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

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

Paul Burridge wrote in message . ..
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.

Filter at 40 MHz with 40 KHz BW means a very high Q, up to 1000.
Cavity filter at 40 MHz is big! You can try active RF tuned stage (at
40 MHz) with a positive feedback, also known as Q-multiplier. In
addition to high selectivity this will also result in a high gain,
which may overload your actual receiver. In this case you can
attenuate the signal at the output of the RF stage. You have to make
sure Q-multiplier would start to oscillate. It is an issue by itself.
73,
Vlad
kb9olm

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.

Paul Burridge wrote in message . ..
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.

Filter at 40 MHz with 40 KHz BW means a very high Q, up to 1000.
Cavity filter at 40 MHz is big! You can try active RF tuned stage (at
40 MHz) with a positive feedback, also known as Q-multiplier. In
addition to high selectivity this will also result in a high gain,
which may overload your actual receiver. In this case you can
attenuate the signal at the output of the RF stage. You have to make
sure Q-multiplier would start to oscillate. It is an issue by itself.
73,
Vlad
kb9olm

In article ,
says...

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


i just read this thread and had a few thoughts. i don't know what kind
of interference you're dealing with, nor do i know what restrictions are
placed on your competitions.

i was thinking about front end overload, also. the types of interference
vary in different countries, but those paging towers and, taxis, etc.
all play hell on a front end. you really need a spectrum analyser to
figure out what you're deaaling with.

as for those ceramic resonators, they're ok, but in the better
receivers, they're followed by if xfmrs to get rid of the spurious
response of the ceramic filter.

since you are thinking of just starting from scratch, here's what i'd do
if you're not bound by rules.

http://www.aerocomm.com
http://www.radiometrix.com

cost? i dunno. i figure if you can afford one, good. then you can spend
more time on the robotics and weaponry.

if you can use these, the question becomes a matter of whether you can
retrofit one of these in time. i'm not ready to look into using them yet
and therefore haven't gotten into the details. so i don't know what
you'd have to do to get a PWM signal in and out, but since they handle a
higher data rate (1 Mbps) than std R/C PWM, you could bust the PWM
signal up into little "chips" and reconstruct it on the receiving end.
maybe an integrator/LPF would be all you need. on the other hand, maybe
you can just send" the PWM directly. I'm sure the applications engineers
could help.

hope this helps.

mike

sorry, i did mean PPM, not PWM

a link was provided earlier. might be this, or it may link to this.

there're links to radios for robot wars. according to this page there
are 2 bands allowed, so maybe those 2.4GHz radios are verbotten. that
sucks! it's a robot competition, not a ham competition.

http://homepages.which.net/~paul.hills/Radio/Radio.html

mike

sorry, i did mean PPM, not PWM

a link was provided earlier. might be this, or it may link to this.

there're links to radios for robot wars. according to this page there
are 2 bands allowed, so maybe those 2.4GHz radios are verbotten. that
sucks! it's a robot competition, not a ham competition.

http://homepages.which.net/~paul.hills/Radio/Radio.html

mike

In article ,
says...

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


i just read this thread and had a few thoughts. i don't know what kind
of interference you're dealing with, nor do i know what restrictions are
placed on your competitions.

i was thinking about front end overload, also. the types of interference
vary in different countries, but those paging towers and, taxis, etc.
all play hell on a front end. you really need a spectrum analyser to
figure out what you're deaaling with.

as for those ceramic resonators, they're ok, but in the better
receivers, they're followed by if xfmrs to get rid of the spurious
response of the ceramic filter.

since you are thinking of just starting from scratch, here's what i'd do
if you're not bound by rules.

http://www.aerocomm.com
http://www.radiometrix.com

cost? i dunno. i figure if you can afford one, good. then you can spend
more time on the robotics and weaponry.

if you can use these, the question becomes a matter of whether you can
retrofit one of these in time. i'm not ready to look into using them yet
and therefore haven't gotten into the details. so i don't know what
you'd have to do to get a PWM signal in and out, but since they handle a
higher data rate (1 Mbps) than std R/C PWM, you could bust the PWM
signal up into little "chips" and reconstruct it on the receiving end.
maybe an integrator/LPF would be all you need. on the other hand, maybe
you can just send" the PWM directly. I'm sure the applications engineers
could help.

hope this helps.

mike