Take a look at lowband filters and duplexers from CellWave, Telewave,
etc.
Your filter is going to be a coaxial cavity about 2mx20cm with piston
for tuning.
I am sure there are some better approaches to your task


Vladimir Vassilevsky, Ph.D.

DSP and Mixed Signal Design Consultant

http://www.abvolt.com


Paul Burridge wrote:

Hi chaps,

I've decided to bite the bullet and try to build an RF filter for
40Mhz. This filter will ideally have a very, very sharp characteristic
at one single spot frequency +-20Khz and attenuate the crap out of
anything either side of this. It'll need to be tunable over a range of
say 200Khz. Can anyone give me a steer on what type of arrangement
would be best suited to fit this purpose?
Thanks,

p.
--

"I believe history will be kind to me, since I intend
to write it." - Winston Churchill

In article , mentioned...

Take a look at lowband filters and duplexers from CellWave, Telewave,
etc.
Your filter is going to be a coaxial cavity about 2mx20cm with piston
for tuning.
I am sure there are some better approaches to your task

How about a helical resonator. They're smaller than a cavity, maybe
not as high Q, but still higher than lumped constant tuned circuit.
They're tunable, but I'm not sure how much.


Vladimir Vassilevsky, Ph.D.

Paul Burridge wrote:

Hi chaps,

I've decided to bite the bullet and try to build an RF filter for
40Mhz. This filter will ideally have a very, very sharp characteristic
at one single spot frequency +-20Khz and attenuate the crap out of
anything either side of this. It'll need to be tunable over a range of
say 200Khz. Can anyone give me a steer on what type of arrangement
would be best suited to fit this purpose?
Thanks,

p.
--

--
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###Got a Question about ELECTRONICS? Check HERE First:###
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On Sat, 26 Jul 2003 21:20:18 -0700, Watson A.Name - 'Watt Sun'
wrote:

In article , mentioned...

Take a look at lowband filters and duplexers from CellWave, Telewave,
etc.
Your filter is going to be a coaxial cavity about 2mx20cm with piston
for tuning.
I am sure there are some better approaches to your task

How about a helical resonator. They're smaller than a cavity, maybe
not as high Q, but still higher than lumped constant tuned circuit.
They're tunable, but I'm not sure how much.

I'd like to live long enough to see a helical at 40MHz that can
'attenuate the hell" out of signals outside the Fr+/-20kHz channel.

On Sat, 26 Jul 2003 21:20:18 -0700, Watson A.Name - 'Watt Sun'
wrote:

In article , mentioned...

Take a look at lowband filters and duplexers from CellWave, Telewave,
etc.
Your filter is going to be a coaxial cavity about 2mx20cm with piston
for tuning.
I am sure there are some better approaches to your task

How about a helical resonator. They're smaller than a cavity, maybe
not as high Q, but still higher than lumped constant tuned circuit.
They're tunable, but I'm not sure how much.

I'd like to live long enough to see a helical at 40MHz that can
'attenuate the hell" out of signals outside the Fr+/-20kHz channel.

Watson A.Name - 'Watt Sun' wrote in message ...
....
How about a helical resonator. They're smaller than a cavity, maybe
not as high Q, but still higher than lumped constant tuned circuit.

I think that's a popular misconception. The resonator Q is
essentially the same as the Q of the same part used as a shielded
inductor, and the shield actually lowers the Q from what it is with an
inductor in free air (so long as it's not large enough to radiate
significantly).

They're tunable, but I'm not sure how much.

They're certainly easily tunable over a few percent, if you need
that...

But the problem as originally stated implies a filter of fairly high
order and low in-band attenuation, which in turn implies resonators of
very high unloaded Q. 20kHz bandwidth at 40MHz in a single tank is a
loaded Q of 2000, and to keep attenuation low, the unloaded resonator
Q should be perhaps 5 times that much. It would be worse for a
multi-pole filter. All this tells me it's silly to even think of an
LC filter. Add to that the extreme difficulty of getting a set of
resonators to tune together. (To get Qu=10000 in a coaxial resonator
at 40MHz would take an air-dielectric line nearly half a meter in
diameter! Just plain silly.)

