EMRFD Message Archive 901

Message Date From Subject
901 2007-07-14 08:24:29 Ed Almos Combining Low-Pass & High Pass Filters
I need to make a bandpass filter with a 3dB bandwidth of 45MHz to
75MHz. Entering the figures into filter design programs produces
strange results and after trying three prototypes (none of which
worked very well) I have decided to try another method.

If I combine a 45MHz high-pass filter with a 75MHz low-pass filter I
should in theory get what I want but I do however have one question.
At the point where the two filters join there are two capacitors, one
in series and one connected to chassis. Is this correct because I
assume that two capacitors connected in this fashion will form a
voltage divider and/or an impedance transformation?

Edward Almos HA6SST
902 2007-07-14 09:39:45 Leon Re: Combining Low-Pass & High Pass Filters
----- Original Message -----
904 2007-07-14 15:47:44 Shawn Upton Re: Combining Low-Pass & High Pass Filters
It's not that uncommon to cascade filters. This will
not create a voltage divider in the passband, as both
filters are looking for 50 ohms in/50 ohms out for
proper operation (or whatever they were designed for).
With just L's and C's, if, in the passband, one had
either voltage step up or voltage step down, it would
require the impedance in/out to have changed.

When in doubt, simulate. Simulations also help to
predict what will happen "if I do this". For example,
if you find you need a 225pF capacitor, what will
happen if I use a 220pF capacitor? What about using
5% tolerance? Or feed and/or terminate with different
impedances? It is also very good for double-checking
a design--maybe you carried wrong in your
calculations, and wouldn't know until after finding it
didn't work.

For the price and performance, it is hard to beat
LTSpice, from Linear Tech.

Shawn KB1CKT

905 2007-07-15 07:47:26 Ed Almos Re: Combining Low-Pass & High Pass Filters
Thank you for the reply Leon. At this point I am prepared to admit
that my problems may be 'pilot error' as I am no expert in using the
Elsie program. Please could you email me the design you created using
the software and I will try this out?

My email address is edwardalmosATgmailDOTcom


Edward Almos HA6SST