Fast-switching frequency synthesizers have important places in a variety of applications, including in antenna measurements and in automatictest- equipment (ATE) systems. Since having the capability of producing flexible modulation schemes is essential in evaluating equipment for most radio communications systems, the engineers at Programmed Test Sources (Littleton, MA) have taken a good thing-the company´s long-proven model PTS 1600 frequency synthesizer-and made it better through the addition of external modulation capabilities.

The PTS 1600 (see figure), which is designated as the model PTS 1600 X-142 with the added modulation capabilities, is based on a combination of direct-analog frequency synthesis and direct-digitalsynthesis (DDS) technology. It can switch frequencies in steps as small as 1 Hz and can tune quickly from 1.000000 to 1599.9999999 MHz. The switching speed is at least 20 s and as fast as 1 s, depending on the size of the frequency jump and the resolution of the step, with a switching in frequency considered the time required to settle within 0.1 radian of a new frequency.

High-speed switching is best achieved through the use of TTL-level binarycoded- decimal (BCD) control signals at the instrument´s parallel port. But for operators integrating the PTS 1600 X- 142 into a test system under GPIB control, the PTS 1600 X-142´s GPIB interface with dedicated control hardware can still provide frequency switching speed in excess of 50 s-much faster than typically found in GPIB-controlled test setups.

The new version of the PTS 1600 allows an external modulation source, generally in the 900-MHz range, to be injected to modulate the synthesizer´s carrier waveforms. A modulation source such as an arbitrary waveform generator can be used to inject amplitude modulation (AM), frequency modulation (FM), pulse modulation, or a combination of modulation formats, resulting in narrowband modulation across the full 1600-MHz range of the PTS 1600 X-142. Both the PTS 1600 and the modulated

PTS 1600 X-142 include numeric keypad for data entry and a liquid-crystaldisplay (LCD) screen to provide output signal information. Output levels can be controlled from +3 to +13 dBm (1-V maximum into a 50-ω load) with ±0.7 dB output-power flatness across the frequency range. Spurious levels are -60 dBc while harmonic levels are -30 dBc. As mentioned earlier, the phase noise of either version of the frequency synthesizer is excellent, specified at -105 dBc/Hz offset 100 Hz from the carrier, dropping to -124 dBc/Hz offset 100 kHz from the carrier, and reaching a noise floor of -142 dBc/Hz.

The PTS 1600 X-142 weighs 40 lbs. and is supplied in a rack-mountable enclosure measuring 19.00 X 5.25 X 18.00 in. It is available with temperature-controlled- crystal-oscillator (TCXO) internal reference or, for somewhat better short- and long-term frequency stability, an oven-controlled-crystal oscillator (OCXO) internal reference. The synthesizer can also run with an external 5- or 10-MHz reference. Programmed Test Sources, Inc., 9 Beaver Brook Rd., P.O. Box 517, Littleton, MA 01460; (978) 486-3008, FAX: (978) 486-4495, e-mail: sales@programmedtest.com, Internet: www.programmedtest.com.