[USRP-users] Is it possible to add DC offset to signal after upconversion?

Marcus Müller marcus.mueller at ettus.com
Wed Dec 10 04:41:33 EST 2014


Hello Jeon,

a quick question first: Which USRP are you using, and if applicable,
which daughterboard? What is the RF frequency you want to work at?

On 12/09/2014 07:25 AM, Seokseong Jeon via USRP-users wrote:
> I'd like make an upconverted signal that doesn't have any negative value in
> time domain.
I'm afraid I don't understand the benefit of that: No antenna will
radiate a dc offset, so this is an unusual request, unless you need
biasing to power an active antenna or go through specific filters or
protectors. Could you please explain your application?

I'm pretty sure that more experienced radio engineers will easily find
an application for biased TX, but I can only think of cases where I need
a DC bias on RX, typically to drive an active antenna (e.g. for GPS
reception) or a block downconverter (satellite TV receivers); hence my
curiosity.

> Mathematically it is simple:upconvert a signal with local oscillator, and
> add DC bias.
Which mathematically means that you then have two spectral components
very far from each other: one at your LO frequency, and one at 0 Hz.
This will not sit well with a lot of the different mixers and amplifiers
we use on our daughterboards.

In fact, there is no daughterboard to allow you to do these things at RF
frequencies; only the LFTX, which doesn't contain a mixer (and DC offset
could be just a signal within the bandwidth of operation).
>
> In practive, however, I'm wondering it is possible to implement only with
> generic USRPs, daughterboards and GNU Radio. I mean, does USRP sink block has an option like DC offset? Or any
> workarounds?

Sorry, none of this is possible in software.

>
> If not, should I make and attach a circuit  like something in fig 6.6.4 this
> page <http://www.learnabout-electronics.org/Amplifiers/amplifiers66.php>
> after daughterboard output?
That *would* be an option if you can impedance-match your Opamps input
to the output of the Daughterboard, so that the energy coming out of the
daughterboard is absorbed by the opamp circuit, and then impedance match
your opamps output to your wiring and antenna. Much harder will be
finding an opamp that has twice the bandwidth that you want to use as RF
frequency (ie. impossible for most cases).

What people usually do when adding a DC bias is acquiring a bias-tee.
This is an analog device, and you will need to specify for which
frequencies it should work, how much distortion and signal energy loss
is ok for you, how much bias you want to add etc.

Best regards,
Marcus Müller

> Regards,
> Jeon
>
>
>
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