Reply
 
LinkBack Thread Tools Search this Thread Display Modes
  #1   Report Post  
Old July 26th 03, 05:06 PM
Paul Burridge
 
Posts: n/a
Default Hi-Q RF filters, anyone?


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
  #2   Report Post  
Old July 26th 03, 08:14 PM
Ben Bradley
 
Posts: n/a
Default

In sci.electronics.design, 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?


I've got that deja-google feeling all over again:


http://groups.google.com/groups?hl=e...com%26rnum%3D6

I like the idea of downconverting to an IF, filtering using
standard IF technology, and (if you want the output to be the same
frequency band as the input) upconverting using the same local
oscillator as the downconverter. Just change the LO frequency (maybe
use a frequency synthesizer for stability) to do tuning.

Thanks,

p.
--

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



  #3   Report Post  
Old July 26th 03, 10:48 PM
Vladimir Vassilevsky
 
Posts: n/a
Default


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

  #4   Report Post  
Old July 27th 03, 06:20 AM
Watson A.Name - 'Watt Sun'
 
Posts: n/a
Default

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@@
  #7   Report Post  
Old July 29th 03, 08:00 PM
Tom Bruhns
 
Posts: n/a
Default

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
  #8   Report Post  
Old July 29th 03, 08:00 PM
Tom Bruhns
 
Posts: n/a
Default

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
  #9   Report Post  
Old July 27th 03, 06:20 AM
Watson A.Name - 'Watt Sun'
 
Posts: n/a
Default

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@@
  #10   Report Post  
Old July 27th 03, 01:54 AM
Tim Shoppa
 
Posts: n/a
Default

Paul Burridge wrote in message . ..
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?


It'll need to be a crystal filter, and your requirement that it must
be tunable means that you will convert down/up to an IF frequency and back
up/down again. (Well, you may not have to convert back up again but you
don't tell us your application).

Problem with the IF and conversion is the production of images. Images
won't be a killer problem because your tuning range is really quite narrow.

If you wanted to really cheapskate out some ceramic IF filters also seem
to meet your stated needs.

Tim.


Reply
Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules

Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Ten-Tec filters K3HVG Boatanchors 0 October 24th 04 07:35 PM
'other' Kenwood SSB Filters : YK-88S1 and YK-88S2 RHF Equipment 0 September 13th 04 01:38 PM
'other' Kenwood SSB Filters : YK-88S1 and YK-88S2 RHF Equipment 0 September 13th 04 01:38 PM
'other' Kenwood SSB Filters : YK-88S1 and YK-88S2 RHF Equipment 0 September 13th 04 01:38 PM
FS: New Crystal Filters $25.00 W4-ham Boatanchors 0 August 10th 03 12:40 AM


All times are GMT +1. The time now is 10:34 PM.

Powered by vBulletin® Copyright ©2000 - 2025, Jelsoft Enterprises Ltd.
Copyright ©2004-2025 RadioBanter.
The comments are property of their posters.
 

About Us

"It's about Radio"

 

Copyright © 2017