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

Ian Buckley ianb at ionconcepts.com
Wed Oct 1 14:20:34 EDT 2014


Urban, a quick question…I just glanced at your flow graph, it's very simple…I'm wondering where did the sample rate conversion occur (2MHz->5MHz) in your example 2) quoted below?  There is no re-sampling block instantiated in your flow graph.

On Oct 1, 2014, at 10:33 AM, Urban Hakansson via USRP-users <usrp-users at lists.ettus.com> wrote:

> First, thank you for responding so promptly. I attach the Gnu Radio flow-graph and the generated python script. It contains two USRP sink objects, one configured for the B210 and the other for the N210. FYI, I use GnuRadio companinion 3.7.1.1 and 3.7.2.2.
> 
> I also attach two example images from the spectrum analyzer looking at the spectrum from the B210.
> 
> Image 1) B210 master clock rate = 5MHz, Sampling rate = 5 MHz, frequency of complex exponential = 1.2 MHz, center frequency = 800MHz.
> No replica at fc-f. As expected.
> 
> Image 2) B210 master clock rate = 5MHz, Sampling rate = 2 MHz, frequency of complex exponential = 750 kHz, center frequency = 800MHz.
> Replica at fc-f.
> 
> The result from the N210 is a replica at fc-f as well so I don't include a image.
> 
> Regards,
> 
> Urban Hakansson
> 
> ----- Original Message -----
> From: "Marcus D. Leech via USRP-users" <usrp-users at lists.ettus.com>
> To: usrp-users at lists.ettus.com
> Sent: Tuesday, September 30, 2014 8:11:34 PM
> Subject: Re: [USRP-users] Frequency translation works differently on N210 and B210.
> 
> On 09/30/2014 06:20 PM, Urban Hakansson via USRP-users wrote:
>> 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: 192.168.2.199
>> | | subnet: 255.255.255.255
>> | | gateway: 255.255.255.255
>> | | 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.
>> 
>> Regards,
>> 
>> Urban Hakansson
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> Could you perhaps share your Gnu Radio flow-graph with us?  It would 
> help in seeing where your problems might originate.
> 
> 
> 
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