Introduction toSDR

Software Defined Radio

By:

Ken Pokigo, KC2AYK

Basic Intro to SDR:

We will limit this introduction to receiver architecture onlyWe wont talk about complex superhetordyne receivers such as double or triple conversionSimple intro, simple math, will stay away from explanations of complex numbers

Radios before SDRThe basis of SDR I/Q

Basic RF Signals:

Basic concept of FM radio:

• We can only hear less than 20 KHz, many of us not even that High• FM radio frequency (RF) band is: 88.0 MHz – 108.0 MHz• Lets take for example a favorite station at 100.7 MHz

How do we get the FM 100.7 MHz radio waves reduced down to audio that we can hear?

Generating an FM signal:

To transmit the audio frequency range over the RF airwaves we modulate the 20 KHz audio signal onto the RF carrier of 100.7 MHz

Time domain FM signal

To get the audio signal onto the carrier we modulate.

FM Signal:

Carrier FrequencyF1

F1-F2 F1+F2

100.7 MHz

The 2 sidebands contain the sound signal

Frequency domain FM signal

Looking at the FM signal across a band of frequenciessuch as seen on a spectrum analyzer

Amplitudes of frequency components….

Basic radio receiver:

Mixer

Local Oscillator

Frequency 1

Frequency 2

F1 + F2F1 - F2

F1 = 100.7 MHz

F2 = 90.0 MHzF1 - F2 = 10.7 MHzF1 + F2 = 190.7 MHz

To get the audio signal back from the carrier we must de-modulate.

Constant we apply

All that’s left is the station’s signal

10.7 MHz, (F2) is called the intermediate frequency, IF

This method has a great advantage –we can us a fixed frequency filter of 10.7 MHz

Filter

Fc=10.7 MHzBW=150KHz

Basic analog radio receiver:

Mixer

Local Oscillator

Frequency 1

Frequency 2

F1 + F2F1 - F2

F1 = 103.7 MHz

F2 = 93.0 MHzF1 - F2 = 10.7 MHz

Filter IF fixed at 10.7 MHZ

Lets say that now we wanted to listen to another station at 103.7 MHz

93.0 MHz

FMDemodulator

Audio AMP

Speaker

Fc=10.7 MHzBW=150KHz

To get the audio signal back from the carrier we must de-modulate.

We have just created the block diagram for the basic Superheterodyne receiver!

Mixer

Local Oscillator

Frequency 1

Frequency 2

F1 - F2

10.7 MHzFilter

IF fixed

93.0 MHz

FMDemodulator

Audio AMP

SpeakerFc=10.7 MHzBW=150 KHz

103.7 MHz

• Very basic version of the superheterodyne or superhet receiver. • Many are far more complicated and can have ave more than one frequency conversion.• Other areas of additional circuitry are added to provide the required levels of performance.• Basic superheterodyne concept remains the same: using of mixing the incoming signal with a locally generated

oscillation to convert the signals to a new frequency.

Items needed for the FM radio transmission and reception:

• Modulator• Mixers• Frequency generator• Filter• Demodulator

Summarizing

• Radio can be totally configured or defined by the software• RF signal is immediately converted to a digital format, and the signal is then processed totally

digitally• Advantage is that the radio can be totally reconfigured for a new application, simply by

changing the software• Common hardware platform can be used across a variety of different products, reducing costs

SDR

Benefits:

• Radio can be totally configured or defined by the software• RF signal is immediately converted to a digital format, and the signal is then processed totally digitally• Advantage is that the radio can be totally reconfigured for a new application, simply by changing the

software• Common hardware platform can be used across a variety of different products, reducing costs

SDR•Software Controlled Radio: Radio in which some or all of the physical layer functions areSoftware Controlled. In other words this type of radio only uses software to provide control of thevarious functions that are fixed within the radio.

•Software Defined Radio: Radio in which some or all of the physical layer functions are SoftwareDefined. In other words, the software is used to determine the specification of the radio and whatit does. If the software within the radio is changed, its performance and function may change.

