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Synthesizers have become indispensable
in many of todays advanced measurement and production systems,
as well as in stand-alone uses. Typical applications range from
ATE and NMR medical imaging to satellite earth station oscillators,
from magnetic storage media testing to crystal production, from mode-locking
of lasers to ECM. Precision timing, radar simulations, Doppler systems,
all make use of synthesizers.
Frequency synthesizers are basically
variable radio-frequency generators which are very accurately
and quickly settable and possess high stability. Within a specified
frequency range they can be programmed either manually or remotely
to practically any output frequency. This output frequency is
as accurate and as stable as a built-in frequency standard, usually
a crystal oscillator, or as accurate and stable as an external
precision standard which may be connected to the synthesizer
in lieu of its own standard. Where very high stabilities are
desired, atomic or molecular standards are often used.
Most commercial frequency synthesizers
use a decimal read-out or indicator system. The least significant
step or digit determines resolution, how closely the synthesizer
can be set to any arbitrary frequency. Resolution ranges from
megahertzs to microhertzs, depending on use; some synthesizers
offer a choice of resolution to match capability (and price)
to users need. (Although read-out or indication of setting
is normally decimal, remote control frequency setting may use
other coding.)
The ideal of a pure frequency, a single
spectral line, is not attained in practical synthesizers. All
produce unwanted frequencies, called spurious outputs, and they
also have, like any oscillator, harmonics. While harmonics are
at least one octave removed and thus not often troublesome, the
suppression of other unwanted frequencies is a major challenge
of synthesizer design; units differ widely in this respect, and
this is of major impact regarding cost. The same is true of the
very close-in noise around the carrier that constitutes unwanted
phase-modulation. These perturbations are variously called broadband
phase noise, spectral density distribution of phase noise, residual
FM, and short term fractional frequency deviation.
Todays synthesizers use three
technologies, singly or in combination, to generate an output
frequency from a reference standard: direct analog, indirect,
and direct digital.
Direct analog synthesis makes use
of a limited number of auxiliary or standard frequencies which
are derived from the reference. The output band is covered solely
by arithmetic operations on these auxiliary frequencies, using
fixed-tuned filters, RF switches, mixers, multipliers and dividers.
The "mix-and-divide" direct synthesis approach permits
the use of many identical modules, producing arbitrarily fine
resolution and low spurious output.
Indirect synthesis uses phase-locked
loops to produce an output frequency. This approach may take
various forms: divide-by-n for one or more digits, fractional-n
with multi-digit capability, and mix-and-divide with loops embedded.
In each case, the loop is governed by some derivative of the
frequency standard. Again, the mix-and-divide approach permits
the use of many identical modules.
Direct digital synthesis makes use
of digital technology. Using adder circuitry, phase is accumulated
at a rate dependent on the frequency selected. Phase value is
then used to address a PROM, which stores discrete values of
the sine function. A D/A converts the digital output of the PROM
to a sine wave which is low-pass filtered to remove the clock
frequency, aliases and D/A glitches. The theoretical maximum
output frequency obtainable is one-half the clock frequency,
although practical filtering considerations limit the output
frequency to less than 45% of the clock.
PTS synthesizers use direct analog
and direct digital technologies. Indirect schemes, although cost-effective
for multi-digit high resolution, are not used because the switching
speed demanded for PTS synthesizers (µseconds) is not attainable.
The most significant digits down to 1 MHz are produced by direct
analog synthesis. When switching speed and signal purity are
considered, there is no better approach. Direct digital synthesis
is faster switching, but at this time the technology does not
provide the low level of spurious outputs demanded by sophisticated
applications at VHF/UHF frequencies.
For the digits from 100 KHz down to
0.1 Hz, PTS offers a choice of repetitive mix-and-divide modules
or direct digital synthesis. The direct analog technology permits
a close match to customer resolution requirements, while direct
digital synthesis provides fast, phase-continuous switching and
allows digital phase modulation.
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