Gain Confidence in mmWave Measurements

Article By : Eric Hsu

While mmWave technology provides ample margin for performance improvement, it also creates many challenges. Here are some strategies to address those issues.

Millimeter-wave (mmWave) technology is a key enabler for next-generation cellular and satellite communications. While it provides ample margin for performance improvement, it also creates many challenges. At mmWave frequencies, path loss makes radio frequency (RF) power limited and costly. Any skew in a flange connection can cause unwanted reflections that degrade signal quality and power.

Millimeter-wave devices are compact and highly integrated, with no connectors or probe points that can be used for conducted tests. Instead, most testing is done over-the-air (OTA). However, this approach introduces excessive path loss, increasing test complexity and measurement uncertainty. To give you a better sense of path loss, Table 1 illustrates antenna apertures and operating frequencies, then calculates the minimum far-field distance and free space path loss. As an example, for an antenna size of 10cm, at 2- and 28-GHz operating frequencies, the path loss difference is up to 45dB. For 43GHz, the difference is up to 53dB.

Table 1. Estimated far-field distance and path loss for different radiating apertures.

Why Path Loss Matters

The excessive path loss at mmWave frequencies between an instrument and a device under test (DUT) results in a lower signal-to-noise ratio (SNR) for signal analysis. Lower SNR makes transmitter measurements, such as error vector magnitude (EVM), adjacent channel power, and spurious emissions challenging.

While engineers can reduce the attenuation of the signal analyzer to improve the SNR, the path loss at mmWave frequencies can be more than 60dB and the input level can be less than -40dBm. Even if you set the input attenuation to 0 dB, the SNR may still be too low for accurate signal analysis. Consequently, minimizing any possible path loss is critical for mmWave testing. Figures 1 and 2 compare the impact on EVM measurements with the same setups for high- and low-input signal levels. As the input signal level decreases, EVM performance increases.

Figure 1. Better EVM performance for a high-SNR 64QAM signal.

Figure 2. Poor EVM performance for a low-SNR 64QAM signal.

Reduce Signal Path Loss

Whether you’re assessing transmitters, troubleshooting receivers, or analyzing OTA signals, the flexibility of signal analyzer hardware and software lets you create the optimum solution. Input signals could be high power to noise-like, low-frequency to THz, and a continuous wave to complex wideband modulation.

Selecting signal paths

To measure the variety of input signals, signal analyzers can apply attenuation at higher power levels or use a preamplifier at lower power levels. Also, signal analyzers provide several RF signal paths—such as a default path, microwave preselector bypass, low-noise path, and a full-bypass path—to lower noise, improve sensitivity, and reduce signal path loss for a better SNR (see Figure 3).

  • Default path: The input travels through the RF attenuator, preamplifier, and preselector before reaching the mixer. It is most useful for measuring low-level signals with a bandwidth under 45 MHz, limited by the bandwidth of the preselector.
  • Microwave preselector bypass: Bypassing the preselector enables wideband analysis and a flat spectrum response over the bandwidth of the digitizer.
  • Low-noise path: Bypassing the lossy switches in the preamplifier path and the preamplifiers.

Full-bypass path: Combining the low-noise path and microwave preselector bypass.

Figure 3. The signal paths of a Keysight X-Series signal analyzer.

Using external mixing

When you build a mmWave test system, cables and accessories in the path between the signal analyzer and the DUT increase insertion loss. Adding an external mixer is a cost-effective way to extend the frequency range of a signal analyzer and move the test plane close to the DUT to reduce insertion loss caused by the long mmWave signal routing.

Harmonic mixer

Figure 4 illustrates an external mixing connectivity setup using a smart mixer. The signal analyzer sends a microwave local oscillator signal to the external mixer for down-converting the DUT’s input signal. The mixer outputs the downconverted intermediate frequency (IF) signal and sends it back to the signal analyzer. The analyzer further processes the IF signal with filtering, digitizing, analysis, and display operations similar to those for internal mixed signals.

Figure 4. The smart harmonic mixer’s connectivity setups.

External mixing provides a cost-effective solution for mmWave signal analysis and moves the test port close to the DUT. However, when measuring frequency outside the mixer’s frequency band, you need to reconnect the test signal to the signal analyzer’s RF input port or another mixer with a different band. Then you need to change the input source from the operation interface correspondingly. These steps increase test complexity and measurement uncertainty. Also, there is no preselector at the front end of the mixer. Strong out-of-band signals may lead to unwanted images in the band-of-interest and degrade measurement accuracy.

Frequency extender

An advanced external frequency extender integrates a preselector and an RF switch into a high-dynamic-range mixer with the seamless operation interface of the signal analyzer. This solution enables unbanded and preselected swept power spectrum from 2 Hz to 110 GHz, without managing band breaks and images, with IF bandwidth up to 11 GHz. Figure 5 shows the test setups for a signal analyzer and an external frequency extender.

Figure 5. The setup for Keysight’s N9042B UXA X-Series signal analyzer and V3050A frequency extender.

More Than a Single Answer

Millimeter-wave testing means not only higher frequencies but also wider input ranges. Engineers need to take extra care to evaluate mmWave components and devices correctly. Understanding your test applications and the test equipment to be used for millimeter-wave testing will allow you to make accurate and repeatable measurements.


Tackling Millimeter-Wave Signal Analysis Challenges

Full Bypass Path for X-Series Signal Analyzers

About the Author

Eric Hsu is currently a Product Marketing Manager at Keysight Technologies Inc. He has over 18 years of experience in wireless applications with Keysight (formerly Agilent Technologies).




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