Microwave frequency synthesizers can be built by stabilizing a number of different oscillators, including those that tune by voltage and those that tune by current. A voltage-controlled oscillator (VCO) offers fast tuning speed, but often with a great deal of unwanted harmonic and spurious signals. A current-controlled oscillator, such as one based on a yttrium-iron-garnet (YIG) resonator, provides a beautifully linear tuning characteristic with low noise, but with slower tuning speed than a VCO and often in a larger package.

The MLSP series of frequency synthesizers from Micro Lambda wireless brings many of the desirable qualities of YIG oscillators to a line of stabilized sources, while minimizing the package size to a workable 5 x 3 x 1 in. These frequency synthesizers feature broad tuning bandwidths within a total frequency range of 0.6 to 20.0 GHz. They boast 1-kHz tuning resolution, healthy output power levels, and the low spurious and phase-noise levels that microwave engineers have come to expect from YIG-based sources.

The MLSP series (Fig. 1) includes models with frequency ranges of 0.6 to 2.5 GHz (model MLSP-0625), 2.0 to 8.0 GHz (model MLSP-2080), 6.0 to 18.0 GHz (model MLSP-6018), and 8.0 to 20.0 GHz (model MLSP-8020). The broadband nature of these sources makes it a simple matter to outfit, for example, an electronic-warfare (EW) or electronic-intelligence (ELINT) receiver, with a frequency range from 2 to 20 GHz using only two synthesizers. For test and modular applications, the frequency synthesizers are small enough to fit into a two-slot PXi chassis. They are equipped with several control interfaces, including a five-line serial control and a standard universal Serial Bus (USB) port.

The lowest-frequency unit in the series is model MLSP-0625, with a range of 0.6 to 2.5 GHz. it delivers +13 dBm output power over the full operating bandwidth, with 4-dB or less variation in output power across the operating frequency and temperature ranges (0 to +60C). As with the other frequency synthesizers in the MLSP series, it tunes with 1-kHz frequency resolution, with tuning speeds of 1 ms for a 100-MHz step, 2 ms for a 1-GHz step, and 3 ms for a full-band step. Model MLSP- 0625 has relatively high harmonic levels, requiring for the additional of output filtering in many applications, although spurious levels are controlled to -60 dBc or better at offsets less than or equal to 500 kHz from the carrier and -80 dBc or better at offsets greater than 500 kHz from the carrier. The phase noise is typically -89 dBc/Hz offset 100 Hz from the carrier, -93 dBc/Hz offset 1 kHz from the carrier, -99 dBc/Hz offset 10 kHz from the carrier, -118 dBc/Hz offset 100 kHz from the carrier, and -142 dBc/Hz offset 1 MHz from the carrier.

Moving higher in frequency, the model MLSP-2080 covers 2 to 8 GHz and also provides +13 dBm output power across its full bandwidth, with 4-dB less output-power variations with frequency and temperature. It also tunes in 1-kHz steps with tuning speeds of 1 ms for 100- MHz step, 2 ms for a 1-GHz step, and 3 ms for a full-band step. Its harmonic performance is somewhat better than that of the lower-frequency model MLSP-0625, at -12 dBc, while spurious levels are controlled to -60 dBc for offsets less than or equal to 500 kHz from the carrier and -80 dBc for offsets greater than 500 kHz from the carrier. The specified phase-noise performance is -82 dBc/Hz offset 100 Hz from the carrier, -95 dBc/Hz offset 1 kHz from the carrier, -95 dBc/Hz offset 10 kHz from the carrier, -117 dBc/Hz offset 100 kHz from the carrier, and -140 dBc/ Hz offset 1 MHz from the carrier.

For comparison, Fig. 2 shows measured phase noise for a model MLSP-2080BD frequency synthesizer at a carrier frequency of approximately 8 GHz (8.000001936 GHz) and output level of about +6 dBm (+5.9640 dBm), with measurements made on a model E5053A microwave frequency downconverter and model E5052B signal source analyzer, both from Agilent Technologies. The phase-noise readings were made with 20-dB intermediate-frequency (IF) gain in the measurement system. The "B" and "D" in the model number refer to optionsfor a synthesizer with an external reference and internal crystal and for RF connectors on the front of the housing, respectively.

Model MLSP-6018 tunes from 6 to 18 GHz with 1-kHz resolution and +10 dBm output power across the full bandwidth, providing similar flatness of output power with frequency and temperature and tuning speeds as the two lower-frequency synthesizers in the MLSP series. The MPSP-6018 follows the trend of improved harmonic performance at increasing frequencieswith a specified level of -15 dBcwhile controlling spurious levels to -60 dBc or better for offset frequencies less than or equal to 500 kHz from the carrier and -80 dBc or better for offset frequencies of greater than 500 kHz from the carrier. The phase-noise performance is specified as -75 dBc/Hz offset 100 Hz from the carrier, -90 dBc/Hz offset 1 kHz from the carrier, -95 dBc/Hz offset 10 kHz from the carrier, -115 dBc/Hz offset 100 kHz from the carrier, and -138 dBc/Hz offset 1 MHz from the carrier.

The highest-frequency synthesizer in the MLSP series is model MLSP-8020, with a range of 8 to 20 GHz. It tunes from 8 to 20 GHz with at least +8 dBm output power. As with the other frequency synthesizers, it maintains output power within a 4-dB window across frequency and temperature, and performs a full-band tuning step in about 3 ms.

Harmonic levels are -15 dBc, while spurious levels are -60 dBc for offsets less than or equal to 500 kHz from the carrier and -80 dBc for offsets greater than 500 kHz from the carrier. The phase noise is specified as -70 dBc/Hz offset 100 Hz from the carrier, -87 dBc/Hz offset 1 kHz from the carrier, -93 dBc/Hz offset 10 kHz from the carrier, -115 dBc/Hz offset 100 kHz from the carrier, and -138 dBc/Hz offset 1 MHz from the carrier. To simplify a comparison of the four YIG-based frequency synthesizers, they are shown with key specifications in the table.

For ease of use, the YIG-based frequency synthesizers feature a Quick Remote Control function with their integral USB interface. The synthesizers are designed for control with a simple Microsoft Windows program running on a personal computer (PC). Setup is as simple as connecting the power-supply input lines and plugging the synthesizer into a computer's USB port by means of a USB cable supplied with each MLSP synthesizer, and waiting for the Windows operating system (OS) to verify that the frequency synthesizer was successfully installed. A control interface is provided by a Micro Lambda Wireless program called MLSP PC Interface, which allows commands to be sent to the synthesizer to control tuning frequency, set power level, and perform other functions.

The four MLSP synthesizers described here are "starting points," since a number of options allow customers to specify different frequencies, internal or external references, connectors, and fixed or variable output-power levels. All of the synthesizers run on supplies of +5 and +15 VDC, with power dissipation increasing with increasing frequency, from about 9.4 W in the model MLSP-0625 to about 23.4 W in the model MLSP-8020. Each unit weighs 15 oz. (426 g) and includes a TTL alarm to indicate frequency-locked status.