

with channel spacing of 200 kHz and 10 kHz, respectively.

TheorTheor

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y of Operationy of Operation

y of Operation

Digital RDigital R

Digital R

eceiver Handbook:eceiver Handbook:

eceiver Handbook:

Basics of Software RBasics of Software R

Basics of Software R

adioadio

adio

entek, Inc.entek, Inc.

entek, Inc. • One Park Way, Upper Saddle River, NJ 07458 • Tel: (201) 818-5900 • Fax: (201) 818-5904 • Email: digrec@pentek.com • http://www.pentek.com

DIGITAL

LOCAL

OSCILLATOR

DIGITAL

LOWPASS

FILTER

AUDIO

AMP

SPEAKER

AMP

ANTENNA

A/D

CONV

DS P

(DEMODU-

LATION)

D/A

CONV

MIXER

TRANS-

LATOR

Figure 3

0fo fs/2 fafs

Aliased image at fs - fa

Figure 4

TheorTheor

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y of Operationy of Operation

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(continued)(continued)

(continued)

Digital RDigital R

Digital R

eceiver Block Diagrameceiver Block Diagram

eceiver Block Diagram

Take a look at the digital receiver block diagram

shown in Figure 3. Note the strong similarity to the

analog receiver diagram—all of the basic principles of

analog receivers still apply.

Right after the RF amplifier and an optional RF

translator stage, we use an A/D (analog-to-digital)

converter to digitize the RF input into digital samples

and all of the subsequent mixing, filtering and demodu-

lation are performed using digital signal processing

elements.

Before we continue, lets first review a theorem

fundamental to sampled data which lays the foundation

for the A/D converter requirements.

Nyquist’s Theorem:

“Any signal can be represented by discrete samples if

the sampling rate is at least twice the bandwidth of

the signal.”

For example, if we use an A/D converter sampling at

70 MHz, then the bandwidth of the analog input must

be less than 35 MHz.

Now let’s see what happens if we ignore Nyquist’s

criterion.

AliasingAliasing

Aliasing

Figure 4 shows a frequency display of a system being

sampled at frequency fs. For all input signals below fs/2,

such as the one at fo, we fully meet the Nyquist crite-

rion. In fact, any number of signals can be present in the

shaded region and all will be correctly represented in the

sampled data.

But if we have a signal present at say, fa, which is

above fs/2, the sampling process will generate an aliased

image which will appear in the sampled data at fs - fa.

This image cannot be distinguished from a true signal

which might have been present at that same frequency.

The point is, that once an aliased image is created in

the sampling process, no amount of further processing

can distinguish between a true signal and an aliased

signal.

Therefore, it is imperative to prevent aliasing before

it occurs.

Digital RDigital R

Digital R

eceiver Handbook:eceiver Handbook:

eceiver Handbook:

Basics of Software RBasics of Software R

Basics of Software R

adioadio

adio

entek, Inc.entek, Inc.

entek, Inc. • One Park Way, Upper Saddle River, NJ 07458 • Tel: (201) 818-5900 • Fax: (201) 818-5904 • Email: digrec@pentek.com • http://www.pentek.com

0fo fs/2 fafs

Low Pass Filter Response

Figure 5

DIGITAL

LOCAL

OSCILLATOR

DIGITAL

LOWPASS

FILTER

AUDIO

AMP

SPEAKER

AMP

ANTENNA

A/D

CO NV

DSP

(DEMODU-

LATION)

D/A

CONV

MIX ER

DIGITAL RECEIVER CHIP

TRANS-

LA TO R

CLOCK

Figure 6

TheorTheor

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y of Operationy of Operation

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(continued)(continued)

(continued)

Anti-Anti-

Anti-

Aliasing FAliasing F

Aliasing F

ilterilter

ilter

The most straightforward way to prevent aliasing is

to use a low pass filter before the A/D converter which

removes all signals above fs/2.

This filter is called an anti-aliasing filter seen in

Figure 5. Now the signal at fa is blocked so the A/D

converter never sees it.

Anti-aliasing filters are often included on the same

board as the A/D converter as a convenience for the user.

As a side note, Nyquist’s criterion can also be met by

limiting the bandwidth of the sampled signal using other

types of filters.

For example, suppose we really wanted to receive

signals between fs/2 and fs in the above diagram. If we

used a bandpass filter with a passband from fs/2 to fs, we

would fully meet the Nyquist criterion because the

bandwidth is equal to one half the sampling rate.

Once the sampling is done, the band of signals from

fs/2 to fs is “folded” into the frequency band from DC to

fs/2. The half-sampling frequency is often called the

“folding frequency.”

This technique is sometimes called “under-sam-

pling” and while this works well in theory, care must be

taken in actual practice to ensure that the A/D

converter supports the higher input frequencies it must

handle.

Digital RDigital R

Digital R

eceiver Block Diagrameceiver Block Diagram

eceiver Block Diagram

Looking again at the overall block diagram, the

digital A/D samples coming out of the A/D converter

are being fed to the next stage which is the digital

receiver chip (in the dotted line as shown in Figure 6).

The digital receiver chip is typically contained on a

single monolithic chip which forms the heart of the

digital receiver system. It is also sometimes referred to as

a digital down converter (DDC) or a digital drop

receiver (DDR).

Inside the digital receiver chip there are three major

sections:

• Local Oscillator

•Mixer

• Decimating Low Pass Filter

Note that the inputs to the digital chip are the

digital samples from the A/D and the A/D sample clock.

With a 70 MHz A/D, samples are fed into this chip and

processed in real time at rates up to 70 MHz!

We will now explore each section of the digital

receiver system shown in Figure 6, starting with the

digital local oscillator.

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redbud04

2012-10-30

没有看到前面的评论，确实只是产品参考设计手册，不少需要的资料，不过，内容还是写得比较清楚。

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