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Is 'Bessel' digital lowpass filter standard?

Started by gretzteam August 24, 2010
On Aug 25, 11:59&#2013266080;am, "steveu" <steveu@n_o_s_p_a_m.coppice.org> wrote:
> >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- Hide quoted text - > > - Show quoted text -
I know Motorola (of course made a TX) and ERG (Australia) made such a TX. Who did you work for? I think Tait (New Zealand) was working on such a design. just curious, Clay

steveu wrote:


>>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.
Our transmitters worked at 150 and 450 MHz bands. The initial plan was frequency upconversion as it could provide for the better spectrum, however the PLL appeared to be much simpler in the hardware. BTW, I have never seen high speed modes and/or 4-level modulation used in the practical installations of POCSAG/FLEX/ERMES. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
> > >steveu wrote: > > >>>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. > >Our transmitters worked at 150 and 450 MHz bands. The initial plan was >frequency upconversion as it could provide for the better spectrum, >however the PLL appeared to be much simpler in the hardware. > >BTW, I have never seen high speed modes and/or 4-level modulation used >in the practical installations of POCSAG/FLEX/ERMES.
I've never even seen an ERMES installation of any kind. :-) I believe SkyTel in the US used FLEX at full speed. They also used reFlex. Perhaps they were the only people who did. Steve
On Aug 25, 1:28&#2013266080;pm, "steveu" <steveu@n_o_s_p_a_m.coppice.org> wrote:
> >steveu wrote: > > >>>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. > > >Our transmitters worked at 150 and 450 MHz bands. The initial plan was > >frequency upconversion as it could provide for the better spectrum, > >however the PLL appeared to be much simpler in the hardware. > > >BTW, I have never seen high speed modes and/or 4-level modulation used > >in the practical installations of POCSAG/FLEX/ERMES. > > I've never even seen an ERMES installation of any kind. :-) I believe > SkyTel in the US used FLEX at full speed. They also used reFlex. Perhaps > they were the only people who did. > > Steve- Hide quoted text - > > - Show quoted text -
Skytel and Pagenet both used 4 level at 6400 bps. Back around 1996 when I did a paging decoder for the feds, I would routinely see a dozen channels of high speed FLEX.
>On Aug 25, 1:28=A0pm, "steveu" <steveu@n_o_s_p_a_m.coppice.org> wrote: >> >steveu wrote: >> >> >>>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. >> >> >Our transmitters worked at 150 and 450 MHz bands. The initial plan was >> >frequency upconversion as it could provide for the better spectrum, >> >however the PLL appeared to be much simpler in the hardware. >> >> >BTW, I have never seen high speed modes and/or 4-level modulation used >> >in the practical installations of POCSAG/FLEX/ERMES. >> >> I've never even seen an ERMES installation of any kind. :-) I believe >> SkyTel in the US used FLEX at full speed. They also used reFlex.
Perhaps
>> they were the only people who did. >> >> Steve- Hide quoted text - >> >> - Show quoted text - > >Skytel and Pagenet both used 4 level at 6400 bps. Back around 1996 >when I did a paging decoder for the feds, I would routinely see a >dozen channels of high speed FLEX.
Couldn't the feds get their feed directly from the paging centres, or was the legality of their actions a bit iffy? I developed a PC based encoder for FLEX around that time, just before the paging market collapsed. :-\ Steve
gretzteam <gretzteam@n_o_s_p_a_m.yahoo.com> 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?
I retract the entire statement.
>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 will give you a filter with no overshoot and a lot of rolloff in the passband, but Vlad correctly states it is not a Bessel. I was confused. Steve
On Aug 25, 2:14&#2013266080;pm, "steveu" <steveu@n_o_s_p_a_m.coppice.org> wrote:
> >On Aug 25, 1:28=A0pm, "steveu" <steveu@n_o_s_p_a_m.coppice.org> wrote: > >> >steveu wrote: > > >> >>>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. > > >> >Our transmitters worked at 150 and 450 MHz bands. The initial plan was > >> >frequency upconversion as it could provide for the better spectrum, > >> >however the PLL appeared to be much simpler in the hardware. > > >> >BTW, I have never seen high speed modes and/or 4-level modulation used > >> >in the practical installations of POCSAG/FLEX/ERMES. > > >> I've never even seen an ERMES installation of any kind. :-) I believe > >> SkyTel in the US used FLEX at full speed. They also used reFlex. > Perhaps > >> they were the only people who did. > > >> Steve- Hide quoted text - > > >> - Show quoted text - > > >Skytel and Pagenet both used 4 level at 6400 bps. Back around 1996 > >when I did a paging decoder for the feds, I would routinely see a > >dozen channels of high speed FLEX. > > Couldn't the feds get their feed directly from the paging centres, or was > the legality of their actions a bit iffy? > > I developed a PC based encoder for FLEX around that time, just before the > paging market collapsed. :-\ > > Steve- Hide quoted text - > > - Show quoted text -
It depends on which feds ;-) CALEA (communication assistance for law enforcement act) allowed the feds to directly connect into call centers etc. But there were cases where the phone # or the esn (pager capcode) was unknown and monitoring (data mining) would allow it to be found. Still in these cases a federal wiretap order was technically required. There were also feds operating outside of the US so they did things surreptitiously. Plus the equipment was also purchased by other governments too. I recall working with border crossing sensors that would directly page agents. The earliest decoders I did was before CALEA was fully implemented, so it also decoded golay, nec, and pocsag. There are still some people making paging equipment nowadays. These include campus wide systems for universities and hospitals. I believe that market is also drying up. I'd just send a text message via the internet to a cellphone - it works way better than paging.
On Aug 25, 1:13&#2013266080;pm, Vladimir Vassilevsky <nos...@nowhere.com> wrote:
> steveu wrote: > >>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. > > Our transmitters worked at 150 and 450 MHz bands. The initial plan was > frequency upconversion as it could provide for the better spectrum, > however the PLL appeared to be much simpler in the hardware. > > BTW, I have never seen high speed modes and/or 4-level modulation used > in the practical installations of POCSAG/FLEX/ERMES. > > Vladimir Vassilevsky > DSP and Mixed Signal Design Consultanthttp://www.abvolt.com
Vlad, tune to 929.6125 MHz and you will see 4 level FLEX and 2400 bps POCSAG. Clay

