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Help selecting a power amplifier for a test LTE base station

Discussion in 'Electronic Design' started by Weland, Mar 6, 2013.

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  1. Weland

    Weland Guest

    Hello,

    The Higher Powers in the company I work for decided we should start
    developing some software related to LTE; in the process of this, we
    have to set up a test base station and carry out some tests, two of
    which involve a TX power of 10 W.

    Being the only one with an EE degree, I was quickly handed out the
    task of coming up with what we need, but by knowledge of RF is rather
    scarce. I did take a course on microwaves in uni, but it was quite a
    while ago and mostly related to burning things, not sending
    information, so I am rather confused by all this telecom finery. 10W
    is about an order of magnitude below anything I studied back then.

    I am currently tring to realize if this PA: http://tinyurl.com/a3fkh5y
    (here's the preview link, too: http://preview.tinyurl.com/a3fkh5y ) is
    adequate as a power amplifier for the TX module.

    Background story: we are going to use an Ettus Research SDR box
    (here: https://www.ettus.com/product/details/UN200-KIT ) with an RF
    frontend also provided by ER (the SBX daughterboard, details here:
    https://www.ettus.com/product/details/SBX ). This is able to put out
    about 0.1 W, which I would like to boost up to the beautiful figure
    of around 10 W (perhaps with some additional stages, since the amp
    I am asking about is fixed-gain). I am looking at one of the LTE
    bandwidth in the 700 MHz spectrum -- not necessarily one in particular
    as long as I fit between 699 MHz and 821 MHz. Yes, I am doing this in
    Europe.

    Unfortunately, I'm stuck with the three following issues:

    - What noise figure should I be looking for (ball-park)? All the
    PAs I've looked for that are supposed to be good for LTE have a
    fairly smaller NF than this one. I presume this would depend on e.g.
    the frequency band, bandwidth and number of carriers, but I have
    no quantitative formulation for this yet :-(.

    - Is there a good rule of thumb regarding the relation between IP3 and
    the 1 dB compression point when LTE is concerned?

    - I'm not sure how I can get from the information provided in the
    datasheet to the EVM and ACLR figures, so that I can tell if I am
    within 3GPP's specs.

    I would very much apreciate if someone could give me a few pointers
    about what/were to read in order to get an answer to these problems.
    I realize this is quite basic; even with my having forgotten most of
    what I learned about microwave systems, I can grok the physical
    phenomenae behind RF circuits, but I have zero experience designing
    telecom systems, and with components being this expensive, I cannot be
    wrong too many times in a row. Unfortunately, prior to this, I simply
    haven't had any interest in mobile communication, so my engineering
    formation can only get me this far.

    If anyone else has had to do this before, I'm glad to hear any other
    tips you may have. If we ever end up in the same area of the globe,
    I'll be more than happy to provide a beverage of your choice :).

    Thank you very much!
     
  2. Weland

    Weland Guest

    Sorry, a small follow-up to my previous post: mini-circuits.com's
    website is a bit borked when it comes to displaying stuff, so I
    cannot properly link to a single model's page.

    The exact model name of the PA I was asking about is ZHL-900A-10W+.
    You can find its datasheet here:

    http://www.mini-circuits.com/pdfs/ZHL-900A-10W+.pdf

    I apologize for messing this up, I only realized it now. I almost
    miss Geocities :).
     
  3. I can't help, (sorry) but have you asked mini-circuits the same
    questions. I've found them to be helpful in the past.

    George H.
     
  4. Weland

    Weland Guest

    I can't help, (sorry) but have you asked mini-circuits the same
    No, not yet -- I thought about it, but I figured I shouldd first get
    some feedback from people who don't care whether I buy from them or
    not :). I'll keep it in mind though; judging by their wealth of
    application notes and guides, they do seem like helpful folks.

    Thanks for the tip!
     
  5. Weland

    Weland Guest

    Mini-Circuits says "Please enter model number or your input is too long".

    Sorry for that -- Mini-Circuit's website screwed me up with its viewing
    system :). The model is ZHL-900A-10W+, you can find its datasheet here:
    http://217.34.103.131/pdfs/ZHL-900A-10W+.pdf . The IP is there because
    of MC's website -- again :-(.
    It's SC-FDMA which, if I understood correctly, is rather tough on linearity
    and spectrum. 3GPP has quantitative requirements that describe "how tough",
    but that is well beyond my puny RF skills.
    This one comes in a nice box; I'd have to get a power supply, connectors
    and a radiator, but I'd probably fire myself if I were to get stuck on
    that. Sure, I write code for embedded systems, but I *am* an EE :).

    I'll look into a rack-mounted amp, too, maybe I can find something useful.
    So far I stayed away from them due to the price (and lack of availability
    in the power range that I want, but then maybe I didn't look hard enough!).
    We're kindda stuck to their boxes for reasons well beyon my control, but
    I can try to do that, too. The 10W requirement itself may be negotiable,
    I can at least try to put it in terms of "well, how far can I go?".
    No, there isn't any compelling reason for that -- in fact, there are two
    compelling reasons *not* to, namely time constraints and my incompetence
    as far as this subject is concerned. I just want to make sure I'm not
    spending money on things that aren't what I need (I'm stuck in one of
    those annoying cases where I can't get a complete description of just
    *what* it is that I need...).

    Many thanks for your suggestions and for the time you took to answer!

    Weland Treebark,
    Wandering Philosopher and Engineer
     
  6. mpm

    mpm Guest

    I can answer most of your questions, but I am pressed for time at the moment. Perhaps over the weekend.

    One thought right away would be for you to use an LTE cell phone booster, of which many are commercially available. Google Wilson Electronics or CellPhone-Mate, for example. These are only 3-watt (typically), but they're usable right out of the box. You'd likely have to pad down your input and perhaps cobble a Z-match, but should otherwise serve your purpose. I assume you're in the USA, or designing for same, so the above applies to LTE in the 700 MHz band. But check around. Some manufacturers, notably Wilson, do offer models for international use.

    Note to lurkers thinking about buying cell phone boosters:
    Google the recent FCC decision regarding the use of these devices.
    In short: Permitted, but requires carrier consent and registration (at least until the new models arrive which comply with the mandated network protection standards - and even then still require registration).

    -mpm
     
  7. Weland

    Weland Guest

    Thanks for your answer!
    The though of using a cell phone booster did cross my mind, but I haven't
    found any that is powerful enough. On the other hand, perhaps I could use
    one to take some measurements on already working equipment that I know
    is good. This could help.

    I'm not in US and, to the extent that relevant information has sank from
    the Higher Powers down to me, I'm not designing for US, either. This is
    planned to be deployed in Europe once the 700 MHz spectrum becomes
    available, which is supposed to happen at some point the near future.
    I don't have too many details at hand, but if you're interested,
    I can look them up tomorrow when I get to work.

    Fortunately for my sorry engineer's ass, this is why I do have some
    flexibility; there aren't any fixed hard requirements just yet, so I'm
    just doing a let's-make-some-700 MHz-tests dance for now.
     
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