More filter talk

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From: Mike Czuhajewski (wa8mcq@u1.abs.net)
Date: Thu May 02 1996 - 08:17:15 EDT


Someone recently suggested that for a 40M filter for the 40-9er, even
the combination of two 10,000 pF capacitors and 0.1 uH inductor would
work. That's true, sort of, but this is one of the more extreme
designs. Remember that posting I made about 3 element filters and
passband ripple, where I went to ripple values as high as 3 dB? This
one is more like 18 dB ripple!

It's interesting to look at this one in the graphic mode of GPLA (or
Touchstone, etc, if you have Big Bucks programs like that available).
It shows a nice, deep and wide dip in the curve below 7 MHz, reaching
over 18 dB at the worst; in fact, it's at least 18 dB between 3.2 and
5.0 MHz--I suppose it makes a nice sub-harmonic filter :-) But as I
said before, W7ZOI says we don't need to worry about passband ripple
in a transmitting low pass filter, so this could still be acceptable.

(There are exceptions to that rule, though. Sometimes people use a
single filter to cover two bands, such as 12 and 10 meters, which are
relatively close to each other; in cases like that you do have to be
concerned with passband ripple.)

This is a decidedly high Q filter. The insertion loss is rather
considerable across the 40 meter band, dipping to 20% and under
across a rather small range. Here are a few selected points--

7.0 2.14 dB (39%)
7.05 1.21 dB (24%)
7.1 0.76 dB (16%)
7.11 0.74 dB (point of lowest insertion loss)
7.15 0.98 dB (20%) and rising continuously beyond that point

The tradeoff that makes all of this worth your trouble is a rather
hefty 42 dB attenuation at 14 MHz! But would someone want to use a
filter like this? The insertion loss is a bit much, but you might be
willing to accept it in some cases. It has at least 50 KHz width
where the loss is 20% or less, and if you have something like an
NE4040 with a fairly restricted tuning range, a rather narrow slot
like that is not a major problem--a fair amount of loss but you do
get it across a range which is adequate for the rig. Tweaking the
values a bit would put that low point anywhere you want in the band.

It's a lot of fun playing with filter design on the computer, but the
proof is in the pudding and in the building, so I'm eagerly awaiting
Alan Kauls results with the 1300/0.69/1300 filter to see how that one
works in the real world! In the meantime, remember that all of my
talk so far has been about bilateral filters, where the input and
output impedances are equal, 50 ohms in this case. If you run your
rig at a power level where the collector impedance deviates from 50
ohms, you will affect the filter performance...and I've seen quite a
few different power levels reported for 40-9er rigs, and each power
level equates to a different collector impedance. (Another fun
complication!)

73 and Queue Our Pea DE WA8MCQ wa8mcq@abs.net


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