Are active filters realistic at 30MHz?

[Joel Kolstad]

Just curious... has anyone used an active filter at 30MHz or higher? I'd be
looking for a second order bandpass filter up there, and using a state
variable topology I'm reading that the GBW of the amplifier needs to be at
least 3*Q*f0... and I can live with a Q of about 10-12.5, so that would
indicate a GBW in the ballpark of a GHz. Analog Devices will such you such
an op-amp, but is it going to very difficult to get the filter to perform
properly when the GBW is the same as the '3*Q*f' formula? Or does that
formula assume you'll spec the op-amp's GBW directly from it?

Other than LC filters, are there other common methods for HF range filters?

Thanks,
---Joel

 

[Joel Kolstad]

Thanks Jim, will do. In fact I already came across your gyrator web page
while Googling -- you had quite the discussion with Win and Peter Brackett!

 

[John Woodgate]

I read in sci.electronics.design that Joel Kolstad <JKolstad71HatesSpam@
Yahoo.Com> wrote (in <OrSdnUccx6jDWpffRVn-rg@comcast.com&gt about 'Are
active filters realistic at 30MHz?', on Wed, 9 Feb 2005:

Other than LC filters, are there other common methods for HF range
filters?

30 MHz is the border between HF and VHF.

Ceramic.

SAW (Surface Acoustic Wave).
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[Joerg]

Hello Joel,

I would crack out Williams filter handbook and do it LC. Much cheaper,
and easier.

Regards, Joerg

http://www.analogconsultants.com

 

[Joel Kolstad]

"Tim Wescott" <tim@wescottnospamdesign.com> wrote in message
news:110m4u55ku7g5cc@corp.supernews.com...

Whatcha trying to do?

Build an electronically tunabe notch filter for the entire HF band.

'Electronically tunable' can mean horrible things like motor driven L/C's,
but I'd really like to avoid that if possible.

---Joel

 

[John Woodgate]

I read in sci.electronics.design that Joel Kolstad <JKolstad71HatesSpam@
Yahoo.Com> wrote (in <UtCdnXH3svRN4ZbfRVn-uw@comcast.com&gt about 'Are
active filters realistic at 30MHz?', on Thu, 10 Feb 2005:

Build an electronically tunabe notch filter for the entire HF band.

Heterodyne up to the 100 MHz band with a fixed oscillator, make a
tunable notch filter for 100 MHz to 127 MHz and heterodyne down again.
Or 500 MHz, if 27% bandwidth is too much.

Or heterodyne up with a tunable oscillator to a fixed 100 MHz notch
filter and heterodyne back down.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[John Woodgate]

I read in sci.electronics.design that Joel Kolstad <JKolstad71HatesSpam@
Yahoo.Com> wrote (in <R6OdnZcxVYCfB5bfRVn-rQ@comcast.com&gt about 'Are
active filters realistic at 30MHz?', on Thu, 10 Feb 2005:

BTW, how were you planning to build your tunable 100-127MHz notch
filter? Varactor diodes in an LC filter?

I left that as an exercise for the student. If you have room, a trough
line or a cavity might be interesting. VERY sharp notch.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk