Are active filters realistic at 30MHz?

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!

May I recommend you take a look at the gyrator-based filter structures on my website? I would think a GHz GBW OpAmp will do just ducky at

...Jim Thompson

Crystal filters, if you need 0.01 to 0.1% relative bandwidths.

I suspect that an LC filter will be more frequency stable than anything with an op-amp (sorry Jim), it'll be passive so you won't have to worry about oscillations, and at 30MHz it won't be much bigger, if at all.

Whatcha trying to do?

Beware that the 3*Q*f0 thing means the GBP at the operating frequency. Many op-amps fall a bit faster than a single pole. If you want the filter to be very accurate, you may have to (gasp) trim it.

The f0 point tends to shift downwards in frequency as the op-amps add excess phase to the system. You may be able to just compensate for the typical values and let the variation in op-amps be a variation in the performance. A little figuring and calculating is needed.

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

I read in sci.electronics.design that Joel Kolstad wrote (in ) about 'Are active filters realistic at 30MHz?', on Wed, 9 Feb 2005:

Ceramic.

SAW (Surface Acoustic Wave).

Hello Joel,

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

Regards, Joerg

Hi John,

Yeah, I've thought of that, and it is quite attractive. The idea of an active filter instead was to introduce less distortion in the whole process, but I have no hard evidence for whether or not that's really the case instead of just a gut feeling.

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

---Joel

I read in sci.electronics.design that Joel Kolstad wrote (in ) about 'Are active filters realistic at 30MHz?', on Thu, 10 Feb 2005:

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.

That has to be the key question.

Whether or not gyrators are suitable up there, gyrators tend to be used where the coil it replaces is way too bulky, and even where the coil may cause stray pickup. These are audio issues, since a big inductor (electrically and physically) can so easily pick up stray 60Hz, and the large inductance can take up space.

At 30MHz, the coils become small, and they have too little inductance to pick up that 60Hz. The active circuitry is likely to use up more space than the inductors they replace. Expecially since a far more complicated filter will be needed at 30MHz to get desired bandwidth.

One also wonders how the gyrators act under strong RF signals. Since one reason for filters at RF is to ensure the active elements don't see too much signal, does using active elements as inductors lower the signal handling capability of the receiver?

When people ask about high frequency gyrators, they seem to generally lack a grounding in RF, and are trying to extend audio concepts that they know to a field they don't know.

If one needs a lot of selectivy, you use a crystal filter at the signal frequency (if you can afford it), or go to a superheterodyne receiver so the selectivity is at a frequency where it can be done easily, with front end selectivity only there to limit what the active components see.

One could always add some regeneration to front end filtering to increase Q, but of course that doesn't help skirt selectivity.

Michael

I read in sci.electronics.design that Joel Kolstad wrote (in ) about 'Are active filters realistic at 30MHz?', on Thu, 10 Feb 2005:

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.