Arbitrary wavefrom generator design

[elektronchika]

Hello,

a signal generator is very useful tool. As I'm interested in the topic I decided to develop my own. A think to use Digilent Nexys 3 board (Spartan 6 FPGA) for the DDS. Actually, I have the dds design running, using a DIY board with TI DAC904, connected to 2 of PMod connectors. The sample rate is 100Msps.

Next steps are to make amplitude and offset control, output ON/OFF. Am looking for a few MHz bandwidth (up to 10 is good enough for me, and 5Vpp amplitude with +/-2.5V offset control). I decided to use TI DAC5862 dual DAC for amplitude and offset control. One of the dacs will control DAC904 reference voltage, the other one will generate offset. Some op-amp and the schematic is done. I have a few questions concerning the design:

- The Nexys 3 board has 4 PMod connector. But due to impedance matching, is it good idea to use them for 100Msps signals? the board also has VHDCI connector with 20 LVDS pairs. But is needs at least 3 or 4 layers board to route all the pins. And I'm not sure is there pairs can be used as single ended (DAC904 has parallel CMOS interface).
- DAC904 accepts reference voltage from 100mV to 1,25V. So I need post amplification/attenuation of -20dB, 0dB and +20dB in order to be able to have output signals as low and 10mVpp and as high to 10Vpp. What output stage circuit will you suggest me: TI VCA820 with another dac for gain control; simple op amps circuits, which are switches with 5V relays; another VGA/PGA or something else?
- Are high speed op-amps with output disable suitable for output ON/OFF function or small relay is better? I've heard some relays ticking on and off in standard waveform generator when changing amplitude, offset, output on/off, so I suggest the relays are the best choice?
- Some other advises you have?

Regards!

[mathias]

You can try to use the PMod connector, making sure that your clock arrives late enough with respect to the data. That way, possible ringing can die out.