[Icom] RE: Filtering at 9 MHz vs. 455 kHz

[Siu Johnny]

Hi Stephen,

Based on your argument, would this mean that a DSP IF filter will not face
the RF radiation problems?

Looking forward to your advice because I am not a radio profession.

TNX & 73

Johnny Siu VR2XMC

----- Original Message -----
From: <k0sd@juno.com>
To: <icom@mailman.qth.net>
Sent: Saturday, September 14, 2002 11:52 PM
Subject: [Icom] RE: Filtering at 9 MHz vs. 455 kHz


> Hello all,
>    One issue I've not seen anyone address in the issue is radiation of RF
> around the filters.   There are traces into and out of the filters, plus
> the pins on the filter.   There is a few pf of capacitance between these
> leads & traces.   The higher you go in frequency, the more blow-by you
> will get around the filter.   This limits the filtering capacity of the
> filter, however good or bad it is.   You will see lot more leakage around
> a 9 mhz filter than what you will a 455 khz (.455 mhz) filter.   That is
> why the low frequency 455 khz filter gives much better ultimate
> rejection.
>    Another way to say this, you can place a couple leads about 3 inch
> long about a 1/2 inch apart in-line in a box between an antenna and
> transmitter, you could pass the rf at 900 mhz with about .05 db loss
> (that's how Poly-Phasar makes part of their EMI lightening protects, I
> have dozens of these on 930 mhz, 500 w paging transmitters).   If you
> tried passing dc through the gap, you have ininity loss, absolutely
> nothing would pass.   The higher the frequency, the more energy it will
> pass.   That's why the FT100 and IC706 with the 9  mhz low IF's (or so)
> does not perform well on a large antenna, with a crowded high power band
> (contest, ect).  The blow-by on the filters becomes very evident.   You
> can hear a S9 +60 or stronger signal over a hundred khz or more of the
> band,  rf is blowing past the filters at this signal level at 9 mhz with
> the few pf of capacitance around the filter.   An ultimate rejection of
> -60 db is irrelevant, by the time you're down to -50 or less, it is
> coupling past the filter.
>    Hope this helps some understand what is going on.
>
> 73's de Stephen, K0SD in NC