Digitizing radio signals:

ADC

Analog RF signal

F1 = 100.7 MHz

Digitalsignal

ADC must be super-fast in conversion so we can get a super-fast sample rate in order to digitize the radio signal !

Once its digitized, we can perform the remaining radio receiver functions in software. (SDR)

Digitizing radio signals:

ADC

Analog RF signal

F1 = 100.7 MHz

Digitalsignal

ADC must be super-fast in conversion so we can get a super-fast sample rate in order to digitize the radio signal !

SDR Functions

The problem is: to do A/D conversion at rates high enough to adequately sample RF, it just gets too expensive. $$$

To sample a signal properly, you must be able to sample at the Nyquist Rate.

This means that whatever amount of frequency bandwidth (BW) you’d like to capture, you must sample at a rate at least 2BW.

Sampling

Digitizing radio signals:

ADC

Analog RF signal

F1 = 2400 MHz

If we wanted to capture a WIFI signal:F1=2400 MHz

Digitalsignal

We can use a mixer to reduce the frequency

Mixer

Local Oscillator

F1 = 2389.3 MHz

F1-F2 =10.7 MHz

Much nicer and less expensive to deal with.

We would still need at least a 22 MHz sampling rate!

There has to be a better way to do this!

SDR Functions

Digitizing radio signals:

ADC

Analog RF signal

Digitalsignal

Mixer

Local Oscillator

F1-F2

Wouldn’t it be nice to get this IF down to 0 Hz?

SDR Functions

F1 = 100.7 MHzF2 = 100.7 MHz

F1 - F2 = 0 MHz

Reducing the IF:F1 = 100.7 MHzF2 = 100.7 MHz

F1 - F2 = 0 MHz

10.7 MHz 0 Hz

0 Hz

Now we only need to sample fast enough to capture our audio frequency BW of 150 KHz

150 KHz

Problem:In the Analog world these negative frequencies don’t exist!

We would actually lose ½ of the signal content.

0 Hz

The concept of using complex numbers:

i

To represent these fictious negative frequencies we create a mathematical figure called “i”, and can eliminate the negative part. (sometimes j)

i represents complex frequencies

0 Hz

The concept of using complex numbers:

i

To represent these fictious negative frequencies we create a mathematical figure called “i”, and can eliminate the negative part. (sometimes j)

i represents complex frequencies

Mathematicians can easily work with the complex numbers and create all kinds of formulas that can be represented and dealt with in software!

We can now use these concepts to allow us to work with low-cost ADCs and be able to digitize and capture without losing any signal content at all.

But how do we make deal with this in hardware so we can use the math.

Quadrature Demodulator:

Mixer

Mixer

LO

90 Phase Shift

RF

LP

Filter

LP

Filter

ADC

ADC

Q

I

I

Q

SDR interface:

• 2 signals are sampled and captured• I (horizontal)• Q (vertical)• They now represent the RF signal

This creates the I/Q signal standard for SDR (similar to mp3 for audio)

The only thing is we will need 2 ADCs in this case but they can be simple, low-cost due to the low sample rate required.

Q/I Information:

I

Q • I/Q data shows the changes in the magnitude (or amplitude) and phase of a wave.• If the amplitude and phase change in an orderly fashion, you can use these changes to encode

the information on a sine wave (or modulate it)

Sine Wave Equation

The only variables you can manipulate to encode or decode the sine wave are: • Amplitude• Frequency• Phase

What does it tell us?

Polar representation of sine wave

• Amplitude and Phase are represented

• Recurrance over time dictates the frequency

Fc of 1Hz = 2 (rad/sec)

Q/I Data:

Polar representation of sine wave Polar representation and I/Q

I/Q data is merely a translation of amplitude and phase data from a polar coordinate system to a Cartesian (X,Y) coordinate system

Using trigonometry can convert the polar coordinate sine wave information into Cartesian I/Q sine wave data and vice versa.

If I/Q data is sampled and captured over a wide range of frequencies, we can re-create the complete signal using math and software.

Frequency domain representationTime domain analog signal

Spectrum Analysis View

What do we mean by SDR:

Software-defined radio (SDR) is a radio communication system where components that have been traditionally implemented in hardware (such as mixers, filters, amplifiers, modulators/demodulators, detectors, etc.) are instead implemented by means of software on computer system.

