| Author |
Message |
Joel Kolstad
Guest
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 Posted:
Thu Feb 10, 2005 8:30 am Post subject:
Are active filters realistic at 30MHz? |
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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
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Ken Smith
Guest
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| Posted:
Thu Feb 10, 2005 8:30 am Post subject:
Re: Are active filters realistic at 30MHz? |
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In article <OrSdnUccx6jDWpffRVn-rg@comcast.com>,
Joel Kolstad <JKolstad71HatesSpam@Yahoo.Com> wrote:
| Quote: | 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?
|
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.
--
--
kensmith@rahul.net forging knowledge |
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Jim Thompson
Guest
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| Posted:
Thu Feb 10, 2005 8:38 am Post subject:
Re: Are active filters realistic at 30MHz? |
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On Wed, 9 Feb 2005 18:30:41 -0800, "Joel Kolstad"
<JKolstad71HatesSpam@Yahoo.Com> wrote:
| Quote: | 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
|
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
30MHz.
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
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Tim Wescott
Guest
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| Posted:
Thu Feb 10, 2005 8:41 am Post subject:
Re: Are active filters realistic at 30MHz? |
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Joel Kolstad wrote:
| Quote: | 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?
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?
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com |
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John Woodgate
Guest
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| Posted:
Thu Feb 10, 2005 8:41 am Post subject:
Re: Are active filters realistic at 30MHz? |
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I read in sci.electronics.design that Joel Kolstad <JKolstad71HatesSpam@
Yahoo.Com> wrote (in <OrSdnUccx6jDWpffRVn-rg@comcast.com>) about 'Are
active filters realistic at 30MHz?', on Wed, 9 Feb 2005:
| Quote: | 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 |
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Joel Kolstad
Guest
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| Posted:
Thu Feb 10, 2005 8:41 am Post subject:
Re: Are active filters realistic at 30MHz? |
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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! |
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Joerg
Guest
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| Posted:
Thu Feb 10, 2005 2:57 pm Post subject:
Re: Are active filters realistic at 30MHz? |
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Hello Joel,
I would crack out Williams filter handbook and do it LC. Much cheaper,
and easier.
Regards, Joerg
http://www.analogconsultants.com |
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Joel Kolstad
Guest
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| Posted:
Thu Feb 10, 2005 9:25 pm Post subject:
Re: Are active filters realistic at 30MHz? |
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"Tim Wescott" <tim@wescottnospamdesign.com> wrote in message
news:110m4u55ku7g5cc@corp.supernews.com...
| Quote: | 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 |
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John Woodgate
Guest
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| Posted:
Thu Feb 10, 2005 10:54 pm Post subject:
Re: Are active filters realistic at 30MHz? |
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I read in sci.electronics.design that Joel Kolstad <JKolstad71HatesSpam@
Yahoo.Com> wrote (in <UtCdnXH3svRN4ZbfRVn-uw@comcast.com>) about 'Are
active filters realistic at 30MHz?', on Thu, 10 Feb 2005:
| Quote: | 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 |
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Michael Black
Guest
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| Posted:
Thu Feb 10, 2005 11:28 pm Post subject:
Re: Are active filters realistic at 30MHz? |
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Tim Wescott (tim@wescottnospamdesign.com) writes:
| Quote: | Joel Kolstad wrote:
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?
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?
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 |
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Joel Kolstad
Guest
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| Posted:
Thu Feb 10, 2005 11:29 pm Post subject:
Re: Are active filters realistic at 30MHz? |
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Hi John,
"John Woodgate" <jmw@jmwa.demon.contraspam.yuk> wrote in message
news:+aW6Q+HHH5CCFwUc@jmwa.demon.co.uk...
| Quote: | 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.
|
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 |
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John Woodgate
Guest
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| Posted:
Thu Feb 10, 2005 11:57 pm Post subject:
Re: Are active filters realistic at 30MHz? |
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|
I read in sci.electronics.design that Joel Kolstad <JKolstad71HatesSpam@
Yahoo.Com> wrote (in <R6OdnZcxVYCfB5bfRVn-rQ@comcast.com>) about 'Are
active filters realistic at 30MHz?', on Thu, 10 Feb 2005:
| Quote: | 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 |
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