I connected a USRP 2901 (B210) to an amplifier I designed. The USRP seems to shutdown every time I try to transmit. I guess I messed up with the input impedance matching on the amplifier. Is it the impedance mismatch that is causing the USRP to shutdown? If so, how high a mismatch can it handle?

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – Kevin Reid AG6YO May 29 '18 at 18:40
  • $\begingroup$ (Folks, don't forget that the question asked is the clear factual one of "how high a SWR mismatch can this transmitter tolerate" and not "please advise me on how to troubleshoot my system". Nothing wrong with that, but it belongs in chat or in a new question.) $\endgroup$ – Kevin Reid AG6YO May 29 '18 at 18:40

The B210 / NI-USRP 2901 has no problems with an open end condition or a short: You can't damage the transmitter with its own power. There's no significant PA on that board. So, it can handle any mismatch.

Also, shutting down (whatever that means) isn't a reaction anyone included in the hard- or software in reaction to an impedance mismatch.

  • $\begingroup$ Every time I switch on my PA, the UHD complains of an IO error(in the GNURadio console) and the transmission stops. If it is not the SWR, do you have an idea of what else could be causing this issue? The schematic to the PA is in this question. Ignore that question. I think I fixed it. The USRP is powered solely by a USB cable if that matters. $\endgroup$ – user11096 May 30 '18 at 5:46
  • $\begingroup$ I'd assume switching on your PA either leads to such a significant drop in grid voltage that your computer's USB port drops, it does something very unhealthy DC-wise to the coax, or it has terrible EMI when switching on. What about having your PA turned on, and only afterwards connecting the USRP to it? $\endgroup$ – Marcus Müller May 30 '18 at 7:18
  • $\begingroup$ Thanks for your suggestion. Connecting an external 6V power supply to the USRP fixed it. I am using a laptop so the USB voltage should not have dropped when connecting the PA. But still the problem is solved and thanks for your help. $\endgroup$ – user11096 May 30 '18 at 7:53
  • $\begingroup$ I realized that my PA does have a terrible EMI but I am not sure if that did anything. $\endgroup$ – user11096 May 30 '18 at 7:55
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    $\begingroup$ Interesting situation: The USRP doesn't pull significantly more power the moment you connect a PA to it, but it's solved by connecting a power supply. Maybe there's a high potential difference between what the PA does to the outer conductor of its input and your USB ground == B210's outer coax conductor? Adding an external supply to that mix would (through security measures in the supply) tie the laptop's ground somewhere closer to earth potential, and might buffer any current spikes on the ground. I really don't know. $\endgroup$ – Marcus Müller May 30 '18 at 7:58

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