[USRP-users] (Mightbe) an impedance mismatching problem

Jeon sjeon87+usrpusers at gmail.com
Tue Jul 28 08:48:38 EDT 2015


I am building a photo-detecting circuit, whose output will be fed into a
USRP N210. (Also have USRP1, E100, B200... But, currently I am using N210.)

Here's a circuit that I built:



To connect with the USRP (drawn as switch in the circuit. It's just a
logical representation whether circuit output is fed into the USRP or not)
I use BNC- Alligator CLIP SP RG58 coaxial cable of 1 meter. Since the USRP
has the SMA port, I also put the BNC JACK to SMA PLUG adapter.
A distributor says that the cable has an impedance of 50 ohm. The pages
below are in Korean. Just in case.

- BNC coaxial: http://eleparts.co.kr/EPX33HV8
- BNCJ-SMAP adapter: http://eleparts.co.kr/EPX33PCF

Back to the circuit, I've tested the circuit with another LED driving
circuit. Without connecting the USRP.



Blue is a state of an LED and yellow is a brightness reading (output of the
op amp).
I think it's not that pretty, but it's not that bad... It's OK...

And then after, I connect the USRP at the output of the op amp. And did the
same test. And the result is:



First, it can be easily found that the blue reading and the yellow reading
are not having a same HIGH/LOW pattern.
In addition, a vertical scale is reduced to nearly 20 %. (The first
oscilloscope has a scale of 500 mV, and the second one ahs a scale of 100
mV.)

With a little knowledge about circuit and hardware, I guess this problem is
mainly caused by impedance mismatch.

In this situation, I am curious whether this phenomenon is an impedance
mismatch or not.
If it is, how can I solve the problem with an arbitrary LED driving
frequency?
(Currently, the LED is driven with a frequency of 500 Hz, but I am planning
to increase the frequency up to 800 kHz and more.)

If it has nothing to do with impedance mismatch, what causes this
phenomenon and how could I solve it?

I apologize in advance if you feel this question does not suit this USRP
users mailing list, but does suit other places such as EE stackexchange.

Regards,
Jeon.
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