Tom - Please see my answers inline.
Tom DeReggi wrote:
Jack,
Is there a technical difference between a notch filter and a bandpass
filter?
Yes. A bandpass filter is designed to pass (with only a dB or two of
attenuation) a "band" of frequencies - hence the name "bandpass".
Everything outside the bandpass frequency range is significantly
attenuated. A notch filter is designed to notch out (attenuate) a rather
narrow frequency range that contains a loud, interfering signal (like
paging).
Is it possible the 930 notch only attenuates 930, and does not help with
the other potential harmful interferers such as the upper 800s, and the
higher than 930 stuff?
Yes, exactly correct. A 930 MHz notch filter only notches out the 930
MHz frequency range and not the upper 800 MHz frequency range where the
cellular band stops (at 885 MHz). Frequencies way above 930 are also not
notched out. This is OK for most WISPs because the interference closest
to the 902-928 MHz band is paging at 929 and 930 MHz.
I'm jsut trying to get an idea of whether a WISP really needs an
assortment of filters in their tool chest to try what helps best, in
absense of a spectrum analizer.
A spectrum analyzer and the knowledge about how to use it is the best
interference reduction tool. The next best tool is a good pair of eyes
hooked up to a brain that can recognize paging and cell sites (which are
often co-located together). When a WISP knows what these sites look
like, the WISP can drive around within a half-mile or so of their
prospective AP location and use their eyes to spot potential paging and
cell sites that would cause overload problems.
It also may matter depending on what unlicensed radio type.
Yes, some radio receivers are better than others in terms of receiver
selectivity. More filtering in the receiver makes a receiver that is
less likely to be overloaded by nearby paging and cell sites therefore
less likely to need an external filter. More filtering also makes the
receiver more expensive. The manufacturer of a PCMCIA card that costs
maybe $20 to make can not afford to spend $75 to include a good receiver
filter; also there may not be enough room on the card for the filter
components.
For example the Ubiquiti 900 card has excellent filtering built in, for
the noise a bit farther away, but not good for the close in adjacent noise.
Maybe the inexpensive 930 notch filter is ideal for use with the
Ubiquiti 900 card?
Yes. Most low-cost receivers would benefit significantly when used with
an external (correctly selected and designed) bandpass or notch filter.
The best use of the notch filter is when there is nearby paging but not
nearby cellular.
Up on a tower, the noise may not just be the close in paging.
Up on the tower, the poor receiver gets hit with everything - in-band
noise from other network sites and out of band noise from paging and
cellular and other nearby transmitters. For best results, a WISP needs
the spectrum analyzer to know what's really out there before choosing an
AP site. Once a site is chosen, it's necessary to "design around" the
noise through proper antenna system selection. While a selection of
filters would help, a better method is to understand the sources of
noise, know how to detect and measure it, then know how to "design
around" it. These are the techniques that I've been writing about and
teaching about since 2001 and practicing since 1993. With practice,
anyone can get good at reducing noise, thereby maximizing WISP
performance and reliability.
jack
Tom DeReggi
RapidDSL & Wireless, Inc
IntAirNet- Fixed Wireless Broadband
----- Original Message ----- From: "Jack Unger" <[EMAIL PROTECTED]>
To: "WISPA General List" <wireless@wispa.org>
Sent: Tuesday, February 20, 2007 4:20 PM
Subject: Re: [WISPA] failing Canopy 900
Because it's a "notch", it attenuates the paging frequencies while
letting the 902-928 band go through with little to no attenuation. You
could use it on any frequency within 902-928.
Brian Rohrbacher wrote:
yes, but I'm still unclear about what center frequency I could use
with it. Thw spec sheet didn't clear it up. I'll have to try to
call them back.
Brian
Jack Unger wrote:
Thank you, Brian. The price is certainly right on that filter.
Brian Rohrbacher wrote:
Sharing the info I am............
Hello, Brian
Thank you for your interest in Microwave Filter Company.
The filter that you inquired about is the:
Notch Filter
MFC P/N 15345
$164.00 unit cost plus shipping
Shipment 1 week after receipt of order
Attached is the specification drawing of the filter that you
requested. If you have any questions or concerns, please contact me
direct.
Regards,
Bob Haytko
Customer Relations
Microwave Filter Co., Inc.
Tel: 315-438-4725 (direct)
Fax: 315-463-1467
Email: [EMAIL PROTECTED] <mailto:[EMAIL PROTECTED]>
Jack Unger wrote:
Brian,
Bandpass filters come in different band "widths". Some are
full-band filters that pass 902-928 MHz and some are
single-channel filters that are narrower and pass only one
channel. The bandwidth of the two Ubiquity filters are a little
narrow to use across the entire band but if you are using 912 or
917 as center frequencies, they should work well. If you're using
a lower center frequency, you'll need a filter that has a bit
wider bandwidth but which still attenuates the paging frequencies
a lot. The availability of good whole-band bandpass filters seems
to have deteriorated a bit in the last year. There are more
filters available but they seem to have poorer characteristics and
sometimes higher prices. I just looked at RFLinx and Hyperlinktech
and I am not happy with their current offerings. I did discover a
notch filter that is tuned to attenuate the paging frequencies
while passing the 902-928 frequencies. I don't know the pricing
but if it's priced reasonably then it looks like your best bet.
Here's the link:
http://www.microwavefilter.com/2ghzRelocation.htm#ism
It's the filter at the bottom of the page. If you call them to get
a spec sheet and to check pricing, please share that info.
Thanks,
jack
--
Jack Unger ([EMAIL PROTECTED]) - President, Ask-Wi.Com, Inc.
Serving the License-Free Wireless Industry Since 1993
Author of the WISP Handbook - "Deploying License-Free Wireless WANs"
True Vendor-Neutral WISP Consulting-Training-Troubleshooting
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Phone (VoIP Over Broadband Wireless) 818-227-4220 www.ask-wi.com
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