Air combat
Of course, the difficulty in transmitting and receiving a clear signal
stems, in large part, from competition for band space. Just as surely as
nature abhors a vacuum, wireless service providers are quickly filling
every available frequency gap from UHF frequencies of 300 Mhz, all the
way up to super high frequencies of 30 GHz and beyond. Even the FCC
can't bend the laws of physics, as useful bands quickly fill to
capacity, forcing manufacturers to develop devices that operate within
tighter and tighter bandwidths.
Take the case of cellular phone service providers, who, despite
utilizing competing multiplexing protocols, still run into conflict when
signal frequencies are adjacent to each other. For instance, the CDMA
passband ends at 888.9 MHz, quickly followed by the GSM 900 passband,
which begins at 890.1 MHz. With only 1.2 MHz of space between the two,
there is little room for error. Harmonics and other spurious signals add
to the challenge of maintaining distinct borders between the two.
The only sentinels in guarding against interference across such
relatively narrow gaps are filters. Using the above example: without
sufficient filtering of GSM signals below 890 MHz, blocking and
intermodulation can occur at the high end of the CDMA passband.
In general, the consequences of inadequate filtering translate into
cross-talk and dropped calls for the cellular phone consumer; and loss
of data and interrupted network connections for wireless LAN and WAN
users. For the service provider, loss of customers and market share
represent the ultimate price to pay for failing to focus on filtering.
Too big a picture?
So how is it that some U.S.-based manufacturers of wireless products
have lost touch with the fundamentals, such as proper filtering?
Consider that much of the design process of subassemblies has now been
farmed out to offshore manufacturers. Few, if any, local manufacturers
can afford to compete on a mass scale when it comes to cranking out
circuit card assemblies that perform simple functions such as frequency
generation.
Rather, many on-shore designers spend their time at the line replaceable
unit (or "box") and system level, combining the various subassemblies to
ultimately create the finished goods. The result is a product that
oftentimes looks good in theory, but fails to deliver when put in the
field. Oftentimes, the problem often arises at the component level, and
in cases of interference, the filter usually lies at the root of
problem.
Back to the component level¾an expert offers solutions
Sam Benzacar is Founder of Anatech Electronics, a Garfield, New
Jersey-based manufacturer that specializes in filters. With three
decades of experience in the RF and microwave filter industry, Benzacar
provides the following tips on improving the performance of wireless
products.
- Don't make the filter an
afterthought.
- "Imagine getting your design out in the field only to discover bad
reception," says Benzacar. "This happens only too often, especially in
active metropolitan areas, and it usually occurs because little
forethought was given to filter performance."
- Benzacar points out that once a defective product is deployed into the
field, the cost to remedy the problem-by sending out field technicians
to swap out the parts at base stations, for example¾almost always
exceeds any up-front expense to upgrade filter performance in the
first place.
- "Instead, at the start of design, look at the frequencies around the
target band and ask what conflicting signals could appear in your
specified time frame," advises Benzacar. "Preparing for interference
at the start of design will more than pay for itself."
- Insist on a filter with sharp cut-off and isolation.
- "A filter screening out a signal just 3 MHz away must be able to
reject over 40 dB of signal strength outside its passband," explains
Benzacar. "Filters today must exhibit sharp cut-off characteristics,
and this requirement becomes all the more important as various
frequency bands close in on each other."
- On duplexers, for example¾which are necessary for any simultaneous
send-receive communication and are essentially two bandpass filters
hooked together¾the filters must be especially sharp with good
isolation and as little insertion or return loss as possible so that
neither the send or receive frequencies will interfere with each
other.
- Don't test the product in too "friendly" an environment.
- In an eagerness to rush a product to market, environmental testing can
take place under artificially ideal conditions. Just because a filter
may breeze through EMF testing within a lab in rural Arizona, doesn't
mean that it won't cause interference, or fail to preclude spurious
signals, on a corner in midtown Manhattan.
- "Avoid making a product 'environmentally dependent,' forcing the
customer to roll the dice when taking the product into new fields,"
cautions Benzacar. "Plan on incorporating filters that ensure proper
performance everywhere, under various conditions."
- Retain a filter manufacturer able to change component
characteristics on short notice.
- Even the best of designs are occasionally comprised by a frequency
conflict that could not have been foreseen. In such instances, it pays
to work with a local component manufacturer who can readily modify its
filters.
- "A manufacturer, may have to change the center frequency of a filter
to improve the performance of its device within a particular band,"
says Benzacar. "A cost effective way to do this without scrapping the
design is to initially work with a filter manufacturer who can quickly
adapt a standard offering."
- For example, a design might have called out a standard filter at 915
Mhz. But once tested in field, it might be determined that what is
actually needed is a center frequency of 920 MHz. Filter manufacturers
like Anatech can quickly and cost-effectively shift the frequency
because they have full, in-house design and manufacturing capability.
It's difficult to obtain that kind of response from a distributor who
might be getting parts from offshore sources.
- For the same reasons, working directly with a local filter
manufacturer can enable last-minute mechanical design changes when
needed. Anatech, for instance, can quickly shift an input/output
connector from one side of the filter to another. Such flexibility can
simplify cabling and lower manufacturing costs.
- Consider filter size with the future in mind.
- "As circuit card 'real estate' gets smaller and performance demands
increase, old algorithms aren't enough anymore," notes Benzacar. "So
it's also important to rely on a manufacturing partner committed to
developing and using the latest mathematical algorithms and techniques
to optimize filter size and performance. To accommodate the need for
smaller component footprints, I would say the bulk of the work we do
is in custom designs. Short production runs are also common."
- Pick a filter supplier that keeps you in the worldwide market.
-
To tap into the burgeoning overseas demand for wireless communication
products, manufacturers must incorporate filters produced by vendors
that take a proactive stance in meeting global environmental
certifications such as ISO-9001-2000 and RoHS compliance.
Since 1990, Anatech Electronics has responded to the changing RF and
microwave marketplace with a product line that includes: bandpass,
lowpass, highpass, and bandstop filters; crystal filters; diplexers
and duplexers, ceramic, cavity and monoblock filters for WiMax
applications; as well as filters for WiFi applications. All components
are ruggedly built to meet stringent military and commercial
specifications. |