Using Proven Commercial Equipment to Generate High Quality Millimeter-Wave Signals
By Erik Diez, Agilent Technologies
In applications that require high speed, short range connections, some developers are considering a shift to millimeter-wave frequencies. Sample applications include satellite-to-satellite links, point-to-point radios and secure communications.
Currently, the attraction of higher frequencies is less-crowded spectrum in the range of 30 to 300 GHz. The companion need is wider modulation bandwidth, which enables faster data throughput.
Moving up to millimeter-wave frequencies and wider bandwidth leads to an increasing degree of difficulty in the measurement and characterization of signals and devices. While it may seem necessary to create in-house solutions, this can be challenging because relatively few labs have the equipment or expertise needed to design millimeter products.
Fortunately, an increasing amount of commercial, off-the-shelf (COTS) equipment is becoming available for millimeter work. For example, external COTS hardware can be used to extend the frequency range of proven microwave signal generators, signal analyzers and vector network analyzers. This saves time and effort, and helps ensure meaningful measurement results.
Working with Millimeter Frequencies
From 30 to 300 GHz, the associated wavelengths range from 10 to 1 mm. At these extremely short wavelengths, antenna dimensions can be very small compared to microwave antennas, and this means transmitter and receiver systems can be very compact. In addition, antennas can be highly directional with small beam widths.
Turning Liabilities into Assets
Millimeter-wave signals have absorption properties that may seem problematic but can instead be treated as advantages. For example, in terrestrial applications these signals are rapidly absorbed by the atmosphere, especially at the resonant frequencies of oxygen, water and carbon-dioxide molecules.
As a result, millimeter signals are most useful for short-range communications. Some of these rely on areas of low absorption: point-to-point radios, wireless backhaul links and high-altitude radio-astronomy arrays.
Others rely on areas of high absorption. For example, the coupling of typical absorption properties with highly directional antennas enables creation of secure communication systems that minimize the chances of eavesdropping.
Handling the Practical Limitations
As frequencies increase, physical dimensions decrease. Thus, all associated hardware becomes smaller and more fragile, and manufacturing tolerances become much tighter. Taken together, these factors mean it is more difficult to fabricate and assemble millimeter-wave devices. Coupling this with today’s modest but growing demand for millimeter products translates into relatively high costs for components, assemblies and devices.
From a measurement perspective, it’s also worth noting the lack of traceable power standards above 110 GHz. Some end users have tried to address this by building power-measuring devices such as bolometers to ensure some level of measurement consistency and repeatability. However, the accurate determination of power levels remains a significant challenge.
Leveraging Available COTS Equipment
Proven microwave signal generators provide a foundation for viable measurement solutions at millimeter frequencies. When used with a compatible signal generator, external devices such as multipliers and upconverters can shift metrology-grade signals to higher frequencies.
Agilent offers a variety of mixers that provide frequency translation for signal analysis up to 110 GHz. Agilent solution partners OML, Inc. and Virginia Diode, Inc. (VDI) also provide a variety of COTS frequency-extension devices that reach up to 1.1 THz for signal generation, signal analysis and vector network analysis. The approaches used by Agilent, OML and VDI for signal generation—multiplication and upconversion—have pluses and minuses that affect their fit with specific applications.
Examining Three Approaches to Signal Generation
Three factors will affect the best choice of solution for signal generation: the application requirements, the pros and cons of the available generation methods, and the cost of the equipment. Naturally, the primary considerations are the application requirements, and four of these often stand out from the rest: frequency range, output power, modulation and bandwidth characteristics, and spurious signals.
Multipliers are a widely used solution, in part because configuration is relatively simple:
connect one or more multipliers to a microwave signal generator and shift the output frequency upward into the range of interest. For example, 6x multiplication can be accomplished by connecting the output signal to a module that contains a doubler and a tripler working in series (Figure 1). If the original signal ranges from 12.5 to 18.4 GHz, the resulting multiplied signal will be in the W-band, covering 75 to 110 GHz.
