[USRP-users] problem using LO offset with USRP1

Stefano Speretta s.speretta at isispace.nl
Sun Jun 3 17:44:59 EDT 2012


unfortunately I discovered the problem while using an antenna + LNA 
connected to the WBX and changing
the gain of the LNA, or the internal WBX gain or disconnecting the 
antenna did not help. To better understand
the problem I terminated the receiver input and, even with a terminated 
input, I still see a spike in DC.
I tried removing it using the DC offset compensation or the LO offset 
with no luck...

I tried this with 2 different USPR1 and WBX boards and I consistently 
see the same thing. Did anybody else experience
the same problem?

Stefano


Il 31/05/2012 15:33, Patrik Tast ha scritto:
> Hi Stefano,
>
>> I am using a USRP1 and I attached 2 50ohm  terminators to the two 
>> antenna
> Ok, as I understand from your post, you don't want to receive 
> anything, just cry?
>
> Remove that 50 ohm thing. Try to connect an antenna and perhaps an 
> amplifier in-between then point at a source, then you wont see that 
> spike...
>
> ....
> P
>
>
> ----- Original Message ----- From: "Stefano Speretta" 
> <s.speretta at isispace.nl>
> To: <usrp-users at lists.ettus.com>
> Sent: Wednesday, May 30, 2012 16:55
> Subject: [USRP-users] problem using LO offset with USRP1
>
>
>> Hello,
>>
>> I am seeing a spurious component at DC using a USRP1 and WBX board.
>> Similar problems were already reported several times in the mailing list
>> and
>> the suggestion was to use an offset in tuning to move this DC component
>> out of band.
>>
>> I am trying to do this but I keep seeing the same result (so that DC
>> component in band). I am using a USRP1 and I attached 2 50ohm
>> terminators to the two antenna
>> ports to eliminate signals coming from outside but the DC component is
>> still there, even if I change the RX tuning frequency.
>>
>> If I do not use the LO offset, in my application I set the RX frequency
>> like this:
>>
>> uhd::usrp::multi_usrp::sptr usrp = uhd::usrp::multi_usrp::make(args);
>> ....
>> double freq = 200e6;
>> uhd::tune_result_t tuned = usrp->set_rx_freq(freq);
>>
>> and if I print tuned.to_string I get this:
>> Tune Result:
>>     Target RF  Freq: 200.000000 (MHz)
>>     Actual RF  Freq: 199.999723 (MHz)
>>     Target DSP Freq: -0.000277 (MHz)
>>     Actual DSP Freq: -0.000277 (MHz)
>>
>> If I do an FFT of the received signal it looks like in the
>> no_lo_offset.png that is attached (span is 20 kHz centered around 200
>> MHz and the spike is 15 dB above noise floor).
>>
>> If I set the offset (5 kHz in this case, I tried with other values too
>> and the result is the same) in the tune function, like this:
>>
>> uhd::usrp::multi_usrp::sptr usrp = uhd::usrp::multi_usrp::make(args);
>> ....
>> double freq = 200e6;
>> uhd::tune_result_t tuned = usrp->set_rx_freq(freq, 5000);
>>
>> printing tuned.to_string gives:
>> Tune Result:
>>     Target RF  Freq: 200.005000 (MHz)
>>     Actual RF  Freq: 200.004932 (MHz)
>>     Target DSP Freq: 0.004932 (MHz)
>>     Actual DSP Freq: 0.004932 (MHz)
>>
>> But the FFT shows again that strong DC peak (check lo_offset.png
>> attached). If you look carefully you will see that there is a Gaussian
>> shaped noise around +5 khz
>> which is exactly the noise around DC, so the offset tuning is working
>> fine (you see the same noise around DC in the other picture).
>> The problem is that the peak around DC is always there.
>>
>> I also tried enabling and disabling the rx_dc_offset (using
>> set_rx_dc_offset) but with no luck.
>>
>> The spurious signal that I see is still there if I tune the the receiver
>> to other frequencies (so I do not think this is an harmonic of the
>> internal oscillator).  The same peak can also be seen
>> by using rx_ascii_art.
>>
>> Is it possible that there is some rounding error in the cordic or
>> decimation stages leading to this DC error? Or in the conversions in 
>> the PC?
>>
>> Does anybody have ideas on what could be the source and how to eliminate
>> this problem?
>>
>> Thanks a lot,
>> Stefano
>>
>
>
> -------------------------------------------------------------------------------- 
>
>
>
>> _______________________________________________
>> USRP-users mailing list
>> USRP-users at lists.ettus.com
>> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
>>




More information about the USRP-users mailing list