Sines, squares and triangles up to 10 MHz

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1 About The Author

1.1 Ibrar Ayyub

A few years ago I needed a function gen

–

erator for my home laboratory. In my

job I had worked with some expensive

commercial models and initially I had

planned to buy one of these. However,

none of them really was what I wanted

– too complex for simple use – and so I

decided to design and build my own. It

turned out to be DDS (direct digital syn

Sines, squares and triangles up to 10 MHzSines, squares and triangles up to 10 MHz

–

thesis) based but that was not the only

component selection issue I ran into. Here

are a few more.

My components, your

components?

After researching various techniques I

settled on a Direct Digital Synthesis (DDS)

based architecture for my project. DDS

employs a digital oscillator with quartz

crystal precision to accurately generate

sinewaves up to very high frequencies.

For the microcontroller I chose one from

Analog Devices. Although well known

for their Digital Signal Processor (DSP)

families, this may not be the first MCU

manufacturer that comes to mind. They

do have a nice family though of 32-bit

ARM controllers supported by something

I like a lot: comprehensive documenta

–

tion. Unlike other MCU manufacturers that

sometimes need more than one thou

sand pages to elucidate their devices,

Analog Devices manages to fit a com

–

plete description of a complex microcon

–

troller into a document of slightly more

than a hundred pages. Analog Devices

also specializes in DDS chips, operational

amplifiers and other analog support chips

and so made for a great one-stop shop

for this project. Their friendly sampling

service quickly got me started without

forking out a lot of money.

The

MCU

I selected was the

ADuC7024B

–

STZ62

, a member of the Precision Ana

–

log Microcontroller family in a 64-pin

package, containing an ARM7TDMI core

running at 44-MHz clock speed. These

MCUs are called analog because they

feature analog inputs and outputs (yup,

ADC and DAC) and an analog compara

–

tor. Besides, they sport PWM, timers and

standard serial ports (SPI, UART, I²C).

Our micro has 8 KB of RAM and 62 KB of

flash memory that can be programmed

in-circuit over a serial port. Note that

some types use I²C for flash memory

programming, so make sure you get the

specified type and not one with a slightly

different part number.

The

DDS chip

for the project is the popu

–

lar

AD9834

. The maximum frequency of

the external oscillator is 75 MHz, allowing

a maximum output frequency of 37.5 MHz

(half the clock frequency). The downside

of such a fast clock signal is a frequency

resolution of just 0.28 Hz owing to the

chip’s 28-bit integer frequency divider.

That doesn’t sound like a big deal, but

it does equate to almost 0.6% at 20 Hz.

To improve this, the clock frequency can

be lowered, albeit at the expense of the

maximum achievable output frequency.

At 1 MHz for instance the resolution

becomes 0.004 Hz, which corresponds

to an error of 0.005% at 20 Hz, but the

maximum frequency is then down to a

measly 500 kHz. In this project the DDS

clock frequency is 75 MHz to ensure a

For more detail: Sines, squares and triangles up to 10 MHz

About The Author

Ibrar Ayyub

I am an experienced technical writer holding a Master's degree in computer science from BZU Multan, Pakistan University. With a background spanning various industries, particularly in home automation and engineering, I have honed my skills in crafting clear and concise content. Proficient in leveraging infographics and diagrams, I strive to simplify complex concepts for readers. My strength lies in thorough research and presenting information in a structured and logical format.

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