A preselector is necessary for two cases. You already covered one (strong ambients, though they could be in-band as well). The other is in the case where you have noise with a high peak-to-average ratio (usually some sort of impulse noise), and the peak-to-peak voltage exceeds the dynamic range of the mixer in the receiver. This is not uncommon with conducted emissions >from power supplies. You could take your cable off the receiver input and connect it to a wide-band oscilloscope set to 50 ohms input impedance. The 'scope needs to have enough bandwidth to take in most of the energy of the signal. Measure the peak-to-peak voltage of the signal. Adjusting the trigger level may help you determine the true peaks for a waveform that is not repetitive, or for which the peaks are infrequent. The peak-to-peak voltage should be equal to or less than the peak-to-peak voltage of a sine wave that the receiver allows. For example, you might set the receiver for 10 dBm maximum input power. This power is 0.7 Vrms, or 2 V peak-to-peak. If your signal as seen on the oscilloscope is no more than 2 V peak-to-peak, you can pretty much get by without a preselector. If you are in doubt, increase the maximum power input setting on the receiver to see if the emissions levels change. Don Borowski Schweitzer Engineering Labs Pullman, WA Ralph McDiarmid wrote on 03/26/2004 11:44:32 AM: > Does anyone in the group have experience with the need for a RF preselector > used in a 3m chamber? I suspect it is really necessary more for an OATS > where out-of-band strong ambients could present a problem for the > attenuator/mixer in a SA. > > Thanks, > > Ralph McDiarmid, AScT > Compliance Engineering Group > Xantrex Technology Inc. This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ieee-pses.org/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
