Paul Burridge wrote in message
...
Hi all,

I need a simple design for a BPF with a centre frequency of 17.2Mhz.,
corner frequencies one Meg or therabouts either side of that and a
reasonably good shape factor (nothing critical here). I don't mind
having to do a bit of tweaking of values myself if anyone has
something close to that they can post. Passive L/C combinations only,
please. (assume the usual 50R nominal Zs.)
Thanks,

p.
--

The BBC: Licensed at public expense to spread lies.


The simplest filter that'll get you by, is this one. (a couple of tuned
circuits with a bit of top-coupling :-) ...

___ ||C1
-|_R_¦------.----.-------||-------.----.-------, O/p
Source |L2 |C2 || |C2 |L2 |
I/p | | | | .-.
C| --- --- C| | |load R
C| --- --- C| |R|
C| | | C| '-'
| | | | |
=== === === === ===
GND GND GND GND GND

F1=16.2e6 'lower limit MHz
F2=18.2e6 'upper limit MHz
R=50 'Ohms source and load impedance
Pi=3.142

C1 = (F1+F2) / (4 x Pi x F1 x F2 x R)
C2 = F1 /(Pi x F2 x (F2-F1) x R)/2
L2 = 2xR /(4 x Pi) x (1/F1 - 1/F2)

The inductors end up being quite small but a couple of loops of wire cost
nothing. At 25megs the output is down to about 3% , which is not bad.

This particular style is the "3 element" design and from the traditional
"Constant K" filter stable. The pleasantly simple to design "constant K's"
and associated "M derived" filters, can be easily and freely used for most
day-to-day electronics work.

Exact Filter design, or 'synthesis' as they nowadays prefer to call it, can
be a nightmare if you're coming in from scratch. Even using a filter design
programme requires a good knowledge of filter types/performance/spec's
before data can begin to be entered.
If not using a prog' you move onto the more vexing method of having to
design an initial (prototype) low-pass filter, either via precalculated
tables or computing the required co-efficients, then mathematically
transforming the low-pass values into a bandpass (etc) design. For higher
order filters it can be ball-acheing drudgery and even then inductor
resistance has a nasty habit of spoiling that pristine work of art. (not a
problem with DSP filters).

I can't suggest a decent book, as the general purpose books seem over
complicated and destined to be written by maths people for maths people.
(The old GPO line-comm's manuals have useful notes on the 'K and M '
filters).
regards
john