[USRP-users] Amplifiers for USRP Transmission

Evan Chavis echavis at umich.edu
Mon Jul 6 18:21:18 EDT 2015

Thanks for the responses everybody.  I'm aware that the receiver I
described is far from ideal, I just wanted to see how clear of a signal I
would be able to receive using just a wire and the audio jack from an old
microphone was one way to allow me to record what the wire was picking up
on my computer.

On Sylvain's points:

Good to know about the filtering that would have to be done if at some
point I do end up using an amplifier.  I had already discovered from
experimenting that I get a much clearer signal if I don't have the gain set
as high as it can be on the daughterboard, and I will keep in mind that if
I do end up using an amplifier I might be better off keeping the signal
between +-.8 rather than +-1.  My only question would be when you say "tx
to the USRP at a higher sample rate than needed" what do you mean?  Maybe
I'm just confused because I'm transmitting from the USRP and not to it?  Do
you mean the original signal I'm giving the USRP to transmit should be
provided at a higher sample rate?  I phrased the sentence with the 31.5 in
it very poorly, I recognize that the daughterboard is NOT transmitting 31.5
dBm, I meant to just ask whether the gains would be added or not and I used
31.5 as the number because it is the upper end of the gain range given by
uhd_usrp_probe and I wanted to illustrate in my question that I was asking
about whether the gain selected on the daughterboard would impact the gain
outputted from an amplifier.  Also, good to know about keeping the power
output below the maximum possible to stay within the linear region.

On Marcus' points,

No worries about the harshness, I too would prefer that I avoid any
unnecessary encounters with the authorities and I appreciate any help I can
get on forums.  I understand that just a plain wire attached to what used
to be a microphone is not an efficient antenna and that since an RF signal
follows the inverse square law, the signal will be only 1/4 as strong at
twice the distance and so on.  Maybe my sense of scale isn't as good as it
should be when dealing with RF, but when I receive my USRP's signal with a
rtl-sdr, a cheap sdr but still a much more efficient receiver than the one
I'm interested in, the range is improved but still not large enough that I
would have thought I was in any danger of melting anything or alerting the
authorities that I'm transmitting.  I have been trying to think
of/experimenting with ways to improve my signal without just adding more
power.  Mostly playing around with filtering through GnuRadio and changing
the gain levels of the daughterboard; If I am missing something obvious I'd
love to know what it is.  I'll see if I can find a copy of the handbook you
mentioned, so far I've been using wikipedia but maybe the handbook will be
a better resource.

On Mon, Jul 6, 2015 at 4:49 PM, Sylvain Munaut <246tnt at gmail.com> wrote:

> Hi,
> > My questions are, what do I need to consider when looking for an
> amplifier
> > to use with a USRP, are there any special problems a beginner would be
> > likely to miss when trying to use those 2 things together?
> Filtering !
> SDR outputs are very often not all that clean and have plenty of
> harmonics and images and stuff like that.
> When you just use the raw output at low power you can often just
> ignore them because they're going to be 10+ dB lower than the main
> signal and so that's low power enough to be ignored.
> However as soon as you start amplifying, those imperfections will get
> amplified too and will start interfering with other transmissions.
> To reduce the images you want to TX to the USRP at a higher sample
> rate than needed and apply sharp digital filtering to your signal
> (sharper than what the USRP itself can do in the FPGA).
> You might also want to configure the analog bandwidth as small as
> possible (although I don't think that's applicable to the USRP1+SBX
> since there is no variable analog bw there).
> To reduce the intermodulation products you want to not drive the
> components too hard. So make sure to only make your signal swing
> between +- 0.8 and not 1.0 for instance. You also want to not put all
> the gain stages at maximum. Back off a bit, like 80% or so, so that
> you keep all of them in their most linear regions.
> Finally to get rid of the harmonics, you want an external band pass
> filter that's center around your frequency of interest. Minicircuits
> has those too. You can for example use two of them, one between the
> SDR and your external amplfier and then one after your amplifier (to
> get rid of the harmonics added by your amplifier). If you only have
> one, then place it after the amplifier.
> > Also, if an
> > amplifier's gain is listed at 20 dB, would that be 20 dB on top of the
> 31.5
> > the daughterboard is capable of outputting, or would the amplifier be
> > useless in that it is capable of providing less power than the
> daughterboard
> > itself?
> 1) Your daughter board is definitely not outputting 31.5 dBm ...
> 2) The gain will de added to the daughter board output as long as
> you're not saturating the amplifier. See the P1db rating for a
> guideline of maximum power (Output power at 1dB compression). For
> instance, the zx60-v63+ you linked to has a P1db of 17.8dBm at 2G ...
> so it's even less than 100 mW.
> 3) You _DEFINITELY_ want to keep the total output power of the
> amplifier like > 3 dB below the rater P1db so that you're well within
> the linear region and your signal isn't distorted and you don't create
> too much harmonics.
> Note that (3) actually depends on the modulation and external
> filtering quality. For some modulation like GMSK (used in GSM), you
> can use amplifiers in their non-linear region and rely on external
> filter to get rid of unwanted harmonics. This is why GSM amplifiers
> are cheap to make. OTOH when working with high PAPR modulation (like
> OFDM), then you actually need to backoff way lower than the P1dB just
> because during the peak of the signal you need your amplifier to still
> be linear. And that's why amplifying LTE is much harder and you get to
> advanced and crazy stuff line pre-distortion ....
> > And third, how is a device such as this one:
> > http://www.minicircuits.com/pdfs/ZX60-V63+.pdf powered?  The data-sheet
> > gives the numbers 5 V and 69 mA but I don't see an obvious way to power
> it
> > aside from just soldering wires onto the marked bumps that say +5 and
> > ground, is that the actual intended way to do it?
> Yes, it is.
> Some other can also be powered by providing a DC bias on their output
> port using a separate bias-tee. That's not the case for this one
> though AFAICT.
> Cheers,
>     Sylvain
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