Filtering and demodulation are handled by the software.

SDR:

Mixer

Mixer

LO

90 Phase Shift

RF

LP

Filter

LP

Filter

ADC

ADC

Q

I

USB

Many SDRs will use USB as a transport to a computer.

Embedded computer or

PC

The computer runs software to do the other functions, like signal display and demodulation

SDR:If we use the quadrature demodconcept along with faster ADCs, we can capture a lot of RF spectrum bandwidth at once.

150 KHz

20 MHz

Like the entire FM broadcast band 87.8 MHz – 108.0 MHz

88.0 MHz 108.0 MHz

We would not need a superfast ADC to digitize the whole FM broadcast spectrum, and all channels at the same time!

150 KHz 150 KHz 150 KHz 150 KHz

Very low cost ($10)

10 MHz

2 MHz

Low cost ($100)

Higher cost ( $325)

RX and TX:

I

Q

• I/Q can be recorded or played back, just like with mp3 files

Storage

ReceiveRecord

I/Q

PlaybackTransmit

I/Q

The Fourier transform does the magic.

A Fast Fourier Transform or FFT is a mathematical technique for converting a signal from the time domain into the frequency domain.

• The RF signal I/Q data is captured in the time domain but can

be converted to the frequency domain.

• In the frequency domain the signal can be displayed as

frequency components.

• This is how the spectrum view is created and displayed

What do we do with this I/Q Data?

For those interested in the Math:

(We wont show you the formula, because we don’t want to loose your interest)

Frequency domain FM signalNarrowband FM has a limited modulation bandwidth.There are positive and negative frequency components.carrier as sin(ωCt)

Closer look at the frequency domain signal:

Time

Sample of broadcast RF spectrum, with visible FM station at 98.1 MHz

Digitized spectrum look from an SDR:

Frequency Spectrum View

Signal Amplitude(at each Freq)

(dBm)

Remember lower negative numbers (are actually higher in signal level.

Waterfall Display

Time

What is the “waterfall”:

The waterfall shows the frequency spectrum view over time…

View of the waterfall in a 3D format

(As viewed from above)

SDR Console Software:STARS UHF Repeater Signal

Signal Amplitude

Frequency Tuned to

TX and RXRX onlyRX only

$10-$25 $100 $300RX only

• Dual Receivers• Gigabit ethernet interface• Tuning: 70 MHz – 6 GHz• ADC: 12 bit• Sample rate: 233 Ksps – 61.4 Msps• Viewable BW: 50 MHz (per receiver)

$1000s

• 1 Receiver or Transmitter• USB interface• Tuning: 1 MHz – 6 GHz• ADC: 8 bit• Sample rate: 2 Msps – 20 Msps• Viewable BW: 20 MHz

• 1 Receiver• Integrated preselector filters• USB interface• Tuning: 1 MHz – 6 GHz• ADC: 12 bit• Sample rate: 2 Msps – 10.6 Msps• Viewable BW: 10 MHz

• 1 Receiver• USB interface• Tuning: 500 kHz – 1766 MHz• ADC: 8 bit• Sample rate: 2 Msps• Viewable BW: 2 MHz

SDR Hardware Available:

You get what you pay for!

SDR Software:

General purpose receive

• SDR Sharp• SDR Console• HDSDR• GQRX• SDRunoAll have waterfall and demodulators for all kinds if signals

SDR Software:

Special purpose

• RTL_433 – listen to sensors• DAB player – decode and listen to digital radio• RTL1090 – decode ADSB and track aircraft

Testing and Analysis• RTL_SDR – wide spectrum analyzer• RTL_Panorama– wide spectrum analyzer• Radio Hacker – hacking digital signals• IMSI-catcher - for mobile phone tracking

Ro

ute

r

SDR Receiver

Computer

USB

SDR receiver control and I/Q data capture

Stars clubhouse network

Remote SDR configuration:

SDR console Server Running

Your Home Hamshack PC

Running SDR Console remote client

THANK YOU http://wb2elw.net/