RF Bandpass Filter
 

If I wanted to make a RF bandpass filter that passed 902-928Mhz and rejected
all else how would I go about that?

Matt

If I wanted to make a RF bandpass filter that passed 902-928Mhz and rejected
all else how would I go about that?

Haven't done it myself, but I think I'd end up doing it in several
stages.

Stage 1: figure out what sort of filter topology you need (e.g.
Butterworth, Cauer, elliptical, etc.) and how high an order you want.
There's going to be a tradeoff between the sharpness of the cutoff,
the flatness of the passband, and the number of components needed to
construct it.

Stage 2: select the component values, using either computer software
or a set of normalized-filter-value tables. If your filter isn't
terribly complex you can probably find tables in the ARRL Handbook
which will give you the correct values.

Stage 3: figure out how you're actually going to construct it. For
high-UHF frequencies like those, using leaded components is probably a
no-no (too much parasitic inductance from the leads). Surface-mount
components may work in some cases, if they can provide an adequately
high Q. My understanding is that most good filter implementations in
these frequency ranges tend to be done by using stripline construction
on PC board, using shorted and open-ended transmission line stubs to
create the necessary inductances and capacitances. The stubs could be
strips etched on the board, or sections of small-diameter coax
soldered to the board traces.

Stage 4: lay out the PC board, etch it, install any discrete
components (i.e. those not implemented via transmission-line stubs)
and any coaxial lines or stubs.

Stage 5: measure the actual response, and if necessary go back and
tweak it to adjust for any parasitic effects you hadn't accounted for.

Another approach entirely would be to check with some of the RF
component vendors. You might find that somebody has a small
surface-mount integrated high- or low-pass filter or two, which could
be hooked up in cascade to create the bandpass response that you want.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
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Go to http://www.digikey.com and search for this part number: TKS5437CT-ND

This is DigiKey's part number for a Toko Helical filter. They have a
pdf datasheet as well. It's listed at $20.90 on line in single
quantities. It is centered at 915 MHz and has a bandwidth of 25 MHz (so
it will pass from 902.5 to 927.5 and will obviously go down to 902 and
up to 928 with minimal loss). This is with an insertion loss of 2.5 dB
and a passband ripple of 1 dB.

Scott
N0EDV



Matt wrote:

If I wanted to make a RF bandpass filter that passed 902-928Mhz and rejected
all else how would I go about that?

Matt

"Matt" wrote in message
...
If I wanted to make a RF bandpass filter that passed 902-928Mhz and
rejected
all else how would I go about that?

Matt

build a highpass for 902mhz
and
a lowpass for 928mhz

"Scott" wrote in message
...
Go to http://www.digikey.com and search for this part number:
TKS5437CT-ND

This is DigiKey's part number for a Toko Helical filter. They have a pdf
datasheet as well. It's listed at $20.90 on line in single quantities.
It is centered at 915 MHz and has a bandwidth of 25 MHz (so it will pass
from 902.5 to 927.5 and will obviously go down to 902 and up to 928 with
minimal loss). This is with an insertion loss of 2.5 dB and a passband
ripple of 1 dB.

Scott
N0EDV



Matt wrote:

If I wanted to make a RF bandpass filter that passed 902-928Mhz and
rejected all else how would I go about that?

Matt



Jeeze, Scott, a hellova fit! Good work!

John

Well, I hope it works for the intended purpose. These filters are nice
in the fact that they are 50 Ohms in and out, so if they are being put
in line with a receiver, all that should be necessary is to tune it
up...the price isn't too terrible either!

Scott
N0EDV

John - kd5yi wrote:
"Scott" wrote in message
...

Go to http://www.digikey.com and search for this part number:
TKS5437CT-ND

This is DigiKey's part number for a Toko Helical filter. They have a pdf
datasheet as well. It's listed at $20.90 on line in single quantities.
It is centered at 915 MHz and has a bandwidth of 25 MHz (so it will pass
from 902.5 to 927.5 and will obviously go down to 902 and up to 928 with
minimal loss). This is with an insertion loss of 2.5 dB and a passband
ripple of 1 dB.

Scott
N0EDV



Matt wrote:


If I wanted to make a RF bandpass filter that passed 902-928Mhz and
rejected all else how would I go about that?

Matt




Jeeze, Scott, a hellova fit! Good work!

John