IEEE Spectrum February, 2008 - 40

radio frequency (say, 1000 kHz). The sum
and difference of these two frequencies
(999 kHz and 1001 kHz) fall just slightly
above and below that of the RF oscillator.
That's why the original audio signal is said
to be "up-converted" to RF. The receiver
for such an AM broadcast would typically
use a similar oscillator to "down-convert"
the RF signal back to audio, using exactly
the same heterodyning principle.
For high-speed data communications at 60 GHz, such operations can
be a nightmare to implement. For one
thing, the oscillator would have to produce two 60-GHz outputs that are exactly
90 degrees out of phase. This is because
the final modulated signal is produced
by combining a sine and cosine. The generation and routing of these two phases,
while maintaining a 90-degree difference
between them, is hard at 60 GHz.
Also, controlling the precise frequency of a 60-GHz oscillator is tricky
because it's running too fast to measure directly, as crystal-controlled frequency standards are limited to about
100 megahertz. The 60-GHz signal must
first be fed into a frequency-divider circuit that reduces the frequency by a large
factor (say, 600). Only then can the output
be compared with a frequency standard,
which indicates whether the rate of oscil-

Delayed
phases

lation is faster or slower than desired, so
that it can be corrected accordingly. The
tactic is simple enough, but the limits on
transistor speed make it difficult to fashion such frequency-divider circuits that
work at 60 GHz.
Fortunately, with a little cunning,
you can make a receiver work using a
40-GHz oscillator instead. The first step is
to mix the output of the 40-GHz oscillator
with the 60-GHz received signal. That
operation down-converts the signal to
the difference frequency: 20 GHz. To
down-convert the signal the rest of the
way, the receiver circuitry need not
incorporate a separate 20-GHz oscillator;
it can simply use the output of a divideby-two frequency divider attached to its
40-GHz oscillator. Because it operates on
40 GHz rather than 60 GHz, such a frequency divider is comparatively easy to
implement. What's more, it is less vexing
to route signals around a chip at 40 GHz
than at 60 GHz. And happily enough for
designers, the transmission path can follow the receiver operations in the reverse
order to avoid 60-GHz oscillators and frequency dividers. That is, the data stream
to be transmitted is first up-converted to
20 GHz and only then raised to 60 GHz.
Using 40 GHz for the oscillator frequency is just one way to dodge some of

Constructive addition

(a)
Delayed
phases

Emitters

(b)

www.speCtrum.ieee.org

Constructive
addition

fEbruAry 2008 * iEEE SpEctrum * NA

57



http://www.speCtrum.ieee.org

Table of Contents for the Digital Edition of IEEE Spectrum February, 2008

IEEE Spectrum February, 2008 - Cover1
IEEE Spectrum February, 2008 - Cover2
IEEE Spectrum February, 2008 - 1
IEEE Spectrum February, 2008 - 2
IEEE Spectrum February, 2008 - 3
IEEE Spectrum February, 2008 - 4
IEEE Spectrum February, 2008 - 5
IEEE Spectrum February, 2008 - 6
IEEE Spectrum February, 2008 - 7
IEEE Spectrum February, 2008 - 8
IEEE Spectrum February, 2008 - 9
IEEE Spectrum February, 2008 - 10
IEEE Spectrum February, 2008 - 11
IEEE Spectrum February, 2008 - 12
IEEE Spectrum February, 2008 - 13
IEEE Spectrum February, 2008 - 14
IEEE Spectrum February, 2008 - 15
IEEE Spectrum February, 2008 - 16
IEEE Spectrum February, 2008 - 17
IEEE Spectrum February, 2008 - 18
IEEE Spectrum February, 2008 - 19
IEEE Spectrum February, 2008 - 20
IEEE Spectrum February, 2008 - 21
IEEE Spectrum February, 2008 - 22
IEEE Spectrum February, 2008 - 23
IEEE Spectrum February, 2008 - 24
IEEE Spectrum February, 2008 - 25
IEEE Spectrum February, 2008 - 26
IEEE Spectrum February, 2008 - 27
IEEE Spectrum February, 2008 - 28
IEEE Spectrum February, 2008 - 29
IEEE Spectrum February, 2008 - 30
IEEE Spectrum February, 2008 - 31
IEEE Spectrum February, 2008 - 32
IEEE Spectrum February, 2008 - 33
IEEE Spectrum February, 2008 - 34
IEEE Spectrum February, 2008 - 35
IEEE Spectrum February, 2008 - 36
IEEE Spectrum February, 2008 - 37
IEEE Spectrum February, 2008 - 38
IEEE Spectrum February, 2008 - 39
IEEE Spectrum February, 2008 - 40
IEEE Spectrum February, 2008 - 41
IEEE Spectrum February, 2008 - 42
IEEE Spectrum February, 2008 - 43
IEEE Spectrum February, 2008 - 44
IEEE Spectrum February, 2008 - 45
IEEE Spectrum February, 2008 - 46
IEEE Spectrum February, 2008 - 47
IEEE Spectrum February, 2008 - 48
IEEE Spectrum February, 2008 - Cover3
IEEE Spectrum February, 2008 - Cover4
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