Gert:
Here are some numbers from my test equipment. An Agilent E4404B analyzer, set to 1.5 GHz and using a 120 kHz RBW, has a noise floor of -72 dBm. An HP-8566B analyzer, set to 1.5 GHz and using a 100 kHz RBW, has a noise floor of -73.3 dBm. Both of those are similar to what you report for your analyzer. If I add an HP-8449B pre-amplifier ahead of the HP-8566B, I get a noise floor of -115 dBm (the 8449B has a measured gain of 37 dB and a published NF of 7 dB). If I were to use my EMCO 3115 horn antenna, with its AF of 25.4 dB @ 1.5 GHz, I could get a sensitivity of (-115 dBm + 25.4 dB) -89.6 dBm/m or 17.4 dBµV/m field strength. >From your pre-amp numbers of 6 dB NF and 24 dB gain, you are getting significantly poorer performance than I am getting with a pre-amp that is 7 dB NF and 37 dB gain. I think that you may not be getting as much gain as you are assuming. OTOH, you could improve things by selecting a pre-amp with even higher gain, like my 8449B has. Now let’s look at the antennas. The EMCO 3115 is a big design compromise; it sacrifices efficiency for broad bandwidth. (I never use my 3115 above 12 GHz, even though it’s rated to 18 GHz; the gain rolls off at around 14 GHz, yielding big antenna factors. I use a custom-built pyramidal horn in the 12 GHz to 18 GHz range; for instance, at 18 GHz, the 3115 has an AF of 46 dB, while my custom horn has an AF of 36 dB. That’s 10 dB of better field sensitivity!) Maybe you can find an antenna that is more efficient than the EMCO 3115. For instance, there are horn antennas available that cover a smaller frequency range, but are much more efficient. A very old Polarad CA-L horn has an AF of 22 dB @ 1.5 GHz, and a Singer EMA-910-1010 horn has an AF of 21 dB. Every dB counts in your budget, so picking up 10 dB with a better pre-amp and several dB with a different antenna can make all the difference you need. OTOH, another problem might be the specification of the limit. Quite often I see radiated emission limits that start with the MIL-STD curve, and then are reduced by X dB to give some equipment a better safety margin. Trouble is, the MIL-STD constrains you to a fixed RBW, so sometimes those reduced limits are just not physically possible. The limit you quoted is nearly on the edge of what is possible, so the customer might not have considered the 120 kHz RBW constraint when they decided to set the field strength so low. Ed Price [email protected] <blocked::mailto:[email protected]> WB6WSN NARTE Certified EMC Engineer Electromagnetic Compatibility Lab Cubic Defense Applications San Diego, CA USA 858-505-2780 Military & Avionics EMC Is Our Specialty From: [email protected] [mailto:[email protected]] On Behalf Of ce-test, qualified testing bv - Gert Gremmen Sent: Tuesday, May 25, 2010 12:17 PM To: [email protected] Subject: Radiated emission testing for automotive at low levels. Colleagues and friends, I have recently been asked to quote testing for automotive car manufacturers ESA specifications. Part of the specification is very low level testing, as low as 20 dBuv/m at 120 Khz BW above 1 Gig. Can any of you shine their lights/contribute 2 cents on the test configuration needed for that. I have run into the following problem: Our R&S analyzer has a noise floor at 120 kHz bandwidth of approximately -71 dBm. Adding a 6 dB preamplifier noise and 24 dB gain lowers the noise floor to 89 dBm (18 dBuV). Our horn antenna (3115) has a AF of >20 dB (@1m) and the resulting noise level field strength is about >38 dBuV/m What is your choice in solving this problem ? Gert Gremmen - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher <[email protected]> David Heald <[email protected]> - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher <[email protected]> David Heald <[email protected]>
