RF generation help!

[saurabh17g]

Hello,
I want to design a circuit which generates a frequency from 1Hz to 20MHz.
amplitude should be 5V p-p.
Moreover the device should have a keypad and LCD interface.
The frequency set by keypad should be outputted by the circuit.
I tried a lot of options using various ICs like AD9833 and other AD ICs. But the current requirement is not sufficient. So please suggest me some thing

 

[Warpspeed]

Only practical way to do something like this is by direct digital synthesis.

ADI - Analog Dialogue | Direct Digital Synthesis (DDS)

 

[JimB]

I want to design a circuit which generates a frequency from 1Hz to 20MHz.
amplitude should be 5V p-p.
I tried a lot of options using various ICs like AD9833 and other AD ICs.
But the current requirement is not sufficient.
So please suggest me some thing

You do not state you current requirements.
An amplifier of some kind seems the obvious solution, but 1hz to 20Mhz is quite demanding, depending on the current of course!

Jim

 

[Vizier87]

I suppose if you use a PIC with a clock of 20Mhz you can get it as close as you want it to be...

 

[saurabh17g]

@Warpspeed
i saw those devices. the Vout is very less around 1V max. so amplification of high freq signal would be a problem.
@Vizier87
how to use pic? do u mean something like creating binary words for phase vs amplitude and appending a DAC.
difficult to get 20Mhz even if i do like that. this is bcos, some instructions(intefacing control signals) will be required and will eat up time. It will have a lot of distortions since its directly generated from microcontroller.
Also, adding LPF to smoothen the sinewave at the end will be a headache.

 

[Vizier87]

well you can increase the clock for the PIC if you want, like 48MHz, 20MHz should be achievable through programming.

 

[mneary]

so amplification of high freq signal would be a problem.
Also, adding LPF to smoothen the sinewave at the end will be a headache.

I would imagine that your instructor has it in mind that you will learn how to solve such problems.

If one chip could be bought to do the whole task, what would you learn?

May we assume that the final project will drive a 50-ohm transmission line?

 

[saurabh17g]

what if i use AD9833 and append an opamp AD8000(works at 1.5GHz and 12 V) to make the output to 10V.
But the question is, whether it would amplify high frequencies at the same amplitude?
also, my friend told me that AD9833 even though datasheet mentions 10MHz, actually gives 100khz which is shocking

 

[Warpspeed]

what if i use AD9833 and append an opamp AD8000(works at 1.5GHz and 12 V) to make the output to 10V.
But the question is, whether it would amplify high frequencies at the same amplitude?(

First get your synthesizer going, then add an output amplifier to it.

There are many video amplifiers that will have sufficient gain bandwidth product to go well beyond 20 Mhz.

 

[mneary]

also, my friend told me that AD9833 even though datasheet mentions 10MHz, actually gives 100khz which is shocking

You would do well to take some time to learn about DDS theory. It constructs the output with samples taken at the Refclk rate. With a Refclk of 25 MHz, a 10 MHz output is going to need a lot of filtering. Your freind is correct in that a 100kHz wave constructed with the same Refclk will appear fairly pure with little filtering.

Keywords: Nyquist, Fourier.

There are more devices in the AD98xx series with higher Refclk values. They'll probably cost more.

 

[Warpspeed]

There are more devices in the AD98xx series with higher Refclk values. They'll probably cost more.

There certainly are, and they are not all that expensive either.

Direct Digital Synthesis (DDS) | RF/IF Components | Analog Devices

 

[mneary]

Wow, prices have come down. Several years ago I was looking at DDS to replace a VCO and the prices killed the project.

 

[Warpspeed]

Wow, prices have come down. Several years ago I was looking at DDS to replace a VCO and the prices killed the project.

Yes, trying to build a stable analog VFO these days is a complete waste of time.
Even decent high quality variable capacitors have become extinct.

A DDS chip, a very basic microcontroller, and a digital shaft encoder, and you are in business.
Nice clean stable sinewaves with digital readout.