A quick calculation will show that if an immunity requirement of 10 V/m is imposed at 105 MHz on a 420 MHz tuned receiver with a sensitivity of -100 dBm, the harmonic content at 420 MHz needs to be on the order of 100 dB down from the carrier. That is quite unrealistic even for a VHF source for which 420 MHz is out-of-band, let alone a broadband amplifier and antenna combination that covers (typically) 80 - 1000 MHz.
From: Bill Owsley <[email protected]> List-Post: [email protected] Date: Tue, 13 Jun 2006 09:23:18 -0700 (PDT) To: Grace Lin <[email protected]>, Bob Richards <[email protected]> Cc: [email protected] Subject: Re: RF Power Amplifiers The harmonic content mentioned adds quite a bit to the measured field strength. As the harmonics are filtered out (a lower level), the field strength drops, so the drive and/or output power is boosted to get the field strength back up, and the harmonic content rises faster than the fundamental, so more filter is added and more power added, and soon there is one BIG power amp required to get the field strength at the fundamental without the harmonics. ps. If the device being tested has a receiver, the harmonic energy can provide some distracting results at unexpected times. ie. The 420 MHz rcvr fails (cannot receive) at 105 MHz. Immunity test freq. pps. The receiver under test was good for -100 dB signals so the harmonics had to rather low to not swamp out the intended signal. Grace Lin <[email protected]> wrote: Dear All, Thank you very much for many members who has replied online and offline. I value your expert comments. So far, I have learned that antenna's VSWR, cable loss (at high frequency), and 80% AM modulation are the factors to count on. The new requirements from the third edition of IEC61000-4-3 are important factors too. Many members suggest to choose an amplifier triple times' power as calculated (i.e. 500W for 80-1000MHz), which I agree with. However, two sales representatives from two amplifier manufacturers suggest a 150W for 80-1000MHz. I get lost. Kindest regards, Grace On 6/10/06, Bob Richards <[email protected]> wrote: Mac, Yeah, good point, I forgot to mention that. The the old 15dB requirement may not be enough. At a lab where I used to work, we tested down to 26 MHz regularly. The difference in antenna factors between 26 and 52 MHz were terrible. When calibrating at 26 MHz at only 1/3 the rated power of the amp, the signal level at 52 MHz dominated. This was with a very common 500w amplifier, at 10v/m level. We ended up using an Amplifier Research 2500w amp for the 10v/m test. Way overkill, but it points out the fact that you can't just calculate the power required to generate a desired field strength and use that figure to size your amp. You may need to double or even triple that figure just to meet the harmonics requirement. Bob Richards, NCT. Elliott Mac-FME001 <[email protected]> wrote: Grace, I agree that trying a demonstrator before you buy is the best approach. One thing that you want to take into consideration besides the compression points that Bob mentioned is the harmonic / distortion requirements mentioned in section 6 of 61000-4-3. The 2006 version of IEC 61000-4-3 is a bit different than the current version of EN 61000-4-3. IEC 61000-4-3:2006 [expected to be published as EN this year] requires that harmonics in the uniform field area field be at least 6 dB below the fundamental. There is a lot of good info in Annex D about how to accomplish / measure this. The current EN 61000-4-3 requires that the amplifier not produce harmonics > 15 dB below carrier. Whichever amp you choose, make sure that you take these considerations into account as well. Good luck! Mac Elliott - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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