I'd opt for a front end with very high dynamic range (esp. low
third-order intermod products), into a good IF filter, etc., and a
communications protocol that optimized whatever performance measure I
needed. Talk to the people who build RF communications sytems that go
on aircraft carriers. Or talk with hams who design receivers with
third order intercepts up in the +50dBm region and higher. By the
way, you may do well by putting an ATTENUATOR on the front end, if
interference (distortion products), and not desired-signal-strength,
is the problem. Distortion products will go down faster than the
inserted attenuation.

Cheers,
Tom

Watson A.Name - 'Watt Sun' wrote in message ...
....
How about a helical resonator. They're smaller than a cavity, maybe
not as high Q, but still higher than lumped constant tuned circuit.

I think that's a popular misconception. The resonator Q is
essentially the same as the Q of the same part used as a shielded
inductor, and the shield actually lowers the Q from what it is with an
inductor in free air (so long as it's not large enough to radiate
significantly).

They're tunable, but I'm not sure how much.

They're certainly easily tunable over a few percent, if you need
that...

But the problem as originally stated implies a filter of fairly high
order and low in-band attenuation, which in turn implies resonators of
very high unloaded Q. 20kHz bandwidth at 40MHz in a single tank is a
loaded Q of 2000, and to keep attenuation low, the unloaded resonator
Q should be perhaps 5 times that much. It would be worse for a
multi-pole filter. All this tells me it's silly to even think of an
LC filter. Add to that the extreme difficulty of getting a set of
resonators to tune together. (To get Qu=10000 in a coaxial resonator
at 40MHz would take an air-dielectric line nearly half a meter in
diameter! Just plain silly.)

I'd opt for a front end with very high dynamic range (esp. low
third-order intermod products), into a good IF filter, etc., and a
communications protocol that optimized whatever performance measure I
needed. Talk to the people who build RF communications sytems that go
on aircraft carriers. Or talk with hams who design receivers with
third order intercepts up in the +50dBm region and higher. By the
way, you may do well by putting an ATTENUATOR on the front end, if
interference (distortion products), and not desired-signal-strength,
is the problem. Distortion products will go down faster than the
inserted attenuation.

Cheers,
Tom

In article , mentioned...

Take a look at lowband filters and duplexers from CellWave, Telewave,
etc.
Your filter is going to be a coaxial cavity about 2mx20cm with piston
for tuning.
I am sure there are some better approaches to your task

How about a helical resonator. They're smaller than a cavity, maybe
not as high Q, but still higher than lumped constant tuned circuit.
They're tunable, but I'm not sure how much.


Vladimir Vassilevsky, Ph.D.

Paul Burridge wrote:

Hi chaps,

I've decided to bite the bullet and try to build an RF filter for
40Mhz. This filter will ideally have a very, very sharp characteristic
at one single spot frequency +-20Khz and attenuate the crap out of
anything either side of this. It'll need to be tunable over a range of
say 200Khz. Can anyone give me a steer on what type of arrangement
would be best suited to fit this purpose?
Thanks,

p.
--

--
@@F@r@o@m@@O@r@a@n@g@e@@C@o@u@n@t@y@,@@C@a@l@,@@w@ h@e@r@e@@
###Got a Question about ELECTRONICS? Check HERE First:###
http://users.pandora.be/educypedia/e...s/databank.htm
My email address is whitelisted. *All* email sent to it
goes directly to the trash unless you add NOSPAM in the
Subject: line with other stuff. alondra101 at hotmail.com
Don't be ripped off by the big book dealers. Go to the URL
that will give you a choice and save you money(up to half).
http://www.everybookstore.com You'll be glad you did!
Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html
@@t@h@e@@a@f@f@l@u@e@n@t@@m@e@e@t@@t@h@e@@E@f@f@l@ u@e@n@t@@