Why Bessel?

[Roger Bourne]

Hello all,

I recently was going over an old electronic design, and I noticed that
the designer used a 4rth order Bessel low pass filter where low pass
filtering was needed. (I know it was a Bessel filter because it so
marked ). The bessel filter was realized with 2 op-amps and many
resistors and a few capacitors. Knowing the designer (designer is
overseas now for a few years...) was always making his designs as
compact as possible, - (had to twist both of to his arms to make him
insert failsafe redundancies ) - why would he use such a bulky
method for low pass filtering?
The bessel low pass filter cuttoff frequency is ~100Hz. It's sharpness
would have to be, hmmm, well pretty sharp. After all it was initially
intended for a medical application. I would hazard a guess that is what
warrants the 4rth order. But why a Bessel topology? I know bessel
filters are usefull in audio applications because of the linear phase
property they have, but this is not an audio application.
Basically, I am asking why Bessel ? Does Bessel have anything special
that I am anaware of?

-Roger

 

[John Popelish]

Hello all,

I recently was going over an old electronic design, and I noticed that
the designer used a 4rth order Bessel low pass filter where low pass
filtering was needed. (I know it was a Bessel filter because it so
marked ). The bessel filter was realized with 2 op-amps and many
resistors and a few capacitors. Knowing the designer (designer is
overseas now for a few years...) was always making his designs as
compact as possible, - (had to twist both of to his arms to make him
insert failsafe redundancies ) - why would he use such a bulky
method for low pass filtering?
The bessel low pass filter cuttoff frequency is ~100Hz. It's sharpness
would have to be, hmmm, well pretty sharp. After all it was initially
intended for a medical application. I would hazard a guess that is what
warrants the 4rth order. But why a Bessel topology? I know bessel
filters are usefull in audio applications because of the linear phase
property they have, but this is not an audio application.
Basically, I am asking why Bessel ? Does Bessel have anything special
that I am anaware of?

-Roger

Bessel filters don't ring when a step is applied to their inputs.
Would that mean anything to this design?

 

[Don Lancaster]

Hello all,

I recently was going over an old electronic design, and I noticed that
the designer used a 4rth order Bessel low pass filter where low pass
filtering was needed. (I know it was a Bessel filter because it so
marked ). The bessel filter was realized with 2 op-amps and many
resistors and a few capacitors. Knowing the designer (designer is
overseas now for a few years...) was always making his designs as
compact as possible, - (had to twist both of to his arms to make him
insert failsafe redundancies ) - why would he use such a bulky
method for low pass filtering?
The bessel low pass filter cuttoff frequency is ~100Hz. It's sharpness
would have to be, hmmm, well pretty sharp. After all it was initially
intended for a medical application. I would hazard a guess that is what
warrants the 4rth order. But why a Bessel topology? I know bessel
filters are usefull in audio applications because of the linear phase
property they have, but this is not an audio application.
Basically, I am asking why Bessel ? Does Bessel have anything special
that I am anaware of?

-Roger

A Bessel filter offers the lowest possible time delay and group delay
distortion. Its falloff rate is an utter atrocity.

See my Active Filter Cookbook.

--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
rss: http://www.tinaja.com/whtnu.xml email: [email protected]

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

[Kevin Aylward]

A Bessel filter offers the lowest possible time delay and group delay
distortion.

Err.. no it doesn't. Your statement is actually a bit vague.

The Bessel filter is a filter invented to produce a linear phase with
frequency (or constant delay) that actually has an *explicit*
mathematical method available to calculate its element values. This is a
key, fundamental point for all of the standard filters. The ability to
actually be able to mathematically calculate component values of a
filter of arbitrary order with some explicit procedure.

If one uses other computer techniques, not available when "modern filter
theory" was being invented, such as brute force least squares fitting it
is strait forward to design filters with better performance then pretty
much any of the standards such as chebychev, Butterworth, Bessel etc.

For example, the Q=2/pi 2nd order is a "better" least mean squares fit
to a linear phase response, i.e. better than the Bessel filter of 2nd
order.

Its falloff rate is an utter atrocity.

Since it wasn't designed to be a frequency filter this is not
surprising. The fact that it rolls off at all is incidental to its
initial invention.



Kevin Aylward B.Sc.
[email protected]
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

"There are none more ignorant and useless,than they that seek answers
on their knees, with their eyes closed"

 

[Adrian Tuddenham]

The Bessel filter is a filter invented to produce a linear phase with
frequency (or constant delay) [...]

Its falloff rate is an utter atrocity.

Since it wasn't designed to be a frequency filter this is not
surprising. The fact that it rolls off at all is incidental to its
initial invention.

Could you tell us who invented it - and a bit more about the design
process they went through if they didn't intend it to be a 'frequency
filter'?

 

[Roger Bourne]

A Bessel filter offers the lowest possible time delay and group delay

distortion. Its falloff rate is an utter atrocity.

How bad is the rolloff (falloff) rate?
We are talking about bessel low passes, rigth? Wouldn't it have to have
a decent rolloff (falloff) rate to be used as a low-pass ?
I assume that the rolloff rate would be ALSO among the parameters that
make or break a filter. Having linear phase is a great property, but it
would also need to attenuate. Rigth?

-Roger

-Roger