USRP B210 problems

[BastiRapp]

Hi there,

I am using an USRP B210 and I basically only need to transmit a rf signal with 2-3 Ghz and I don't seem to get the hang of it. I installed everything according to the getting-started guide and when I use uhd_find_devices it finds my board. Now when I want to transmit (gnuradio unfortunately doesn't work which is another problem) I use tx_waveforms or tx_samples_from_file. Here I have 2 questions:

1. What exactly is the difference between the center frequency and the wave frequency here and why do I need to specify both when I just want e.g. a signal of 2 GHz? The wave frequency seems to be limited by the sample rate, so how exactly does it translate to the center frequency?

2. When I use tx_waveforms and I plug it in to the oscillator even with maximum gain of 89 the signal amplitude is very low, am I doing sth wrong? I am typing in: 

tx_waveforms --freq 915e6 --rate 5e6 --gain 89 --wave-type SINE --ampl 1 --wave-freq 1e6

So in essence, what is a simple way to get a nice 2-3 GHz rf signal?

Thanks in advance

Bastian 

[ni_hj]

Hi Bastian,

You can set carrier frequency from 70 MHz – 6 GHz, where you set now to 915MHz (--freq 915e6). But for bandwidth, USRPB210 can support bandwidth up to 56MHz, where you set it to 1MHz now (--wave-freq 1e6).

So, I hope you understand difference between carrier frequency and bandwidth.

BTW, the sample rate you set as 5Msps (--rate 5e6), can be increase up to 61.44Msps.

- Try to decrease gain value to 20 or 30. (I think you can not set gain value to 89, the max gain is around 25 to 30 for different USRP hardware)

- Please check if you connect antenna to correct TX port.

- What do you mean by "I plug it in to the oscillator"?

Do you have any error when you run this command, if yes, you could share for more discussion. You may also share picture of your hardware setup to check hear. 

[BastiRapp]

Hi,

thanks for the reply. Ok so the center frequency is that of the rf signal and the wave-freq here is the bandwidth around it?

Sorry, I meant oscilloscope. I connected the output tx port to it to see the signal. When I select a small gain I see no real signal above the noise in the FFT, only when I apply the maximum gain of 80-90. I will attach some pictures.

setup:

setup

gain=80:

gain=80

gain=20: no clear signal

gain=20 no signal above noise

[ni_hj]

It is not safe to directly connect RF output of USRP to oscilloscope, it is better to use attenuator in between.

set the carrier frequency to 915MHz and the bandwidth of the oscilloscope to the same value you have in your command, they must be same.

again, 80-90 is seems not correct value for gain.

try to use other TX port, may be you are receiving leak signal from other TX!

[BastiRapp]

Ok hm. I also tried it with an antenna and picked up the signal with a network analyzer. Here it was the same problem that the signal was very weak. The tx/rx port seems to be the right one indicated by a LED.