Passive Bessel Filter

Discussion in 'Electronic Basics' started by Samuel Groner, Jun 9, 2004.

1. Samuel GronerGuest

Hi

I want to design a passive Bessel LP-filter, 2nd order @ 200 kHz.
Source imp. is 50 ohm, terminated with 250k ohm.

From Zverevs "Handbook of Filter Synthesis", p. 323, I get the
normalized values L1=1.3617 and C1=0.4539. And now: how do I
denormalize this stuff?

I got the math for 1/Rsource<=10, but with 1/Rsource=INF, I don't know
how to choose the reference impedances.

Thanks for the help
Samuel

2. Joe McElvenneyGuest

Hi,

It is easier with odd order networks. Change the design to a
3-pole filter (||C, series-L, ||C), cut it in half, transform the
hi-Z end only (C becomes C/R and L becomes R x L), stitch it back
together (adding the series components) and then do the usual
frequency and impedance scaling.

This works (according to Geffe) on Butterworth and
Tchebycheff, so it should do the same for low-order Bessel.
Alternatively, with a 50-ohm source, 250K is practically
open-circuit and most filter tables give values for this.

Cheers - Joe

3. John LarkinGuest

C' = C / (W' * Z')

L' = L * Z' / W'

where W' is cutoff, 2*pi*200KHz

Z' = 50

250K is close enough to an open circuit, so just use a 50-to-infinity
design. The prototypes are common in the filter book tables. Ends are
freely flippable.

Oh, I have a cute, simple program that designs bessels up to 5th
order. Not exactly "designs", really just stashes the prototype values
and normalizes for you. I could post to a.b.s.e. if there was popular
demand.

John

4. Samuel GronerGuest

Z' = 50

That was it! Thanks for the help...
Samuel