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.