Is 'Bessel' digital lowpass filter standard?

Started by gretzteam August 24, 2010
"Vladimir Vassilevsky" <nospam@nowhere.com> wrote in message 
news:Sqmdnb5lg99iZe7RnZ2dnUVZ_qKdnZ2d@giganews.com...

> I can make a filter as close to a brick wall as required without any > overshoot or ringing.
How? Any brick wall lpf is going to ring if an input transition has components outwith the pass-band.
> It could be done with linear or nonlinear phase; there is no connection.
Agreed (but in the negative). Nonlinear phase has asymmetric ringing. Pete
gretzteam <gretzteam@n_o_s_p_a_m.yahoo.com> wrote:

> I'm now trying to find a good-enough solution, that solves > the problem. I don't know what is wrong with that. Bessel gets > me pretty much there, and I wonder if there could potentially > be better.
A cascade of identical single-pole filters is a Bessel filter (true whether they are continuous-time or discrete-time). If a particular filter type is known to be good for the problem at hand then it may make sense to just use it. You might also compare the delay spread of said Bessel filter with other proposed soluations such as linear-phase FIR's. Steve

Steve Pope wrote:


> A cascade of identical single-pole filters is a Bessel filter > (true whether they are continuous-time or discrete-time).
Wrong whether they are continuous-time or discrete-time. VLV
Vladimir Vassilevsky  <nospam@nowhere.com> wrote:
> >Steve Pope wrote:
>> A cascade of identical single-pole filters is a Bessel filter >> (true whether they are continuous-time or discrete-time).
>Wrong whether they are continuous-time or discrete-time.
Okay, I retract that. Steve
> > >Clay wrote: > > > >> It could be that the >> Bessel filter is good enough for your application. If need be, just >> increase the filter order to get the overshoot under control. > >The Bessel filters of the higher orders have higher overshoot. > >> The >> magnitude response of Bessel filters (also called Thompson filters) > >IIRC the Thompson filter is some kind of compromise between Butterworth >and Bessel responses. > >> is >> described by some as sort of "droopy." So you don't see them often, >> but I do know of some protocols where they are speced in. For example, >> Flex and ERMES specify 10th order Bessel filters > >9th order, Fc = 3.9kHz. Just advised as an example, not prescribed. > >> for intersymbol interference rejection! > >for adjacent channel spillage reduction. > >> Probably not the best answer by the protocol >> committee but it was its simplicity to specify as opposed to some >> optimized numerical result that made it get speced that way. > >I was surprised by such an advice also; actually, I used FIR filter.
An actual filter in the product? It was easier to do the implementation with a simple lookup table. I was also puzzled by that spec, but it might have been someone's in joke. None of the people putting the ERMES spec together really took it seriously. It dragged on for ages, poorly resourced. I think the first FLEX transmitter from Motorola just reused their ERMES prototype, when the ERMES spec stalled and they decided to proceed with FLEX. Steve
On Aug 25, 4:39&#2013266080;am, "steveu" <steveu@n_o_s_p_a_m.coppice.org> wrote:
> >Clay wrote: > > >> It could be that the > >> Bessel filter is good enough for your application. If need be, just > >> increase the filter order to get the overshoot under control. > > >The Bessel filters of the higher orders have higher overshoot. > > >> The > >> magnitude response of Bessel filters (also called Thompson filters) > > >IIRC the Thompson filter is some kind of compromise between Butterworth > >and Bessel responses. > > >> is > >> described by some as sort of "droopy." So you don't see them often, > >> but I do know of some protocols where they are speced in. For example, > >> Flex and ERMES specify 10th order Bessel filters > > >9th order, Fc = 3.9kHz. Just advised as an example, not prescribed. > > >> for intersymbol interference rejection! > > >for adjacent channel spillage reduction. > > >> Probably not the best answer by the protocol > >> committee but it was its simplicity to specify as opposed to some > >> optimized numerical result that made it get speced that way. > > >I was surprised by such an advice also; actually, I used FIR filter. > > An actual filter in the product? It was easier to do the implementation > with a simple lookup table. I was also puzzled by that spec, but it might > have been someone's in joke. None of the people putting the ERMES spec > together really took it seriously. It dragged on for ages, poorly > resourced. I think the first FLEX transmitter from Motorola just reused > their ERMES prototype, when the ERMES spec stalled and they decided to > proceed with FLEX. > > Steve- Hide quoted text - > > - Show quoted text -
Yes , I actually met with the chair of the Flex protocol committee. He said they simply followed the ERMES spec so FLEX could be used on an ERMES capable transmitter. Of course the FLEX spec had its own issues which have nothing to do with Bessel filters. And believe it or not, FLEX is still being used. I heard it on my scanner just the other night. You can hear the "keep alive" burst at the top of every minute on 929.6125MHz. Clay

steveu wrote:
>> >>Clay wrote: >>
>>>Flex and ERMES specify 10th order Bessel filters >>9th order, Fc = 3.9kHz. Just advised as an example, not prescribed. >> >>>Probably not the best answer by the protocol >>>committee but it was its simplicity to specify as opposed to some >>>optimized numerical result that made it get speced that way. >> >>I was surprised by such an advice also; actually, I used FIR filter. > > > An actual filter in the product? It was easier to do the implementation > with a simple lookup table.
Yes, the actual transmitter. It was more complicated as it was DDS modulated with PLL multiplication of the frequency. Pulse shaping was achieved by loop filter altogether with DSP FIR filter; it took some time to optimize this combination for proper pulse shape and clean output spectrum. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
>Vladimir Vassilevsky <nospam@nowhere.com> wrote: >> >>Steve Pope wrote: > >>> A cascade of identical single-pole filters is a Bessel filter >>> (true whether they are continuous-time or discrete-time). > >>Wrong whether they are continuous-time or discrete-time. > >Okay, I retract that. > > >Steve >
What needs to be retracted? The whole thing or just the part about continuous-time or discrete-time? Are you saying that I could simply cascade a few single-pole filters to get something similar to the Bessel both in regards to overshoot and attenuation? Dave

gretzteam wrote:

>>Vladimir Vassilevsky <nospam@nowhere.com> wrote: >> >>>Steve Pope wrote: >> >>>>A cascade of identical single-pole filters is a Bessel filter >>>>(true whether they are continuous-time or discrete-time). >> >>>Wrong whether they are continuous-time or discrete-time. >> >>Okay, I retract that.
>
> What needs to be retracted? The whole thing or just the part about > continuous-time or discrete-time? > > Are you saying that I could simply cascade a few single-pole filters to get > something similar to the Bessel both in regards to overshoot and > attenuation?
A cascade of single pole filters is what is called real pole filter. It does not overshoot, however it is not a linear phase and its frequency response is even more sluggish then that of a Bessel filter. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
> > >steveu wrote: >>> >>>Clay wrote: >>> > >>>>Flex and ERMES specify 10th order Bessel filters >>>9th order, Fc = 3.9kHz. Just advised as an example, not prescribed. >>> >>>>Probably not the best answer by the protocol >>>>committee but it was its simplicity to specify as opposed to some >>>>optimized numerical result that made it get speced that way. >>> >>>I was surprised by such an advice also; actually, I used FIR filter. >> >> >> An actual filter in the product? It was easier to do the implementation >> with a simple lookup table. > >Yes, the actual transmitter. It was more complicated as it was DDS >modulated with PLL multiplication of the frequency. Pulse shaping was >achieved by loop filter altogether with DSP FIR filter; it took some >time to optimize this combination for proper pulse shape and clean >output spectrum.
That sounds fairly similar to the deign we ended up with. We had 150, 280 and 930MHz versions. Steve