Software Defined Radio

How I learned to love DSPTom McDermott, N5EG

October 2, 2008

How we’d like to build a software receiver

7,021 kHz

Problems:• A-to-D Converter in our PC only samples at 48,000 samples per second.

We need > 16 Million Samples per second.• ADC in our PC only has 80~85 dB dynamic range (about 14 effective bits).

We need > 130 dB of dynamic range (about 22 effective bits).

1 kHz

Direct Conversion Receiver

VFO

3 kHz LowPass Filter

Mixer

AudioAmplifier

7,020 kHz

7,021 kHz 1 kHz7,024 kHz 4 kHz

1 kHz

Problem:Hears both upper and lower sideband at the same time. QRM! QRM!

7,019 kHz 1 kHz7,016 kHz 4 kHz

1 kHzA-to-D

Converter+ DSP

Solution:Analog mixer downconverts RF to Audio.

We can sample at low rate.Low pass filter limits bandwidth (helping dynamic range)

Single-signal Direct Conversion ReceiverUsing the Phasing Method

VFO

3 kHz LowPass Filter

Mixer

7,020 kHz

7,021 kHz 1 kHz7,024 kHz 4 kHz

1 kHz7,019 kHz 1 kHz7,016 kHz 4 kHz

1 kHz

Solution:Two analog mixers using phasing technique

downconverts RF to Audio. I = In-phase VFO, Q = Quadrature phase VFO.

Use PC Left-audio-channel to digitize IUse PC Right-audio-channel to digitize Q.

I

90° PhaseShift

3 kHz LowPass Filter

Q90° Phase

Shift7,021 kHz 1 kHz7,024 kHz 4 kHz

1 kHz7,019 kHz 1 kHz7,016 kHz 4 kHz

1 kHz1 kHz1 kHz (inverted)

(The PC can do this in software)

1 kHz

Softrock receiver implementation

4-phaseVFO

TayloeMixer

0°

90°

180°

270°

LPF

LPF

LPF

AudioAmplifiers

I

I*

Q*

Q

Left

Right

Low Pass Filters:• Need to filter < ½ the computer soundcard sample rate.

About 20 kHz for 48,000 sps.• Provides + 20 kHz and - 20 kHz, or total 40 kHz wide

spectrum without tuning the VFO (the computer can do it all in software).

A 24 bit soundcard works much better than 16 bit.A 192,000 sps soundcard and 80 kHz LPF would give

~160 kHz spectrum without tuning the VFO.

7,020 kHz

Softrock transmitter is just the inverse.

Softrock Kit KC0WOX: Let's Build A Softrock 40m/80m RXTXv6.2

Building & testing the kit – a portion at a time

• Installing parts one section at a time.

• View of board top and bottom after each step.

• Test each portion after that assembly step.

KC0WOX: Let's Build A Softrock 40m/80m RXTXv6.2

Phasor Phaser

A SINE wave is the vertical component of the rotating phasor. A COSINE wave is the horizontal component of the rotating phasor. Whether the phasor rotates clockwise or counterclockwise, any single view looks identical for either.

Thus: any one waveform is the SUM of ½ the positive frequency plus ½ the negative frequency. It’s why mixers give us sum and difference products.

If we know both components over the same period of time then we can tell which direction the phasor is rotating. Now we have one, not two frequencies, and separate USB from LSB.

These are the I (In-phase) and Q (Quadrature-phase) components. jejPhasor sincos)( (Euler’s formula, 1748)

jejPhasor sincos)(I

Q

cos

sin