[USRP-users] Amplifiers for USRP Transmission

Marcus D. Leech mleech at ripnet.com
Mon Jul 6 19:49:33 EDT 2015


On 07/06/2015 06:56 PM, Evan Chavis via USRP-users wrote:
> 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.
>
So, would it be reasonable to describe your set-up as:

USRP-->TX antenna  ~~~~~~~~~ AIR GAP ~~~~~~~~~  your RX antenna--->PC 
sound card         ????

If you are getting demodulation there of your transmitted audio it is 
only because of two things:

(A) slight imbalances in the delivered 400MHz (or whatever) sinusoid 
will lead to tiny amounts of envelope detection of your AM signal as seen by
       the low-pass filter that is in all sound cards.

(B) the semiconductor inputs on your sound-card  input have 
non-linearities that also effect a kind of envelope-detection, which is then
       sampled by your sound card.

Ideally, you want that "envelope detection" to happen before the sound 
card, which will be nicely sampling an audio signal (perhaps weak, but 
an audio signal).

So, what you want for a receiver is more along the lines (and I think 
the other Marcus already discussed some of this):

Antenna-->RF filter--->detector-diode--perhaps-some-audio-gain---->sound 
card

There are other topologies that will work as well (just as in software, 
there are many different ways to solve any given problem in analog RF 
hardware).

Now the other effect, which we can talk about, but which is unlikely in 
this scenario, is band-pass sampling, leading to aliasing being seen by 
the sound card.
   When you under sample signal, it gets aliased back into the pass band 
of whatever is sampling, and the details depend on the sample-rate, but 
ALSO on
   the analog bandwidth out in front of the sampler--sound cards 
typically have analog bandwidth going up to only 50-80kHz or so, 
depending on sound-card
   design and ADC design.

See this Wikipedia article about Nyquist:

https://en.wikipedia.org/wiki/Nyquist_rate





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