>-----Original Message----- >From: Gordon,Ian [ mailto:ian.gor...@bocedwards.com] >Sent: Monday, August 11, 2003 8:33 AM >To: 'IEEE EMC-PSTC GROUP' >Subject: Temperature effects on conducted emissions? > > > >All >Can anyone suggest a means by which the indicated signal from a LISN + >transient limiter + receiver combination can result in a 30dB >change over >one month? I used the same "reference source" and test >configuration on both >occasions to generate emissions. However, the source is merely >a piece of >standard equipment and not intended as a calibration reference. >The temperature has varied considerably over the last month >but I would not >have thought this could result in a 30dB variation. >Alternatively, can anyone suggest a means of constructing a >reference source >which may be connected to the LISN input? >Thanks >Ian Gordon >
Ian: You didn't say if the latest measurements were higher or lower than the original. However, 30 dB is a huge variation, and room temperature shifts shouldn't cause that at all. You need to build confidence in your setup. First, verify the cal of your analyzer. Easiest way is to use the front panel cal output at maybe 100 MHz or 300 MHz. Or inject a signal from a signal generator into the analyzer input. One thermal effect could be that you have fried the analyzer's input attenuator. Now, put the limiter on, and repeat the above. Now inject into the head end of your coax. You should be down only by the expected coax loss. Now, inject into the input of the attenuator that you normally use on the LISN. You should be down by 10 dB. Last, check your LISN. That's relatively easy. Assuming that you are using an LISN that works from 9 kHz to 10 MHz or so, just connect a 50-ohm signal source to the LISN power output terminal and the case ground. (You did disconnect the LISN input power?) Also connect a high impedance oscilloscope probe to this point. Now connect the second oscilloscope channel to the LISN signal output port, using the 50-ohm termination option in the oscilloscope. (I use a Tek TDS640A.) Inject enough RF signal to get a nominal value on Channel 1, typically 1 Vrms. Now look at the Channel 2 50-ohm signal port level. It should also be almost exactly 1 Vrms, except for the frequency range below about 100 kHz. If this reading is significantly lower than it should be, then you are seeing a bad coupling capacitor in the LISN. If too high, then a blown LISN resistor is the cause. BTW, as you sweep the injected signal down towards 9 kHz, you should see a rise in the signal port loss, until you see about 5 dB or so at 9 kHz. This is normal, and something that you should have been adding in to you acquired data. (It's the typical loss associated with the RC voltage divider. The loss will be very high at the power frequency, and infinite at DC.) The last thing to consider is the attenuator and limiter. Both of these may have been subjected to extreme physical shock (as in I dropped it on the concrete floor). This could cause an intermittent problem. You might want to tap or slap them while you watch the signal loss through them. Also try wiggling and moderately pulling on the various coax fittings and connectors. Regards, Ed Ed Price ed.pr...@cubic.com WB6WSN NARTE Certified EMC Engineer & Technician Electromagnetic Compatibility Lab Cubic Defense Systems San Diego, CA USA 858-505-2780 (Voice) 858-505-1583 (Fax) Military & Avionics EMC Is Our Specialty >
