Two-three years ago I made my own hobby 0 - 18 MHz function generator with a MAX038 and a PIC MCU.
The power supply scheme was a classic linear dual ± 5V using 7805 & 7905 took out from the drawer.

The decision to use a linear power supply was taken after performing these tests with a linear PS and a MC34063 (the one I had at the time).

Test with the linear PS:

Test with the MC34063 followed by 7805:

As the MC34063 test shows, the signal waveform (with probe to LM7171 output which of course amplifies all the PS garbage - output from MAX038 was the same) was pretty noisy of EMI and I already was expecting this since I know that SMPSs are noisy.

Do you think it's possible to achieve a less noisy waveform using a DC/DC a converter different (and probably better) then the MC34063 ? If so (as I think but depends also on the PCB layout and shielding), which IC would you suggest ? I don't think I would spend around ± 100 $ or more for a SMPS unit.

Thank you in advance.
Regards,
Simon

 

I think you're picking up radio stations on your scope leads. Is the impedance of your measuring circuit low enough to avoid that?

If it is, paralleling ceramic capacitors with your electrolytics will extend your power supply filtering to the MHz range.

 

You could reduce the noise of a DC/DC converter with carefully added LC filtering and ground plane layout, but why do you need the greater efficiency of a DC/DC for such a relatively low power requirement if the linear supplies already works?

 

but why do you need the greater efficiency of a DC/DC for such a relatively low power requirement if the linear supplies already works?

Well, it's just the sunday-idea. Just to make the PCB smaller if possible.

Is the impedance of your measuring circuit low enough to avoid that?

No. The tests were performed with the MAX038 + MCU + opamp (so the minimal setup) on board.

I understand it's better to think to another project and leave the generator as it is.

 

How is your circuit constructed? On a PCB with a ground plane, I hope. I would be interested in seeing a picture of your generator.

Function generators are a hobby of mine. I am working on the design of a 100 MHz one right now.