Want a chip to block AC contamination

The problem with traditional lumped LC filters at UHF frequencies is that both the L and the C have parasitic capacitances and inductances, so there could be quite bad self resonances at frequencies that are hard to predict.

Some ferrites intended for RFI suppression not only increase the inductance of the wire passing through it, but are also quite lossy at higher frequencies, thus effectively damping any parasitic resonances. I would not be too surprised, if some ferrite manufacturers would offer such inductances as surface mountable components.

In addition a low inductance and low loss (at UHF) chip capacitor should be mounted as close as possible to the inductor and the other end directly to a solid ground plane.

Paul

Great, Paul,

Could you please recommend me a few manufacturers for ferrites intended for RFI suppression, as well the manufacturers for low inductance and low loss (at UHF) chip capacitor?

Thanks.

John

Unfortunately I have no idea about the current market situation, but when I was working with 430 and 1300 MHz equipment for more than a decade ago, I used Fair-Rite ferrites and some "high-Q" chip capacitors from various manufacturers.

It should be noted that the PCB track layout is critical, since 1 mm of wire or PCB track will have an inductance about 1 nH, which has an inductive reactance about 10 ohms, making any filter quite inefficient. Multiple chip capacitors from Vcc to a solid ground plane will effectively put the inductances in parallel, thus reducing the inductive reactance.

For narrow band filtering, various resonant 1/4 or 1/2 wave stubs can also be used, however, these might be an overkill in this case.

Paul

Thank you very much, Paul,

It reminds me that a senior RF guy in our group once adopted wave stubs to improve the performance, now I understand why!

Johnson

for

loss

Hello John,

As Paul suggested, Fair-Rite is a good source. But there are dozens of other manufacturers. Ferrites are also offered by companies like Philips, TDK, Tokin, Kaschke, Epcos, Dongying Xinyi, Fenghua and many others. Look for the expression "ferrite bead" when searching. If for some reason you cannot obtain samples fast enough contact Amidon. They carry Fair-Rite cores and it is where I usually got my EMI stuff when running out a needed core size. And buy a bag full of cores since when people find out how useful they are they are consumed faster than rice cakes.

As for caps it's the usual sources like Panasonic, AVX etc. Best is to check with a distributor in your area since that is probebly the only way to obtain samples quickly. They'll probably carry several brands. Digikey should work, too.

Regards, Joerg

Thank you very very much for sharing the treasure with us!

Johnson

for

loss

I will second that. Digikey is, to my knowledge, the only distributor that actually carries, in stock, all the capacitors shown in the catalog.