Help selecting a power amplifier for a test LTE base station

[Weland]

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!

 

[Weland]

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 .

 

[George Herold]

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 .

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.

 

[Weland]

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.

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!

 

[Weland]

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 :-(.

I'm not very familiar with LTE (I had to look it up on Wikipedia to see
what it was, in fact). But if it's one channel of TDMA then the PA specs
aren't terribly critical as far as link performance goes. If it's more
than one channel of TDMA or if it's CDMA, then distortion matters, but
I'm not sure how much.

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.

There are rack-mount power amplifiers that would probably be a better bet
than buying a module from MiniCircuits and trying to mount it to a board,
or even buying one of their boxes (remember, I couldn't make your link
work).

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!).

I'd call up Ettus Research and say
"We're thinking about using your SDR, but before we buy it we need to be
confident that we can boost the power to ten watts. How do we do that?"
Asking the question that way not only puts the ball in their court, but
it makes it clear that if they can't tell you how to make a system that
produces your desired power, you're going to go shopping someplace else.

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?".

For one-off lab realizations of things like this, particularly when I'm
on thin ice as far as my own expertise goes, I always look to a
collection of boxes roped together with cables first. You will always
end up with a price tag that makes you gag, yet is less than what you'd
spend in engineering hours trying to make something less polished work.
Only if there is some compelling technical reason to roll your own would
I step out of that paradigm.

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

 

[mpm]

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!



--

Weland Treebark,

Wandering Philosopher and Engineer

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

 

[Weland]

Thanks for your answer!

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.

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.