Frequency and Wavelength Relationship

Frequency and Wavelength Relationship

In 1939, Major Edwin Armstrong introduced FM (frequency modulation) into radio
broadcasting in the United States. FM had the advantage over AM (amplitude modulation)
of the capture effect. Provided sufficient signal strength was available at the
receiver, the signal would experience gain through the demodulator, delivering a significant
improvement in signal-to-noise ratio. The deeper the modulation depth (that
is, the more bandwidth used), the higher the gain. Additionally, the capture effect
made FM more resilient to (predominantly AM) interference. Toward the end of World
War II, the U.S. Army introduced FM radios working in the VHF band. The combination
of the modulation and the frequency (VHF rather than shortwave) made the FM
VHF radios less vulnerable to jamming.
Fifty years later, CDMA used wider bandwidth channels to deliver bandwidth gain
(rather like wideband FM processor/demodulator gain). Rather like FM, CDMA was,
and is, used in military applications because it is harder to intercept.
A shortwave or VHF portable transceiver in 1945 weighed 40 kg. Over the next 50
years, this weight would reduce to the point where today a 100 gm phone is considered
overweight.
Parallel developments included a rapid increase in selectivity and stability with a
reduction in practical channel spacing from 200 kHz in 1945 to narrowband 12.5, 6.25,
or 5 kHz transceivers in the late 1990s, and reductions in power budget, particularly
after the introduction of printed circuit boards and transistors in the 1950s and 1960s.
The power budget of an early VHF transceiver was over 100 Watts. Atypical cell phone
today has a power budget of a few hundred milliWatts.
As active and passive device performance has improved and as circuit geometries
have decreased, we have been able to access higher parts of the radio spectrum. In
doing so, we can provide access to an ever-increasing amount of radio bandwidth at a
price affordable to an ever-increasing number of users.
As RF component performance improved, RF selectivity also improved. This resulted
in the reduction of RF channel spacing from several hundred kHz to the narrowband
channels used today�"12.5 kHz, 6.25 kHz, or 5 kHz (used in two-way radio products).
In cellular radio, the achievement of sensitivity and selectivity is increasingly
dependent on baseband performance, the objective being to reduce RF component
costs, achieve better power efficiency, and deliver an increase in dynamic range. The
trend since 1980 has been to relax RF channel spacing from 25 kHz (1G) to 200 kHz (2G
GSM; Global System for Mobile Communication) to 5 MHz (3G). In other words, to go
wideband rather than narrowband.
Handset design objectives remain essentially the same as they have always been�"
sensitivity, selectivity, and stability across a wide dynamic range of operational conditions,
though the ways in which we achieve these parameters may change. Likewise,
we need to find ways of delivering year-on-year decreases in cost, progressive weight
and size reduction, and steady improvements in product functionality.
In the introduction, we highlighted microcontrollers, digital signal processors
(DSPs), CMOS (complementary metal-oxide semiconductors) image sensors, and displays
as key technologies. We should add high-density battery technologies and RF
component and packaging technology. RF component specifications are determined by
the way radio bandwidth is allocated and controlled�"for example, conformance standards
on filter bandwidths, transmit power spectral envelopes, co-channel and adjacent
channel interference, phase accuracy, and stability.

Historically, there has also been a division between wide area access using duplex
spaced bands (sometimes referred to as paired bands) in which the transmit frequencies
are separated by several MHz or tens of MHz from receive frequencies, and local
area access using nonpaired bands in which the same frequency is used for transmit
and receive. Some two-way radios, for example, still use single frequency working
with a press-to-talk (PTT) key that puts the transceiver into receive or transmit mode.
Digital cordless phones use time-division duplexing. One time slot is used for transmit,
the next for receive, but both share the same RF carrier.
One reason why cellular phones use RF duplexing and cordless phones do not is
because a cellular phone transmits at a higher power. Acordless phone might transmit
at 10 mW, a cellular handset transmits at between 100 mW and 1 Watt, a cellular base
station might transmit at 5, 10, 20, or 40 Watts. For these higher-power devices, it is particularly
important to keep transmit power out of the receiver. 7