New Microwave Signal Generators that Adapt to Wide-ranging Tasks

New Microwave Signal Generators that Adapt to Wide-ranging Tasks

Tektronix Inc.
Beaverton, OR

Flourishing digital wireless technologies, in addition to bread-and-butter RF applications such as radar design and electromagnetic susceptibility testing, have created a need for simpler, more cost-effective RF signal sources. As a result, new solutions have emerged that can be tailored with the exact functionality needed for a given application. Integration and configurability are key features to the success of this concept.

A case in point is the new Rohde & Schwarz SMR series signal generator family, which can create digitally modulated, multicarrier signals at frequencies up to 40 GHz. The series comprises three models (SMR20, SMR27 and SMR40) designed as CW generators with built-in pulse modulation capability. The standard factory configurations address frequency ranges of 20, 27 and 40 GHz, respectively. Output power, spectral purity and phase noise performance is in keeping with demanding wireless, radar and EMS applications. In addition, a broad selection of optional features allows any SMR series instrument to be optimized for all of these applications.

A Solid Foundation of Signal Purity

The heart the SMR generators is a fractional-N synthesis subsystem that drives a low noise yttrium iron garnet (YIG) oscillator. These two elements form a phase-locked loop (PLL) that produces a spectrally pure output signal from 2 to 10 GHz. A doubler/divider, modulator, amplifier (DMA) module then processes the PLL signal to produce the 1 to 20 GHz frequency range. Figure 1 shows the signal generator's block diagram. Throughout the frequency range, the tuning resolution is a constant 1 kHz. (A 0.1 Hz tuning resolution is available as an option.)

The DMA module also contains a pulse modulator, an AM modulator for the modulated signal generator models, and an output amplifier that ensures high output levels (> +10 dBm at 20 GHz). The gain of the output amplifier can be varied over a range exceeding 65 dB to perform automatic level control (ALC).

A switched filter bank forms the basis of the SMR generators' < 55 dBc harmonic performance. In addition, the filter bank provides switching to route the 10 MHz to 1 GHz signal from the available low frequency extension option to the instruments' RF output.

Strong Basic Features with a Broad Array of Options

Many users require nothing more than a good basic CW generator, and the standard SMR generator is an easy-to-use instrument that provides CW functions at an affordable cost. In their standard configuration, the SMR series generators offer ample features and performance for conventional RF measurement applications.

All SMR models feature a pulse modulator with an on/off ratio of greater than 80 dB. Rise and fall times are less than 10 ns, and pulse widths of 20 ns are possible. These specifications make the SMR an ideal signal source for use in development, production and maintenance of radar equipment. The optional SMR-B14 pulse generator provides single and double pulses with repetition rates up to 10 MHz. The pulse generator can be triggered internally or externally, or operated in the external gate mode. Pulse width is programmable across a range of 20 ns to 1 s with pulse delays from 40 ns to 1 s.

The SMR signal generators are equipped with a high precision, frequency-compensated level control for levels above -20 dBm. The RF attenuator option (B15 for SMR20/27 and B17 for SMR40) extends the setting range to -130 dBm.

The SMR base models also include frequency sweep functions. The standard digital sweep (with step times from 1 ms to 1 s) allows convenient frequency response measurements. A 20 dB level sweep range makes it equally easy to perform compression measurements.

The SMR generators' configurability extends their usefulness far beyond pure CW measurement applications. Options provide a cost-effective way to transform the instruments into application-specific tools.

The frequency extension option (SMR-B11) generates the 10 MHz to 1 GHz frequency band by dividing a 0.5 to 2.5 GHz signal from the DMA module. This signal is further divided by a programmable binary divider, then amplified for sufficient output power and filtered to suppress harmonics. Like the DMA module, the frequency extension option also contains a pulse modulator and variable-gain amplifier for producing AM and for performing the ALC function.

The analog ramp sweep option (SMR-B4) is functionally similar to the analog sweep of traditional sweep generators. However, an essential difference exists: The frequency sweep is controlled continuously by the PLL over the entire span. Linearity errors are virtually eliminated -- even at sweep speeds up to 600 MHz/ms. The analog ramp sweep permits the SMR generator to be used as a stimulus source for a scalar network analyzer. This pairing is the most effective toolset for real-time adjustment of microwave filters. As with the digital step sweep, the start and stop frequencies can be set arbitrarily within the entire range of the generator. The sweep time can be varied from 10 ms to 100 s.

For either the digital step sweep or the analog ramp sweep, 10 user-selectable frequency markers are available. Two types of markers can be used: a pulse or video marker at the marker output with HCT/TTL-compatible logic level, or an RF amplitude marker modulated on the RF level. In addition, a Z-axis signal is provided for use with appropriate scalar network analyzers.

AM/FM capability can be added to any SMR model by specifying option SMR-B5, a versatile built-in AM/FM/scan modulator. The amplitude modulator's bandwidth ranges from DC to 100 kHz, and FM bandwidth ranges from DC to 5 MHz with good carrier-frequency stability. Scan (logarithmic AM) allows radar antenna rotation simulation. The SMR-B5 option also includes a low frequency generator with a range of 0.1 Hz to 10 MHz and 0.1 Hz resolution. The low frequency generator can produce sine or square wave signals. Table 1 lists key characteristics of the generators.

An Integrated Upconverter for Simplified Microwave Measurements

Wireless designers need a way to convert lower frequency modulated signals into much higher frequency stimulus signals while preserving the modulation characteristics. Until now, cumbersome and costly external upconverters were the only option. Unique among RF signal generators, the SMR units offer an optional integrated upconverter.

The IF input option (B23 for SMR20 and B24 for SMR27/40) handles the upconversion chores. This option consists of a broadband mixer integrated in the instrument's RF output path. When the function is turned off, two RF relays bypass the mixer. Thus, the microwave performance of the generator is not degraded by the mixer. If the function is activated, the generator acts as a local oscillator for the mixer. Any DC to 700 MHz signal at the IF input can be upconverted to the 1 to 20 GHz  (SMR-B23) or 2 to 40 GHz frequency range (SMR-B24).

A typical application is shown in Figure 2 . Here, a multicarrier RF signal source (the Rohde & Schwarz model SMIQ03B with AMIQ modulation generator) creates the IF drive for the SMR generator's upconverter module. The SMR generator upconverts the digitally modulated signal and delivers it to the RF output. If necessary, an external filter can be used to reject unwanted signals that result from the mixing process.

Conclusion

The basic architecture of the SMR generator family addresses a variety of high performance RF signal source needs cost-effectively. A diverse set of options is available to equip the instruments for applications ranging from wireless R&D to radar maintenance to real-time adjustment of RF components on the production line. The generators' integrated upconverter is unique among tools within similar instruments and provides a means to deliver complex modulated RF signals at frequencies up to 40 GHz.

The SMR signal generators provide high value not only through their initial cost, but also through low cost of ownership with a recommended calibration cycle of three years. A built-in diagnostics system simplifies troubleshooting, reduces mean time to repair and further minimizes the total cost of ownership. Prices: $19,950 (SMR20), $28,950 (SMR27), $34,950 (SMR40). Delivery is eight weeks (ARO).

Tektronix Inc.,
Beaverton, OR
(800) 426-2200.