DDS project

[elektronchika]

Hi all,

I have an idea from several years, but I don't have time to complete it. This is the idea - 1 or 2 channel DDS with output frequency 10 to 20MHz, 1Hz precision. The ability to control the amplitude and offset with 1mV precision, with ranges 10Vpp/ +/-10V. Some king of display is needed and buttons to control the frequency, amplitude and offset.
It sounds like an ordinary signal generator. I've done the DDS in Spartan 6 FPGA so far, but I'm thinking about how can I control the amplitude and offset. Here are some ideas:

AC Signal Offset and Amplitude Control Using a Dual Channel Multiplying DAC and a Single I/V Converter
Amplitude Control Circuit for AD9834 Waveform Generator (DDS)


I've worked with Agilent and Stanford research signal generators. But I can hear inside a relay switching when changing the amplitude or offset. Does anyone have an idea how is made the amplitude and offset control in these signal generators? I've seen Dave's teardown Hantek I think generator, but he doesn't say anything about this, or I saw the circuit of it.

So, have a nice day you all!

[dannyf]

DDS with output frequency 10 to 20MHz, 1Hz precision. The ability to control the amplitude and offset with 1mV precision, with ranges 10Vpp/ +/-10V.

You mentioned some fairly common spec (10-20Mhz, 1Hz precision), you mentioned some difficult spec (10Vpp/, or even +/-10v). You mentioned some impossible spec (1mv precision).

You then failed to mention some really important spec, like what kind of distortion you expect the dds to generate, what kind of wave form you want it to generate. what kind of separation you expect ...

As to amplitude control: it can be dome sometimes with the chips selected. Otherwise, you can do it outboard, in analog or digital. The analog approach typically requires a pga + an amplitude detection. The digital approach would be to read the amplitude, adc it and then use that to control a digital pga.

The spec you picked unfortunately is quick difficult to achieve.

[elektronchika]

Well, these are just some numbers, the spec is not critical, because this is a hobby project. Yes, maybe 10mV resolution (I used precision, but I'm wrong) is easier to achieve. My concerns are how to make the amp and offs control.

[dannyf]

PGA or digital pots can be used to control gains and add shifts, assuming more reasonable spec. or old resistive pot - nothing wrong with that in my view.

Resolution is fairly easy to deliver, precision is much harder.