steveu wrote:

>>On Aug 25, 1:28=A0pm, "steveu" <steveu@n_o_s_p_a_m.coppice.org> wrote: >>>>steveu wrote: >>> >>>>>>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. >>> >>>>Our transmitters worked at 150 and 450 MHz bands. The initial plan was >>>>frequency upconversion as it could provide for the better spectrum, >>>>however the PLL appeared to be much simpler in the hardware. >>> >>>>BTW, I have never seen high speed modes and/or 4-level modulation used >>>>in the practical installations of POCSAG/FLEX/ERMES. >>> >>>I've never even seen an ERMES installation of any kind. :-) I believe >>>SkyTel in the US used FLEX at full speed. They also used reFlex. > > Perhaps > >>>they were the only people who did. >>> >>>Steve- Hide quoted text - >>> >>>- Show quoted text - >> >>Skytel and Pagenet both used 4 level at 6400 bps. Back around 1996 >>when I did a paging decoder for the feds, I would routinely see a >>dozen channels of high speed FLEX. > > > Couldn't the feds get their feed directly from the paging centres, or was > the legality of their actions a bit iffy?
It is mandated that they have direct feeds from all kinds of telecoms at home; however, it has to work abroad, too.
> I developed a PC based encoder for FLEX around that time, just before the > paging market collapsed. :-\
Swissphone used proprietary encryption over POCSAG. There were (are?) quite interesting paging systems with PSK modulation over the AM broadcast carriers. VLV
> > >steveu wrote: > >>>On Aug 25, 1:28=A0pm, "steveu" <steveu@n_o_s_p_a_m.coppice.org> wrote: >>>>>steveu wrote: >>>> >>>>>>>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. >>>> >>>>>Our transmitters worked at 150 and 450 MHz bands. The initial plan
was
>>>>>frequency upconversion as it could provide for the better spectrum, >>>>>however the PLL appeared to be much simpler in the hardware. >>>> >>>>>BTW, I have never seen high speed modes and/or 4-level modulation
used
>>>>>in the practical installations of POCSAG/FLEX/ERMES. >>>> >>>>I've never even seen an ERMES installation of any kind. :-) I believe >>>>SkyTel in the US used FLEX at full speed. They also used reFlex. >> >> Perhaps >> >>>>they were the only people who did. >>>> >>>>Steve- Hide quoted text - >>>> >>>>- Show quoted text - >>> >>>Skytel and Pagenet both used 4 level at 6400 bps. Back around 1996 >>>when I did a paging decoder for the feds, I would routinely see a >>>dozen channels of high speed FLEX. >> >> >> Couldn't the feds get their feed directly from the paging centres, or
was
>> the legality of their actions a bit iffy? > >It is mandated that they have direct feeds from all kinds of telecoms at >home; however, it has to work abroad, too. > >> I developed a PC based encoder for FLEX around that time, just before
the
>> paging market collapsed. :-\ > >Swissphone used proprietary encryption over POCSAG. >There were (are?) quite interesting paging systems with PSK modulation >over the AM broadcast carriers.
Nokia and others made quite a lot of pagers that decoded RDS. These, and the AM radio ones, were rather bulky and heavy compared to a POCSAG pager, though. I never saw encrypted POCSAG. Salesmen generally told customers that normal paging was secure, and engineers seldom likes to spoil the illusion. :-) Steve