What happens when PIM testing is for 20W, but it's considerably higher?

Discussion in 'The PIM Corner' started by Jason Fortier, Dec 22, 2011.

  1. Jason Fortier

    Jason Fortier Industry Observer

    If the top end PIM specification requires you to meet 20W performance power levels, how do you know that there wouldn't be a failure at higher power levels especially as networks grow?

    In example, even though the newly touted lightRadios could be about 5W each, they would still be stacked together in many locations and an 8 stack would provide 40W.

    I've been in the business for over five years, but I'm a newbie with PIM. So any information would help.
  2. canderson

    canderson Friend of the Community

    Their software might take that into account. But it's an interesting question for the PIM test gear companies. It may not even be a problem.
  3. TBell

    TBell Industry Observer

    I just recorded a webinar for TESSCO that addresses this topic. Once it is live on their site Iwill post a link. The key things to remember are:

    1) PIM levels do change as power changes. This is why the IEC testing specification says that "comparitive tests" should be done at the same power level. This gives the industry a way to compare "apples to apples."

    2) For mobile communications systems, the IEC specification recommends 20W test power. A lot of effort went into developing that specification by RF equipment manufacturers and operators more than a decade ago. (First published in 1999.) When 20W was recommended there were >100W macro BTS on the market. The availability of 40, 60 or even 80W remote radio heads has no impact on whether or not 20W is an appropriate test power.

    3) Signal to noise ratio of the PIM signal you are measuring is the driving factor to determining the appropriate test power. More signal is better (better measurement accuracy) but less power is also better (safety, equipment reliability, power consumption, size, weight, etc.) Generally speaking, 10 dB SNR is the minimum needed to achieve an accurate measurement. With 20W test power most manufacturers achieve >20 dB SNR for the signal levels when measuring -107 dBm (factory PIM test level.) With field PIM tests where -97 dBm is a common test level, the equipment on the market achieves >30 dB SNR! More power isn't necessary (regardless of the marketing spin you are hearing!)

    4) Something that everyone often forgets is that most cellular systems are not directly impacted by IM3. If you look at the frequency combination it is more typically IM9, IM11 or even higher order modes that actually fall in an operators band. By reducing IM3 to a prescribed level, we are also drive the higher order modes (that actually fall in-band) to an even lower level, often below the BTS receiver noise floor.

    The webinar covers this in detail and hopefully will help everyone to understand how we got to where we are today and understand that the logic that went into selecting 20W more than a decade ago is still valid today.
  4. meisner22

    meisner22 Friend of the Community

    If the availability of 40 W or more has no bearing upon the results why is Anritsu coming out with a 40W analyzer when most companies are at 20W?
  5. TBell

    TBell Industry Observer

    Excellent question. I have a theory that new entrants in the PIM testing market looked at the original Summitek factory PIM test brochure and saw 40W as an available option and designed their field units to support that power level without asking the question WHY Summitek offered a 40W option.

    To answer that question... Summitek has offered a 40W option to RF equipment manufacturers for many years so that they can use long cable runs between the PIM test equipment and antennas under test inside a remote RF chamber. By outputting +46 dBm at the test set we can accomodate 3dB of cable loss between the test set and the antenna and still be testing with +43 dBm (20W) at the antenna input connector.

    I have never seen this as a field requirement though.
  6. TBell

    TBell Industry Observer

    Something else to think about.... Recent ads I have seen claim that PIM testing should be done at 40W to "simulate real-world conditions." Put asside for a moment that the 20W PIM test specification was released back in 1999 when some macro BTSs were pumping out >100W... and ask yourself this,... "If simulating real-world conditions is important for RF testing, why do SiteMasters & FieldFoxes test Return Loss (RL) at 0 dBm?" That's not exactly "simulating real world" power levels.

    The answer is the same answer given above for PIM testing. When trying to measure a 30dB return loss you don't need more test power to achieve an accurate measurement. Assuming a -50 dBm broadband receiver noise floor, 0 dBm test power and the desire to accurately measure a 30 dB return loss, the signal level seen by the test equipment receiver would be on the order of -30dBm...that's a SNR of 20 dB. More test power isn't necessary to acheive an accurate measurement..... Sound familiar?
  7. Jason Fortier

    Jason Fortier Industry Observer

    That makes sense Kaelus, but I hope some RF engineer reading this doesn't decide that from now on all testing will have to be done with a 40W box. Great for the manufacturer, but terrible for the contractor. Leasing looks more attractive every day.
  8. TBell

    TBell Industry Observer

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