[USRP-users] Amplifiers for USRP Transmission

Evan Chavis echavis at umich.edu
Mon Jul 6 18:56:18 EDT 2015

I think my initial description of my receiver was pretty misleading so I'll
try again:

I'm not trying to feed the received signal into a microphone and use it as
a speaker or anything.  I was just using parts from an old microphone so
that I could record what was being received on the wire I'm using as an
antenna on my computer; I was using the audio jack from what used to be a
microphone and attached a wire(the receiving antenna) where the part of the
microphone that picked up audible noises used to be.  I guess I really
should have left any mention of microphones out of my initial question.

On Mon, Jul 6, 2015 at 6:21 PM, Evan Chavis <echavis at umich.edu> wrote:

> 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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