Return-Path: Received: (qmail 3951 invoked from network); 5 Jun 2000 12:50:48 -0000 Received: from unknown (HELO post.thorcom.com) (212.172.148.70) by teachers.core.plus.net.uk with SMTP; 5 Jun 2000 12:50:48 -0000 Received: from majordom by post.thorcom.com with local (Exim 3.02 #1) id 12ywCN-0007h1-00 for rsgb_lf_group-outgoing@blacksheep.org; Mon, 05 Jun 2000 13:41:43 +0100 Received: from helios.herts.ac.uk ([147.197.200.2]) by post.thorcom.com with esmtp (Exim 3.02 #1) id 12ywCM-0007gw-00 for rsgb_lf_group@blacksheep.org; Mon, 05 Jun 2000 13:41:42 +0100 X-Priority: 3 X-MSMail-Priority: Normal Received: from [147.197.200.44] (helo=gemini) by helios.herts.ac.uk with esmtp (Exim 3.11 #1) id 12ywCJ-0005Hp-00 for rsgb_lf_group@blacksheep.org; Mon, 05 Jun 2000 13:41:39 +0100 Message-ID: <23462.200006051241@gemini> X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1106 From: "James Moritz" Organization: University of Hertfordshire To: rsgb_lf_group@blacksheep.org Date: Mon, 5 Jun 2000 13:48:45 +0000 MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII; format=flowed Content-Transfer-Encoding: 8bit Subject: Re: LF: Bessel bandpass filter? In-reply-to: <393881DE.3D9D@xtra.co.nz> X-Mailer: Pegasus Mail for Win32 (v3.11) Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group Sender: Dear LF Group, ZL2CA wrote: > But the intended thrust of this email is to ask about if anyone can find a > reference to a BESSEL BANDPASS filter. Text books all say that the Bessel > low pass filter has the most linear phase response (best group delay) of > the basic range of filters (Butterworth, Chebyshev, elliptic, Bessel). > But I can not find any reference to a BANDPASS variant of the Bessel > filter. I have a feeling that it is not realisable mathematically, and > that is why it is obvious by its absence as a text book band pass filter, > but if there is a filter theory guru on the reflector I would dearly like > to hear a response. Not sure if I could be called a 'filter guru', but I have a fair bit of experience designing filters. It is a huge area, and difficult to scratch the surface, but here are some useful bits of info: You won't find tables of Bessel or other bandpass filter values very often - this is because the standard filter design procedure starts with a normalised low pass design (ie. designed for 1ohm source & load, 1rad/s cut off frequency) and transforms it into whatever low- high-, bandpass, or bandstop filter you desire. So the Bessel low pass tables are where you need to start to design a Bessel bandpass filter. For passive LC filters, these are usually given as a table of L and C values, but active filter designs usually start from the tables of pole locations (these effectively specify the cut off frequency and Q of each section of the filter). The reason for doing this is that the possible permutations of bandwidths and centre frequencies are more or less infinite, so tabulating all the values would be impossible. An alternative is the 'coupled resonator' approach, which is normally reserved for passive bandpass filters with bandwidths less than 5 or 10% of the centre frequency. This uses tables of k and q values for different types of filter response. As to how to do it, the A.B. Williams / F.J. Taylor 'Electronic filter design handbook' is excellent for passive LC and active designs, and includes many worked examples and formulas for different types of circuit suitable for different applications. Later editions have a section on digital filters too. The bible for passive designs including crystal filters is A.I. Zverev's 'Handbook of filter synthesis'. These are the two references I have used most, but there are others too. As to different types of filter response, there is always compromise. Filters with sharp cut-off always have poor transient response, those with good transient response have lousy skirt selectivity. So Chebyshev and eliptic filters have lots of ringing and overshoot, but good selectivity, while Bessel, Gaussian, linear phase etc. have little ringing and overshoot, but poor selectivity. Increasing the number of poles (in a bandpass filter the number of resonators) will improve the selectivity / transient response trade- off, but require higher Q for each element, and greater precision in component values, and have higher insertion losses. There are also many compromise filter responses, which have both reasonable transient response and selectivity. These include the Butterworth, and various 'transitional' responses. I have had good results with 100 and 250Hz, 5 pole CW filters using a 'transitional Gaussian to -6dB' response; this has a rounded response like a Bessel filter until it falls of by 6dB, when the skirts get steeper like a Chebyshev filter. I was impressed by the 'crispness' of CW through these filters. In general, any passive filter design can also be implemented using an active filter and vice versa. Passive filters have lower component count and better dynamic range, and work better at high frequencies. Active filters do not require inductors, and are better suited to very narrow band filters where very high Q is required. It seems to be difficult to design a passive CW audio filter with a bandwidth much less than about 50Hz, due to the finite Q of inductors available. This is fairly easy with active filters, but requires the use of multiple op-amp filter sections, rather than the single op-amp multiple feedback circuits. It gets increasingly difficult to make good active filters at high frequencies, but the audio range is no problem. Whether active or passive, it is usually neccessary to trim component values with narrow filters. For audio filters, BiFET op amps like TL071/81, LF351 etc and their dual and quad variants are a much better choice than 741 type op amps; their lower bias currents make for more choice in the range of resistor and capacitor values. Some of the newer CMOS op-amps are good too, but relatively costly. The most important op-amp parameter for filters is the gain-bandwidth product - the higher it is, the higher the filter Q that can be obtained with a given circuit. There are usually many ways of producing a given filter performance; it's best to have a read of one of the handbooks before you start. Hope this E-mail is legible - something funny has happened to the text wrap on this software! Cheers, Jim Moritz 73 de M0BMU