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

Matt Ettus matt at ettus.com
Wed Sep 11 19:50:40 EDT 2013


If the unwanted signal is at the same amplitude as the desired signal, then
there is something wrong in the code, and you are generating a bad signal.
 As a simple test, try just generating a plain complex tone and see if it
looks right.

Matt


On Wed, Sep 11, 2013 at 11:18 AM, Wallace, Frank L CIV NSWCDD, Q41 <
frank.l.wallace1 at navy.mil> wrote:

> I have run the calibration routines to do the I/Q imbalance.  However, if
> it was the I/Q imbalance, I thought the image would be at a much lower
> amplitude than the desired signal.  On the spectrum analyzer, they are at
> the same amplitude.
>
> Does the set_rx_bandwidth() and set_tx_bandwidth() do anything on the SBX
> daughtercards?   The get_tx_bandwidth_range() just returns 40e6.  When I
> got a get_tx_bandwidth() after I do a set_tx_bandwidth(), it returns what I
> sent in.
>
> Thanks,
> Frank
>
> -----Original Message-----
> From: John Malsbury [mailto:john.malsbury at ettus.com]
> Sent: Wednesday, September 11, 2013 2:08 PM
> To: Wallace, Frank L CIV NSWCDD, Q41
> Cc: Marcus Müller; usrp-users at lists.ettus.com
> Subject: Re: [USRP-users] Getting rid of one of the sidebands on transmit
>
> 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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>
>
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