Millimeter wave measurement technology and the advantages of using high-performance mixers

1 Introduction

At present, attractive millimeter wave applications are mainly in the E-band and V-band. The E frequency band corresponds to the frequency range of 60 GHz to 90 GHz. In this frequency band, the line of sight transmission (LOS) method can only be adopted due to the influence of atmospheric attenuation. In fact, many molecules in the atmosphere, such as oxygen, water vapor, or nitrogen, can absorb energy at specific wavelengths in this frequency band. However, in practice, sufficient spectrum resources available in these frequency ranges are still driving the industry to apply future technologies to these frequency ranges. Similarly, the V band corresponds to 40GHz～75GHz and is widely used in satellite communications.

There are 3 key applications that are being developed on these frequency bands. They are: mobile backhaul, automotive radar, and Wi-Gig (ad).

This application relies on the fact that the current ultra-heterogeneous network is full of multiple small base stations, which greatly increases the demand for the transmission capacity of the backhaul line. The network must handle large amounts of data being transmitted to each node in a specific area. Therefore, based on these connections of millimeter-wave wireless links with a bandwidth greater than 1 GHz, we can meet the backhaul requirements of modern and future networks and provide a solution that is more than optical fiber. Both mobile backhaul and automotive radar are important applications. The 79 GHz band is likely to become the standard frequency for FMCW (Frequency Modulated Continuous Wave) radar technology. This technology can work with signals up to 4Ghz bandwidth, so as to achieve the required accuracy when detecting targets in the mobile environment of the car. , Wi-Gig is a new WLAN 802.11 standard, which has been developed for very high rate transmission services, such as uncompressed high-definition television (HDTV) and instant music and image transmission. It works at 60GHz frequency and Occupies 2GHz bandwidth.

In view of the characteristics of transmission on these frequencies, appropriate measuring instruments will be required to ensure the realization of all these technologies. These instruments will need a dynamic range to cope with highly attenuated signals and the ability to measure ultra-wideband signals.

2. Challenges of millimeter wave equipment and different measurement solutions

2.1, harmonics

The equipment of the harmonic mixer works in such a way that the limited local oscillator (LO) frequency involved in the mixing process is affected by the harmonic components. The main advantage of using these types of mixers is the simple and cost-effective solution it provides.

However, there are two main problems from these systems. First, the multiple harmonics used to affect the local oscillator signal proportionally introduce loss as the frequency increases. Therefore, the dynamic range of the solution becomes very poor. Secondly, the influence of the mirror reaction is very important here, because multiple frequency components will be unwelcomely mixed in during the process. The mirror response that affects the measurement results will be displayed at the 2x offset position of the intermediate frequency (IF). As an example, if 1 set of frequency spectrum plus 1 set of harmonic mixer designed to work at 1.58 GHz IF frequency measures the 4GHz bandwidth signal from FMCW radar, some important test items, such as frequency error, occupied bandwidth or The transmit power cannot be measured because there will be an image response that overlaps the actual radar signal. In some cases, this problem may be solved by image suppression methods. However, this solution is ineffective in the case of FMCW continuous wave modulation, because the transmission frequency is constantly changing.

2.2, typical down-conversion configuration

The typical way to overcome the image response of a harmonic mixer-based solution is to use a classic down-conversion setup to connect to a spectrum analyzer. On the one hand, due to the configuration used by the basic mixer, harmonics are not used to affect the local oscillator signal. An ideal intermediate frequency can be designed according to the test and bandwidth. Basically, a continuous wave combined with a multiplier will down-convert the signal to provide the required local oscillator signal.

On the other hand, a system needs to be composed of multiple components such as mixers, locals, multipliers, filters, and gain amplifiers. Obviously, because the above-mentioned equipment needs to be configured, calibrated and maintained during use, it can be understood that the down-conversion configuration will be time-consuming.

