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?
Thanks,
Ales
I have seen the same issue in the last couple of days.
My setup:
Ubuntu 10.04 laptop
UHD + GNUradio 3.3
USRP2 with WBX
TX parameters:
15dB gain, 1090MHz center frequency
4MS/s transmission
RX parameters:
25dB gain, 1090MHz center frequency
4MS/s acquisition
transmitted signal:
U(t) = 10mV + 5mV * sin(2pif*t)
f = 100 kHz
I observe unexpected sinusoidal oscillations in the amplitude at about
400kHz
Oscillation amplitude is on the order of 1mV.
Dave
Aleš Povalač wrote:
868 MHz, 200 kSps, gain 15 dB
loop issue?
–
View this message in context:
http://old.nabble.com/WBX-CW-transmit-problem-tp31963722p31964536.html
Sent from the GnuRadio mailing list archive at Nabble.com.
On 06/30/2011 07:13 AM, Aleš Povalač wrote:
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
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 E. [email protected]:
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 P. [email protected]:
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 P. [email protected]: