Extending Millimeter-wave Measurement Systems with Harmonic Mixer Technology
Emerging applications in the millimeter-wave (mm-wave) band, which occupies the 30 to 300 GHz spectrum (wavelengths from 10 to 1 mm), now span radio astronomy, communication, imaging, space research and homeland security. Market forecast and limited av...
Figure 1 The harmonic mixer converts the DUT mm-wave (RF) to a predefined IF frequency.
Harmonic Mixer Primer
To overcome frequency limitations in available instrumentation, frequency extension accessories based on harmonic mixer technology are used to down convert the mm-wave spectrum into the signal analyzer's bandwidth for analysis. In a typical external mixer setup, the harmonic mixer bridges the gap between the mm-wave output from the DUT and the lower frequency spectrum analyzer input (see Figure 1). In this way, the harmonic mixer provides the enabling technology for mm-wave measurements. This setup functionally relies on an external mixer option in the spectrum analyzer for the necessary LO and IF interconnects to the harmonic mixer and automatically displays the desired signal parameters. Once connected, the nth harmonic of the LO frequency mixes with the mm-wave frequency (RF) to produce the predefined IF frequency. The conversion loss of the harmonic mixer is proportional to the multiplier factor, n. This popular setup depends on a diplexer for signal separation, which can be either external or internal to the spectrum analyzer.
With an external mixer option, the harmonic mixer operation with the spectrum analyzer is transparent to the user. The harmonic mixer with waveguide interface can conveniently connect to the mm-wave output of the DUT or to a waveguide antenna. On the opposite side of the harmonic mixer, a reasonable length coaxial cable (such as 1 meter) offers efficient access to the spectrum analyzer, including the diplexer. After selecting the corresponding waveguide band on the spectrum analyzer, engineers can use their familiar instrument to conduct mm-wave measurements on their DUT. For accurate amplitude measurements, additional offset features are available in the spectrum analyzer to manually compensate for the conversion loss of the harmonic mixer. In this way, this frequency extension accessory offers an attractive value proposition to engineers with mm-wave requirements.
Figure 2 A typical mm-wave measurement set-up.
A typical mm-wave measurement setup includes the microwave spectrum analyzer, harmonic mixer and diplexer (see Figure 2). Cabling is efficient and unobtrusive. The inlay shows the close-up interconnects between the diplexer and the IF and LO inputs provided with the external mixer option.
As background, the spectrum analyzer's external mixer option enables substitution of the harmonic mixer for its own RF front-end design to overcome the mm-wave measurement limitation. After substitution, the later stages in the spectrum analyzer's receiver chain are still utilized for the remaining signal analysis capabilities. Harmonic mixer suppliers use spectrum analyzer manufacturer's designated LO, IF, and multiplier factor to characterize their harmonic mixers (with bias, if available).2 The correction process is easy to implement using the supplier's final test data.
Figure 3 Popular 50 to 325 GHz mm-wave spectrum by waveguide bands.
Once connected, the harmonic mixer design down converts the RF signal by mixing the nth harmonic of the LO to generate the predefined IF of the existing instrument. The RF input and the harmonics from the LO drive the mixer to produce the IF that satisfies the equation n(LO) – (RF). As an example, the high performance spectrum analyzer with predefined IF of 321.4 MHz has multiplier values that can range from n = 14 for WR-15 to n = 48 for WR-03 (see Figure 3).3 Typically, firmware automatically handles the multiplier factor so the displayed start and stop frequencies are the desired mm-wave RF spectrum. In addition, offset compensation is possible so displayed amplitude corrects the conversion loss of the harmonic mixer. In a typical measurement scenario, the display readout offers actual results with real-time updates when using the harmonic mixer technology with the spectrum analyzer.
Below 50 GHz, commercially available instrumentation using coaxial connections are available for convenient and affordable signal analysis, as well as reasonable cable losses. Over the 50 GHz threshold, rectangular waveguide is often implemented for its low-loss transmission of mm-wave frequencies. In particular, popular waveguide band segmentation allows engineers to translate their application into the proper frequency extension accessory that is based on these same industry standard waveguide terminologies.
Figure 3 also contains the key rectangular waveguide information for the TE10 propagation mode, including the aperture size, both dimensionally and visually, for relative comparisons. The cutoff frequency indicates the frequency above which electromagnetic energy will propagate in the corresponding waveguide. The dimensions are proportional to the wavelengths, which decrease with higher frequencies. The multiplier factor (n) is a harmonic mixer value chosen by the analyzer manufacturer that down converts the millimeter to the microwave spectrum for easier analysis in modern spectrum analyzers.
Figure 4 Representative conversion loss of a single unbalanced harmonic mixer.
For most convenient readouts, modern spectrum analyzer features can compensate for the external harmonic mixer attributes, so amplitude and frequency readouts are accurate. For amplitude readouts, the harmonic mixer manufacturer supplies the typical amplitude correction factor (that is conversion loss) value, which is largely influenced by the nth harmonic of the LO signal needed to down convert the RF to the predefined IF for signal analysis. As one might expect, the conversion loss increases with higher multiplier values. Figure 4 shows the representative conversion loss of a single diode, unbalanced harmonic mixer versus the mm-wave frequency range for the Agilent PSA Signal Analyzer (model E444xA). As predicted, the multiplier factors are overlaid with the typical conversion loss values to show how conversion loss increases with the multiplier value. These results are typical for the predefined LO, IF capabilities of the PSA. Results may vary when using other spectrum analyzers, due to different settings for LO, IF, and multiplier factors. For simplified frequency readouts, the spectrum analyzer contains preset settings, selectable by waveguide band, to compensate for the multiplication factor so the frequency scale reads RF instead of LO or IF.
Independently verifying the operation of the harmonic mixer requires a mm-wave source with a known power level. Simply set the RF source to a value in the harmonic mixer's linear range avoiding input compression. Using Figure 1, apply this "reference" RF signal to the input of the harmonic mixer and complete the LO and IF connections to the spectrum analyzer (an external diplexer may be necessary). After properly configuring the spectrum analyzer for external mixer operation, the readout will display a measured value that includes the reference signal level and the harmonic mixer's conversion loss. By entering the conversion loss as an offset, the spectrum analyzer will display the corrected power level.
The following factors should be considered by test engineers when using harmonic mixers to extend their measurement system into mm-wave frequencies:
Damage Level: The maximum input power is typically 20 dBm, where nominally +15 dBm is allocated to the LO signal. Maintaining composite power levels below +20 dBm ensures damage will not occur to the harmonic mixer diode(s).
Linearity: Mixers are inherently nonlinear devices, so careful selection of power levels will help optimize the results. Position measurements in the linear input range, which, practically speaking, mean to avoid applying input signals within 10 dB of the 1 dB compression point. Below -30 dBm input power, single diode unbalanced harmonic mixers typically provide both accurate and repeatable measurements when using high performance spectrum analyzers.
Mixer Topology: Balanced mixers are popular for their increased linearity performance. However, they also fundamentally limit harmonic mixing to only even products due to the balanced properties in this topology. This may be a good selection as long as the spectrum analyzer utilizes even harmonic multipliers in their external mixer option. In contrast, the single diode mixer offers more versatility to use both even and odd products with less LO power, which are the reasons for their popularity in mm-wave applications. The single diode topology also requires bias, which can be useful to "peak" responses and further optimize results.
Image Rejection: There will be numerous mathematical intersections occurring where these harmonic currents, m + n, combine to produce responses within the spectrum analyzer's IF bandwidth. Furthermore, the strongest of these IF responses can, in turn, be combined with other m + n products to produce additional IF responses. Do not be alarmed that the results on a spectral display look like a "picket fence." Instead, most high performance spectrum analyzers offer "image rejection" features to eliminate "false" from the "desired" results, thereby simplifying the signal analysis task.
Conversion Loss and Dynamic Range: Frequency extension using harmonic mixer technology is a valuable tool for measuring fundamental characteristics of mm-wave signals, but not without some trade-offs. As a general observation, the higher conversion loss versus higher frequency behavior reduces measurement dynamic range and might be an obstacle when measuring low-level signals (such as intermodulation distortion products, discrete spurious or noise figure). As a tip, it is important to analyze after external mixing (taking into account the conversion loss) whether sufficient dynamic range (that is signal-to-noise ratio) exists in the spectrum analyzer for accurate measurements. Generally speaking, accurate measurements require greater than 10 dB signal-to-noise ratio.
Figure 5 Diplexer design for external mixing.
Diplexer Characteristics: The diplexer is essential to the successful operation of the harmonic mixer, especially in single diode harmonic mixers. Although it is more convenient when the diplexer is integrated into the spectrum analyzer, this is not always the case. For example, the Agilent PSA (model E488xA) requires an external diplexer as part of its external mixer setup. In this case, the predefined IF is 321.4 MHz and the available LO range is 2.9 to 7 GHz. The diplexer design for external mixing is optimized for signal separation and harmonic mixing performance at the predesigned IF and available LO range of 2.9 to 7 GHz and will ensure these frequencies will flow unimpeded and with adequate signal separation to optimize performance for mm-wave spectrum analysis (see Figure 5). The diplexer characteristics are occasionally worthwhile to consider in the setup because they constitute hardware constraints.
The harmonic mixer technology enables the practical measurement of millimeter-wave signals. This primer describes harmonic mixer technology, including the typical conversion loss versus the millimeter waveguide bands for single diode harmonic mixers. This primer and tips will ensure that engineers can explore the mm-wave frontier using the terminologies and frequency extension accessories as practical tools. This technology is also the foundation for additional frequency extension accessories deployed in mm-wave signal generation, scalar, and vector network analysis.
- M. Sayed, "Millimeter Wave Tests and Instrumentation," 65th ARFTG Conference Digest, June 2005, pp. 28-37.
- "Using a Millimeter Wave Harmonic Mixer to Extend the Frequency Coverage of a Spectrum Analyzer," OML Inc. Application Note, 42-010124, January, 2001.
- "External Waveguide Mixing and Millimeter Wave Measurements with Agilent PSA Spectrum Analyzers," Agilent Technologies Application Note #1485, 5988-9414EN, October, 2007.