0

I'm trying to send a signal from a Red Pitaya's fast analog output through a very long RG174 cable. The signal comes out quite noisy and somewhat attenuated on the other side (purple is the signal at the Red Pitaya output, yellow is the other side of the cable):

enter image description here

When I send a similar signal from a function generator through the same cable, it comes out perfectly clean (signal at the function generator output is purple, yellow is the other side of the cable):

enter image description here

I'm wondering if anyone can help me understand what the difference between the Red Pitaya and the function generator outputs are (both are 50 ohm impedance outputs), and how I can produce such a signal from the Red Pitaya.

Zurn
  • 23
  • 3
  • 1
    It must be terminated with 50 Ohms to prevent baseband reflections, it's a DSO option on most, otherwise a BNC T connector allows inserting a 1/4W resistor near 50 ohms. The 2nd signal is actually lower BW so echos are not ringing as much – Tony Stewart EE75 May 09 '22 at 20:20
  • Repeat with "the terminator" ;) enabled and expect cleaner waveforms at half amplitude. Unterminated coax acts like an antenna to radio waves and reflections at 1/4wave length – Tony Stewart EE75 May 09 '22 at 20:33
  • with 4 cycles per 1us in coax at c=2e8m/s $\lambda/4 = c/4f$ = 2e8m/s /1e6cyc/s= 200 m cable for a 1/4 wave resonance at 1MHz, 3,5,7 etc. The cable itself depends on size and quality will also have dB/100m loss vs f specs or dB/100ft – Tony Stewart EE75 May 09 '22 at 20:40
  • This looks like classic reflections issues. Look at the rise time on the troubled signal vs the sine wave. – Aaron May 09 '22 at 22:08
  • Aha... you're certainly right about the reflections not present due to the sine vs square wave rising times. I've added a 50 ohm resistor to the receiving end and it looks much cleaner, though still attenuated. A promising development! – Zurn May 10 '22 at 15:02

0 Answers0