[USRP-users] Getting rid of one of the sidebands on transmit

John Malsbury john.malsbury at ettus.com
Wed Sep 11 14:08:00 EDT 2013


Frank,

I might have misinterpreted this email thread, but what you're seeing is
caused by I/Q imbalance, which will be present to some degree in any direct
conversion transmitter.  We do have some self-cal utilities you can run to
minimize I/Q imbalance.  Another potential option, depending on the
bandwidth of your signal/chirp, is to use offset tuning to move the
opposite sideband out of your frequency of interest.

http://files.ettus.com/uhd_docs/manual/html/calibration.html

http://files.ettus.com/uhd_docs/manual/html/general.html#tuning-notes

-John


On Wed, Sep 11, 2013 at 10:55 AM, Wallace, Frank L CIV NSWCDD, Q41 <
frank.l.wallace1 at navy.mil> wrote:

> I do radar and have a LFM chirp
> f(t) = f0+k*t
>
> If I move fo up a little bit so that it is a little off the carrier.  I
> can see the LO leaking through on the spectrum analyzer.
>
> Then I do exp(j*2*pi*t*f(t):
> Real = cos(2*pi*t*f(t));
> Imag = sin(2*pi*t*f(t));
>
> Still, when I look at the output on the spectrum analyzer, it shows an
> upper sideband and a lower sideband.
> Right now, I am using f0=(receive sample rate)/4 trick so that the
> exp(j*fo*t) becomes a series of 1,j,-1,-j, etc.
> This moves one of the sidebands to DC and I lowpass filter to get rid of
> the original DC and the other image.
>
> The single sideband stuff that I have looked at (Wikipedia, etc.) all end
> up with a real signal.  So, I wasn't sure if I just do the exp(j*stuff) and
> go with it for the ETTUS radios or how he I and Q DACs get mixed together
> on the daughercard.
>
>
> Thanks,
> Frank
>
>
>
>
> -----Original Message-----
> From: USRP-users [mailto:usrp-users-bounces at lists.ettus.com] On Behalf Of
> Marcus Müller
> Sent: Wednesday, September 11, 2013 11:37 AM
> To: usrp-users at lists.ettus.com
> Subject: Re: [USRP-users] Getting rid of one of the sidebands on transmit
>
> Hi Frank!
> > When I try to transmit a signal that is not at baseband, I get an upper
> sideband and a lower sideband.
> I reckon you're trying to transmit a signal that is frequency shifted
> bandpass signal; that's baseband. You actually can't do anything else using
> your daughterboard.
> > How can I get rid of one of the sidebands?
> Is this really what you want to do? It seems as you are trying to transmit
> a real baseband signal s_r, meaning that the spectrum S_r is always
> symmetric to the f=0 axis: (beware, ASCII art)
>           ^ |S_r|
>       ----|----
> ----/    |    \----
> _________|___________>f
> If you're really just trying to have your real signal on a carrier
> frequency, you might just shift it in frequency domain, giving you a
> complex signal s_c
>
>           ^ |S_c|
>   --------|
> /        \----    ----
> _________|____________>f
> by multiplying it with exp(jt) (in time domain), before applying a low
> pass filter, cutting off any frequency that is far away from DC. That is
> equivalent to using a band-pass filter to filter out one halfband*.
> > Is the data for the send() call for the tx streamer really complex?
> Yes. The USRP has a two-channel I/Q DAC, therefore complex baseband is
> what you do. You really need to grep all the complex baseband theory to do
> useful stuff  with it ;)
> > I am pretty sure it is from one of the tx examples.  Right now, I am
> processing it on the receive side to remove one of the sidebands, but I
> would like to get rid of it on the transmit side.
> Good choice, since throwing away half your signal is wasteful in means of
> signal power and spectrum usage.
> If you do know, however, how to throw away half of the baseband on the rx
> side, why don't you just do exactly the same on tx? There is no
> "mathematical" difference between samples going into and samples coming out
> of the USRP...
>
> Greetings,
> Marcus
>
> *in fact, band passes can be constructed as frequency shifted low passes,
> which just multiplies the filter taps with exp(jt)
>
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