From: Cortland Richmond [mailto:[email protected]]
Sent: Saturday, April 21, 2007 4:25 AM
To: [email protected]
Subject: IF filter response and EMC receiver accuracy
Recently, I and some co-workers worked overtime running down a problem with a
new EMC receiver. It consistently read some 3 dB lower than one we bought a
few years ago.
I finally slowed things down and we looked at the filter response shape (tiny
steps, lots of them, and a narrow span). Aha! Where the old German receiver
had a nice rounded curve, the new one was OK at the -6dB points, but looked
like a dog's canine above that. If we weren't RIGHT on a carrier, it'd be
measured lower.
CISPR 16-1 specifies masks for IF bandwidth curves; I don't recall having seen
one for MIL-STD tests, and that's probably why the German set was well
behaved. I expect this is the beginning of an exciting discussion with the
manufacturer. And another Banana Skin for our collections.
Cortland Richmond
KA5S
Cortland:
Beyond the MIL-STD-461E Table II Bandwidths, Paragraph 40.3.10.1 talks further
about IF bandwidth.
"The bandwidths specified in Table II are consistent with the recommended
available bandwidths and the bandwidth specification technique for receivers
contained in ANSI C63.2. Existing receivers have bandwidths specified in a
number of different ways. Some are given in terms of 3 dB down points. The 6
dB bandwidths are usually about 40% greater than the 3 dB values. Impulse
bandwidths are usually very similar to the 6 dB bandwidths. For Gaussian
shaped bandpasses, the actual value is 6.8 dB."
Do you have a link to a picture of the observed response?
Ed Price
<mailto:[email protected]> [email protected] WB6WSN
NARTE Certified EMC Engineer & Technician
Electromagnetic Compatibility Lab
Cubic Defense Applications
San Diego, CA USA
858-505-2780 (Voice)
858-505-1583 (FAX)
Military & Avionics EMC Is Our Specialty
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