[USRP-users] Frequency translation works differently on N210 and B210.

Urban Hakansson uhakansson at tecore.com
Tue Sep 30 18:20:53 EDT 2014

Hi everybody,

I have a problem. I include below 1) General information, 2) Introduction to my problem 3) Detailed description of my problem.

General background information about my environment: 
Fedora 17 Linux; GNU C++ version 4.7.0 20120507 (Red Hat 4.7.0-5); Boost_104800; UHD_003.007.001-0-unknown 

N210 informationfrom uhd_usrp_probe: 
| Device: USRP2 / N-Series Device 
| _____________________________________________________ 
| / 
| | Mboard: N210r4 
| | hardware: 2577 
| | mac-addr: 00:80:2f:0a:e6:15 
| | ip-addr: 
| | subnet: 
| | gateway: 
| | gpsdo: none 
| | serial: F4A09C 
| | FW Version: 12.4 
| | FPGA Version: 10.1 

B210 information from uhd_usrp_probe: 
| Device: B-Series Device 
| _____________________________________________________ 
| / 
| | Mboard: B210 
| | revision: 4 
| | product: 2 
| | serial: F571B5 
| | FW Version: 4.0 
| | FPGA Version: 3.0 

Introduction/background: It is my understanding that outputting a real-valued baseband signal x(t) = a*sin(2*pi*f*t) on an RF carrier fc should result in two components at fc+f and fc-f, but outputting a complex-valued baseband signal x(t) = exp(2*pi*f*t) should only result in an fc+f component. The complex exponential can be used for frequency translation but is causing me serious problems on the N210 + SBX daughterboard.

Detailed Problem Description: Using GnuRadio I output a simple baseband complex exponential at frequency +f centered at the RF center frequency fc. Now on the N210 in addition to the sinusoid at fc+f there is a unexpected replica at fc-f about 13-14 dB below fc+f. When I run the same script on the B210 and set master clock rate equal to the sample clock rate, I only see the desired frequency component at fc+f. There is no replica at fc-f as in the case of the N210. This is the correct behaviour as I understand it. However, if I don't set the master clock rate equal to the sample rate on the B210 I do get the undesired replica at fc-f 13-14 dB below the signal at fc+f just as I did on the N210.

Why does it only work if the master clock rate is set equal to the sample clock rate on the B210? I have read somewhere on the mailing list that the FPGA including CIC and HB filters are bypassed in this case. 

Question: How can I output a simple complex-valued baseband signal x(t) = exp(2*pi*f*t) on an RF carrier fc on the N210 and only get an fc+f component so I can use this mechanism to perform frequency translation?

Thanks for you consideration.


Urban Hakansson 
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