Re: [Discuss-gnuradio] WBX CW transmit problem

[Ales Povalac]

Well, it seems I have solved the problem for myself. Until now I did
not know that USRP allows to set center freq in steps much smaller
than PLL synthesis using DSP processing. This may work well for most
signals but during CW (I=1, Q=0) the resulting signal is not perfect,
maybe because of non-ideal carrier suppression. I have patched
tune.cpp from UHD to set both RF and DSP policy to manual and dsp_freq
to 0. Now the output is nice.

Just an additional question: is there any version of UHD prepared for
open-loop power control on WBX?

Thanks,
Ales


2011/7/1 Ales Povalac <address@hidden>:
> Sorry, I checked the signal at MODOUT again and it is _not_ stable as
> I wrote before. So the AGC loop is probably OK and just reacts to
> wrong signal on its input.
>
> The signal is weird even at the DAC outputs. Enclosed is the screen of
> IOUTN_A and IOUTP_A obtained with ./tx_waveforms --rate 250000 --freq
> 868000000 --gain 15. When I try the same command with 600MHz output,
> there are two nice stable lines. I am running linux; GNU C++ version
> 4.4.3; Boost_104000; UHD_003.001.002-release.
>
> Any ideas why is it acting like this? Just on several output frequencies...
>
> Thanks,
> Ales
>
>
> 2011/7/1 Ales Povalac <address@hidden>:
>> Hi Matt,
>>
>> thanks for the answer. The problem is not in analyzer setup. Using
>> large RBW and zero span, it acts as an envelope demodulator and shows
>> the signal in time domain. Since I used the USRP2 to generate CW
>> signal only (I=1, Q=0, constant in time), there should be stable
>> transmitted power over the time. This works well on most frequencies
>> but not always in the 900MHz band.
>>
>> It seems like a problem with AGC of ADL5386. RF signal at MODOUT is
>> nice and its level is stable. Voltage levels at U501/VSET and
>> U501/VREF are constant. The voltage at U501/VDET_VCTL is oscillating
>> with the same shape as the RF signal envelope (obviously, it is also
>> the input of the attenuator).
>>
>> Regards,
>> Ales
>>
>>
>> 2011/7/1 Matt Ettus <address@hidden>:
>>> On 06/30/2011 07:13 AM, Aleš Povalač wrote:
>>>> Dear all,
>>>>
>>>> I have been testing USRP2 with WBX board (#957). Using GNU Radio, I
>>>> have created a simple CW TX system. The problem: output signal is
>>>> oscillating when I set the TX frequencies to 900MHz band. Signal
>>>> output (in time domain) at TX/RX connector from the FSL analyzer is in
>>>> the attachment.
>>>>
>>>> Settings:
>>>> 868 MHz, 200 kSps, gain 15 dB
>>>>
>>>> Same results with:
>>>> ./tx_waveforms --rate 250000 --freq 868000000 --gain 15
>>>>
>>>> The oscillation with ca. 1dB amplitude is present for almost all gain
>>>> settings except the lowest values. I have observed the problem at
>>>> several frequencies from 700M to 1G, sometimes with amplitude over
>>>> 2dB.
>>>>
>>>> Any ideas about the cause of this problem? Some PA instability or AGC
>>>> loop issue?
>>>
>>>
>>> There is DC offset in the signal.  Since you are using a 10 MHz RBW, you
>>> are seeing both the DC offset and the desired signal beating with each
>>> other.  If you take your spectrum analyzer out of zero-span mode and
>>> zoom in on the signal you will see that the amplitude is constant.
>>>
>>> Matt
>>>
>>
>