2.3, high-performance basic mixer

The figure below shows the vision of Anritsu's high-performance basic mixer. MA2808A and MA2806A, which work in E-band and V-band respectively, can be understood as an integrated down-converter, based on waveguide technology and built-in single-stage multiplier, low-noise amplifier, and filter design. These devices provide a solution to the problems discussed earlier: they have excellent dynamic range, the mirroring reaction occurs at a long distance from the required signal, and only one connection is required between them and the spectrum analyzer to work.

On the one hand, high-performance basic mixers have two main advantages over harmonic mixers: better sensitivity or DANL, thanks to lower conversion loss; and better image response suppression, which benefits It uses 1.875GHz intermediate frequency. In addition, the internal mixing/filtering technology and the polarization shift function make it feasible to measure millimeter wave signals with a bandwidth of 4GHz. On the other hand, high-performance basic mixers have the following advantages over traditional downconverters: they allow a simple configuration or connection to a spectrum analyzer, and the conversion loss can be added from the USB memory simply by one-button operation, providing a Better 1dB compression point performance than commonly used downconverters. There is no doubt that this compact test system can simplify the design and layout of the manufacturing site, while reducing the maintenance and calibration costs of measuring instruments.

3. Typical measurement items for millimeter wave equipment

The measurement of millimeter wave devices can be divided into two different parts: RF output characteristics (following ETSI EN 302 264-1) and modulation or signal characteristics (depending on the actual technology to be tested). In the next part, we will explain how Anritsu's high-performance millimeter wave solution shows its outstanding advantages in each part.

3.1. Transmitting power, frequency error and stray radiation under sufficient sensitivity

In many cases, due to the nature of the signal at these frequencies-greatly affected by reflection, attenuation or material absorption, the transmit power and spectrum emission mask of millimeter wave devices need to be tested under Over The Air (OTA). Therefore, the test equipment needs to have good sensitivity. For example, if the test antenna is 50 cm away from the DUT, the free space loss of the 79 GHz signal will be around 65 decibels. Since the EIRP defined by ETSI EN 302 264-1 requires measurement of <-40dBm/MHz, considering the test antenna gain of 23 dBi, the requirement for the test equipment at 79 GHz will be approximately 142 dBm/Hz.

In general, a typical harmonic mixer is characterized by a conversion loss of about 15dB to 20dB. When it is combined with a spectrum analyzer, we can estimate that the displayed average noise level (DANL) is about -135 dBm/Hz to 140 dBm/Hz, which will make it difficult to meet the above requirements. However, the combination of the new MS2840A spectrum analyzer with excellent noise floor performance and the MA2808A high-performance basic mixer can achieve at least 8dB in transmit power and spurious emission sensitivity.

3.2. Broadband modulation test

To test millimeter wave quality, the phase noise performance of a spectrum analyzer is very important. For example, when testing FMCW automotive radar, the phase noise characteristics and the frequency linearity of the DUT must be verified. When the time and frequency difference between the transmitted and received signals is small, the phase noise performance of the spectrum analyzer is poor, because the received signal may be hidden in the phase noise of the transmitted signal, and the two signals cannot be distinguished, as shown in the following figure. Show.

Combining MS2840A and MA2808A, the phase noise performance below -100 dBc (100 kHz offset) and below -110 dBc / Hz (1 MHz offset) at 79 GHz can meet the requirements of automotive radar technology at least -90 dBc / Hz ( 100 kHz offset) and -100 dBc/Hz (1MHz offset) phase noise performance requirements.

4. Summary

With the upcoming promotion of 5G networks and ADAS, the demand for millimeter wave systems is also increasing. To test these ultra-wideband technologies, a spectrum analyzer with an external mixer must avoid the problem of image response, must provide sufficient sensitivity for OTA testing, and must have sufficient modulation phase noise performance for modulation analysis. The combination of MS2830A/MS2840A spectrum analyzer and MA2808A high-performance waveguide mixer is an ideal solution to meet these needs.