>-----Original Message-----
>From: richwo...@tycoint.com [mailto:richwo...@tycoint.com]
>Sent: Wednesday, March 20, 2002 11:14 AM
>To: emc-p...@majordomo.ieee.org
>Subject: Tuned Stub Filter
>
>
>
>We are considering using a tuned stub filter to suppress the
>carrier so that
>we do not overload our spectrum analyzer while measuring
>spurious emissions.
>One question we have concerns the Q of the filter - i.e., will it
>sufficiently suppress the modulation sidebands. So, let me ask
>the following
>questions. In the frequency ranges of 860-1000 MHz and 2.45
>GHz, how much
>suppression should I expect from a stub filter and what design
>steps can be
>taken to have the necessary bandwidth? Or, am I going down the
>wrong path
>for a filter?
>
>Richard Woods
>Sensormatic Electronics
>Tyco International
>
Rich:
I have tried using double-stub tuners to do this job, and it's usually not
very good. I seem to recall getting about 35 dB of rejection from a
double-stub, and the Q is not great. I now use a combination of K&L tunable
band-rejection filters (which have a 5% wide >55 dB rejection band) and
high-pass / low-pass filters.
One problem with resonant band rejection filters is that if you tune them to
notch out the fundamental, you also get rejection at harmonic frequencies
too. So this makes measuring harmonic content a problem. I typically use the
band rejection filter to notch out the fundamental while looking near the
fundamental for spurious emissions. Looking below the fundamental for
spurious, I use a low-pass filter. Above the fundamental, I measure
harmonics with a high-pass filter that has a cutoff just below the second
harmonic. Once above the second harmonic, you can usually use the high-pass
filter for measuring spurious emissions too.
Needless to say, after a while, you build up a decent collection of filters.
(I especially like some high-pass filters I have made from waveguide; they
give >80 dB well below cutoff, the cutoff is sharp, and the passband is just
about zero dB loss. Actually, waveguides are band-pass filters, and you need
to switch to a waveguide of smaller size as you progress up in frequency.)
And remember, you have to characterize all of your filters.
Ed Price
ed.pr...@cubic.com
Electromagnetic Compatibility Lab
Cubic Defense Systems
San Diego, CA USA
858-505-2780 (Voice)
858-505-1583 (Fax)
Military & Avionics EMC Services Is Our Specialty
Shake-Bake-Shock - Metrology - Reliability Analysis
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