High-Speed Synthesizer Spans 2.25 To 18 GHz

This wideband, low-phase-noise frequency synthesizer provides 3-s frequency tuning speed over an instantaneous bandwidth of 2.25 to 18.0 GHz.

Sometimes specifiers become suppliers, especially when trying to meet demanding requirements. Such is the case with the Receiver Systems Division of Wide Band Systems (Rockaway, NJ). Often associated with military receiver components, and a long-time user of frequency synthesizers, the company has turned its attention to designing and supplying broadband, high-speed synthesizers for a variety of applications, including in test equipment, simulator systems, and as local oscillators (LOs) in advanced receiver systems. The first model operates from 2.25 to 18.0 GHz with 3-µs frequency switching speed and only 22 W DC power consumption, in a package with volume of only 334.3cm3 (20.4 in.3).

This fast-switching synthesizer (typically 1000 times faster than similar devices) provides a clean RF output spectrum with an absolute accuracy better than 10 kHz. Based on wideband frequency locking of a voltage-controlled oscillator (VCO) to a stable clock reference source, the patent-pending design represents a new class of microwave synthesizer. In the design, a pair of suboctave VCOs are frequency doubled and then frequency doubled again to provide single-band frequency coverage from 2.25 to 18.0 GHz. The small size (12.7 × 195 × 135 mm) and low power consumption, coupled with the switching speed and accuracy, make it attractive for a wide range of applications (see table).

The frequency synthesizer is available in three different physical configurations (Fig. 1), all of which incorporate a basic circuit design: a 2U rack-mount chassis, a modular replacement synthesizer, and an integrated package. All three configurations share the same circuits (Fig. 2). The 2U rack chassis was configured to meet a specific installation requirement; the replacement synthesizer package was designed to replace an earlier phase-lock synthesizer design; and the integrated synthesizer configuration is a component of an airborne radar-warning-receiver (RWR) system.

Frequency accuracy is better than 10 kHz for all three synthesizer configurations (Fig. 3), while standard frequency resolution is 1 MHz; resolution as fine as 3.9 kHz is available. Harmonic and spurious content is controlled to −60 dBc, while phase noise is at least −86 dBc/Hz offset 10 kHz from a 2674.5-MHz carrier (Fig. 4) and better than −76 dBc/Hz at the same offset from a 3801-MHz carrier (Fig. 5). The phase noise improves to better than −95 dBc/Hz offset 1 MHz from either carrier frequency.

With respect to tuning time, the synthesizer blanks the RF output during an output frequency transition. When a frequency synthesizer is employed as an LO in a narrow-bandwidth intermediate-frequency (IF) tuner, failure to blank the LO output when changing frequency may produce an uncontrolled spurious frequency in the tuner's IF filter. The tuner would then have to wait until this spurious filter response dies out, wasting valuable receiver time. Since the synthesizer operates by comparing the current VCO frequency to an input-selected RF signal, digital processing automatically blanks the synthesizer RF output as soon as a strobe is received loading new frequency data. Blanking is released when the processor determines that the synthesizer output frequency is within a preset margin (such as 1 MHz) of the programmed frequency. If a fault should occur in the synthesizer (where the automatic measurement function does not detect the output frequency to be within this preset margin), the synthesizer will remain blanked and an output error flag will be set.

Since the processing and control of the VCO is accomplished digitally using high-speed field-programmable gate arrays (FPGAs), the synthesizer is capable of not only providing the selected output RF, but can also provide output of programmed patterns of output frequencies. These patterns can be a sequence of frequencies, a sequence of RF pulses, a sequence of amplitudes, or any combination of frequency, timing, and amplitude sequences desired.

The circuit topology of Fig. 1 was adopted with revised RF and digital circuitry to improve the VCO output spectrum, including suppression of harmonics and spurious content, while simultaneously improving the set-on timing. Two suboctave VCOs were used in parallel, avoiding the problem of VCO harmonics and reducing the tuning sensitivity required of each VCO. The selected baseband VCO output is provided to a digital implementation of an ambiguous frequency correlator, measuring the rate of change of phase of the selected VCO output and comparing this measurement to that of the desired RF output frequency. The difference between the measured and predicted rate of change of phase is used to correct the VCO frequency. Switches then select the desired multiple of the corrected VCO frequency. The output RF power level is detected and used to control the voltage variable attenuator (VVA), providing RF output leveling. In this particular implementation, multiple parallel RF outputs are used to tune and calibrate a wide band receiver system. For other applications, the output switch and power divider array are replaced with a programmable RF amplitude modulator.

Another Application
Another application for this synthesizer design is the replacement of digitally tuned oscillators (DTOs) in electronic-warfare (EW) systems and system simulators. A DTO consists of an oven-stabilized VCO with open-loop control. The open-loop control corrects the VCO input digital control data, using a memorized digital calibration. Unfortunately, DTOs are not without problems, including the power required for oven stabilization, post-tuning frequency drift, and the need for recalibration over time. The new synthesizer design does not employ oven stabilization, does not exhibit post-tuning output frequency drift, and does not require recalibration. With a total power requirement of 25 W, a volume of 20 in.3, and the ability to quickly tune, in any frequency sequence, over the 2.25-to-18-GHz band, a single synthesizer may now replace multiple DTOs for significant cost savings and performance advantages. Wide Band Systems, Inc., 389 Franklin Ave., P.O. Box 289, Rockaway, NJ 07866; (973) 586-6500, FAX: (973) 627-9190, e-mail: [email protected] tems.com, Internet: www.widebandsystems.com.

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