Help...Cannot work with fast changing signals.

[ronyang@RP]

Hi all:

I have experience with Xilinx FPGA, but I am a newbie to Red Pitaya. I followed the fpga examples on Github and by simply writing
always @(posedge dac_clk_1x)
begin
dac_dat_a <= adc_dat_a;
end
The acquired data from ADC is passed to DAC directly. This simple bypassing program works well for a 15MHz sin wave. However, here's what I get for a 1MHz square wave input.


Seems part of the board does not work with TTL pulse, or, any signal with large time derivative.
Ten I did a second experiment: I feed a smooth 2MHz sin waveform into the ADC. Output signal this time is generated by a one-to-one mapping from the digitized input signal: a lookup table. I did this just to control the time derivative of the output signal.
I tried several lookup tables such as numerical inverse and exponential function. It turns out that oscillation occurs only when the output signal has a large time derivative. For instance, the following image shows Output = 1-log(Input)


I got really confused by this as I was about to use the board for some MHz application. Can anyone offer an explanation?
Thanks!
-Ron

[Nils Roos]

Hi Ron,

do you have the appropriate termination on the DAC output at the point where you measure?

Regards
Nils

[ronyang@RP]

Hi Ron,

do you have the appropriate termination on the DAC output at the point where you measure?

Regards
Nils

Hi Nils:

Thanks a lot for reminder. I added a 50Ohm terminator to the oscilloscope and it helped!
The terminator removes oscillation at the second case, where input is smooth while output is wiggly, but it doesn't help when I feed a square wave into the board. I tried both the 50Ohm and HighZ output port on my function generator and they look the same. Do you think it's still a impedance mismatching problem?

Best,
Ron

[Nils Roos]

Hi Ron,

this ringing looks like what you get with an uncompensated oscilloscope probe. Do you connect your function generator directly to the Red Pitaya or do you use a probe connected to the RP's input ?
The easiest way to get rid of it is to measure a calibration source - most conventional oscilloscopes have one built in - with the oscilloscope app on the Red Pitaya. If the signal shows the same ringing, adjust the compensation of the probe to minimize it while still showing good signal edges.
Then use the compensated probe on the 50ohm output of your generator (with termination).