Bandpass filter with adjustable Q

There has been interest in the past concerning filters with the property that some parameter of the filter can be adjusted (with, say, a potentiometer) without peturbing other parameters. The latest issue of Electronic Design magazine has such a design in the Ideas for Design section.

formatting link

I'm not sure but what I may have seen this one before, but I thought I'd call it to the attention of the group.

Reply to
The Phantom
Loading thread data ...

It's probably a case where the professors in question had a need for the circuit, figured it out, and decided to share it with others. Most of what ends up in trade magazines such as this is not meant to be new material, rather it's meant to present useful techniques to the working engineer. Hence, a lot of the same ideas are re-printed again and again...

Reply to
Joel Kolstad

some

peturbing

design in the

it to the

The "Twin-T" approach is the pitts. For how to do it right see all the gyrator-based filters on the S.E.D/Schematics page of my website. These feature independent adjustment of bandwidth and center frequency.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
 Click to see the full signature
Reply to
Jim Thompson

There are three functions being served by articles like this:

  1. For a mag to get the cheap periodicals mailing rate, they have to have a certain percentage of "editorial content", but would rather not pay for such, so they accept most anything. Without that requirement, they'd be all ads.

  1. Foreign engineers and academics - a huge fraction of the presenters

- get published in a "prestigious" US mag.

  1. The staff of the mag, journalism majors mostly, can just cut and paste the stuff and not have to worry their pretty heads over writing about electronics, about which they know very little.

Hell, bootsrapped twin-tees have been around for 70 years roughly.

John

Reply to
John Larkin

Yeah ... Just like the newsgroups...

;-) Rich

Reply to
Rich Grise

But I think the point of the article is that their thing has constant gain at the center frequency while you change the bandwidth. A Q mult. has increasing gain as the Q, well, multiplies. IOW, it's the "constant gain" feature that's their selling point.

At least that's how I see it.

Thanks, Rich

Reply to
Rich Grise

My gyrator-based filters have unity gain at fo, irrespective of bandwidth.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
 Click to see the full signature
Reply to
Jim Thompson

Hello John,

It's probably older than that. I built Q-multipliers when I was a kid. Not to brag about it but because I didn't have the dough for a decent filter in those days. Or, ahem, to be honest, I didn't want to tap into my brewsky funds for that.

Regards, Joerg

formatting link

Reply to
Joerg

that some

without peturbing

a design in the

call it to the

Used the same method with a Wein bridge network about 5 years ago. Idea came straight out of a electronics-for-beginers' book. Also saw similar idea in a Wireless World article from 50 years ago. This is "trivial" stuff, why does it take 3 of 'em to reinvent a wheel?. Have the uni's gone soft?. regards john

Reply to
john jardine

A bit like the US patent office then?

martin

Reply to
martin griffith

what

You're right of course. If they'd generated 'original' work then for sure, they'd be publishing it in some learned tome. Kneejerk on my part. Ie, If I can grok what they're saying then it wasn't worth them saying it. I'm just having a grumpy day :). regards john

Reply to
john jardine

Hello Rich,

IIRC the Q multiplier also had constant gain. You just hung a gain stage to the first filter and brought it really close to oscillation. Probably very similar to Jim's gyrator thing. It did not amplify anything. I do not recall having to adjust the volume when I changed the bandwidth. It was actually a pretty cool thing but not new at all. When I built it I basically copied from a tube based schematic in a ham radio magazine. Judging from the type of tube the original must have been a 40's or 50's design.

Regards, Joerg

formatting link

Reply to
Joerg

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.