[USRP-users] DC biased output from LFTX DB

Marcus D. Leech mleech at ripnet.com
Mon Jul 20 16:26:25 EDT 2015


On 07/12/2015 09:01 AM, Jeon via USRP-users wrote:
> I've built a such GNU Radio flow graph:
>
>
>
> I've connected USRP N210 with LFTX - SMA to alligator clip cable - 
> oscilloscope to see what comes from the USRP. Nothing else is 
> connected with them.
>
> And I've got the following waveform:
>
>
>
> Please ignore the blue readings.
>
> Yellow Max - Min says 800 mV and the amplitude will be 400 mV. Well, 
> it's not that good, but it is okay and acceptable.
>
> But the offset doesn't seem to be right. I expect a square wave form 
> swinging from -400 mV to 400 mV, but what I've actually got is, a 
> square wave form swinging from -180 mV to 620 mV.
>
> Since, my application doesn't need a highly DC balanced signal, it's 
> not a big deal but I am just curious about it.
>
> In addition, if I increase amplitude in the flow graph, a square wave 
> form becomes very unstable. It fluctuates and goes up and down when it 
> is supposed to show high voltage.
>
> And this problem makes me feel bad. I want to increase the amplitude 
> as high as possible.
>
> Is my LFTX DB in some troubles?
>
> Regards,
> Jeon.
>
>
Two immediate issues I see here:

You're running the USRP sink at 32ksps, which isn't a valid sample rate 
for N2xx.

I wouldn't expect to get more than about 2V P-P from the LFTX, since 
it's largely just giving you (buffered) what the DAC can produce.

Not sure about the DC offset, but fix the sample-rate issues first, and 
then see where you're at.



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