Multiplier modules are easy to set up and use, typically requiring an external power supply and just one RF input cable connected to the output of the signal generator. To enhance this simplicity, some signal generators are compatible with millimeter source modules, providing the ability to enter the multiplication factor and then present the correct frequency on the instrument display.
The pros and cons of multiplier modules will help clarify how suitable they are for an application. On the plus side, this approach has four important attributes:
• Simple to set up and use, as noted above
• Work well with continuous-wave (CW) and pulse-modulated signals
• Designed for fixed output power; some modules offer a variable attenuator mounted at the output
• Readily available from commercial manufacturers
On the negative side, four factors arise with most of these modules:
• Operate with saturated output power
• Have issues with most types of modulated signals
+ Not suitable for most digitally modulated signals, especially those that involve amplitude changes (due to saturated output operation)
+ Alter angle modulation (e.g., multiply the deviation of FM and ΦM signals)
• Are inherently nonlinear
+ Severely distort AM (including QAM) due to saturated output
+ Create spurs at harmonic, sub-harmonic and non-harmonic frequencies
• May alter the rise and fall times of pulse-modulated signals (e.g., may be sharper than the original signal)
• Multiply the phase noise of the signal generator
This last problem can be quite prominent because the phase noise of the signal generator is multiplied upward, rising by 6 dB with every doubling in frequency (i.e., by 20*log(n)). Figure 2 shows how the phase noise of a microwave signal generator increases when used with a variety of source modules at frequencies up to 450 GHz.
Using Simple Upconversion
Compared to multiplication, the configuration for upconversion is more complex because even the simplest case requires a mixer and two signal generators, one providing the LO signal and the other producing the modulated IF signal (Figure 3). Some upconversion configurations include the use of a multiplier in the LO path, making it possible to use a lower frequency—and therefore lower cost—signal generator as the LO source.
On the plus side, this approach has four important qualities:
• Works well with modulated signals
• Can support wide-bandwidth signals
• Provides reasonable output power
• Better preserves the phase noise of the IF and LO signal generators
Unlike the multiplier approach, upconversion maintains a high degree of modulation fidelity, even with wideband signals. Depending on the conversion loss of the mixer, this configuration can provide enough output power for most applications.
On the down side, three factors affect the usefulness of upconversion:
• The configuration is more complex and potentially more expensive than with multiplication
• Mixing produces an image signal at twice the IF frequency either above or below the desired signal
• Mixing produces spurious signals at harmonic, sub-harmonic and non-harmonic frequencies
As another possible negative, amplitude control is limited. A modest amount of control can be achieved at the mixer output by varying the strength of the LO and IF signals. However, pushing the power too high can overload the mixer, potentially causing not just distortion but also physical damage. When greater control is needed, a variable attenuator can be connected to the RF output.
One last point: Although a large number of upconverting mixers are available, it can be difficult to find a model with a specific combination of LO, IF and RF frequencies along with a low enough conversion loss to deliver the required output power.
Enhancing Upconversion with Dedicated Devices
The next level of integration beyond a simple mixer is a dedicated upconverter. Figure 4 shows a custom unit designed to produce 802.11ad signals in the 57 to 66 GHz range. The rear panel has inputs for the LO and IF signals as well as a frequency reference and DC power; the front panel has a V-band waveguide output flange and control knobs for manually operated attenuator settings.
This upconverter takes an LO signal between 10 and 12 GHz and mixes it with a 5 GHz IF signal. To minimize conversion loss, the LO signal is multiplied by 4 and then filtered to help prevent sub-harmonics and other spurious signals from reaching the mixer.
Because the WiGig standard defines a very wideband OFDM signal (e.g., 2 GHz wide), careful selection of individual components is essential to ensuring that the upconverter maintains adequate bandwidth and a reasonably flat frequency response throughout the signal path. This helps maintain signal fidelity to a level that ensures a WiGig receiver can accurately demodulate the signal.
Access to innovative measurement solutions is essential to early research into next-generation wireless technologies and systems. For those working at millimeter-wave frequencies, the increasing availability of COTS equipment will help save time and effort while ensuring meaningful measurement results.
For more information about millimeter-wave signal generation, please see www.agilent.com/find/SG_mmwave.
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