[ARC5] Using SCR-274N active filter ?

[Bruce Long]

Active filter IF filters
There is a somewhat unconventional active filter circuit that uses two op-amps to synthesize the equivalent of a single pole LC resonant filter  referred to (In the "Electronic Filter Handbook" Author Williams  as a Dual Amplifier Bandpass  DARP configuration  first introduced by Sedra and  Espinoza  --  reference infoa added incase anybody wants to look it up    that is excellent for such an application as it is very undemanding of op-amp performance and component tolerance.  Cener frequency and Q can each be adjusted independantly with separate poteniometers.

You can cascade stages to generate any filter response you like - Butterworth Chebyshev Bessel etc- using conventional LC filter design techniques.  I have been wanting to find some time and design a good 80 kHz Bessel filter ( Bessel = low ringing) for use in a BC-453 or as part of a complete receiver with up convertion to 45 MHz followed by narrow low cost high quality, adjustable filtering at 50-80 KHz ( somewhere in this range).

Of course i have not had the time.

If anyone is interested I am willing to scan and send the approbriate pages from the Williams filter design handbook.

I have used this filter several times and have always gotten exactly what I expected out of it.

bruce






________________________________
 From: Bill Cromwell <wrcromwell at gmail.com>
To: ARC-5 Maillist <arc5 at mailman.qth.net> 
Sent: Friday, March 29, 2013 10:53 AM
Subject: Re: [ARC5] Using SCR-274N
 
On Fri, 2013-03-29 at 09:54 -0700, Dennis Monticelli wrote:
> I have fancied going the crystal filter route with the BC-453 (with
> converter ahead for the ham bands).  Unfortunately, 85KHz resonators
> are not only hard to find, then are very high in impedance. The hi-Z
> makes them hard to graft into the IF chain without a lot of loss.  It
> can be done, but it takes more work to do it in such a manner that it
> is easily reversible.  A better route is probably using a fast op amp
> configured as an active filter.  You can use a modern hi-speed op amp
> these days at 85KHz and still get good Q.  It would be small and could
> run off a step-down from the DC filament voltage.  I would experiment
> with this if I had the time.
> 
> 
> Dennis AE6C


Hi Dennis,

The IF strip in a BC-453 or R-23 is already pretty narrow with the tabs
pulled all the way up..their narrowest setting. I get single signal
reception and when I plug in the NEScaf filter is chokes right down to
narrow CW bandwidths - less than 100 hurts. I am thinking of the xtal
filters (or Q-multipliers) for the 80 and 40 meter receivers. A guy on a
Harley rode right through on my 40 meter receiver (evil grin). He passed
a VW bug that was already coming through the IF.

73,

Bill  KU8H


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