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Frequency Banded Solutions for VNA Millimeter-wave Measurement

Frequency Banded Solutions for VNA Millimeter-wave Measurement
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by Brian Walker, Senior RF Design Engineer, Copper Mountain Technologies

As Radio Frequency (RF) technology evolves, frequencies of operation climb to accommodate wider data bandwidths for communications and greater spatial accuracy for radar applications. 5G networks will use millimeter-wave frequencies from 24 to 28 GHz, 37 to 40 GHz, and 64 to 71 GHz. Automotive radar uses frequencies from 77 to 81 GHz. Full body scanners, the bane of air travelers, operate between 70 and 80 GHz and some medical applications utilize high power RF between 40 and 70 GHz. 

With explosive growth in the millimeter-wave arena comes the need for test instruments which operate at these frequencies to facilitate the design, field validation, and ongoing periodic maintenance. Vector Network Analyzers (VNA) are an important tool for the design of the amplifiers, phase shifters, filters, and antennas which go into the products mentioned above. 5G communication systems utilize beam steering and MIMO antennas to improve signal to noise ratios. A VNA is essential to validate the amplifiers, mixers, splitters, and phase shifters in the signal chain feeding the antenna array initially in the 24 to 28 GHz band and later from 37 to 40 GHz. 

Millimeter-wave systems employ these high frequencies for broadcast, but the signal processing is done at much lower frequencies where the components are less expensive, and the circuit layout is less critical. The 24 to 28 GHz 5G signal might be mixed down to less than 1 GHz and then converted to digital for demodulation. Similarly, the modulated transmit signal is developed at a relatively low frequency before being mixed or multiplied up to millimeter wavelengths. These systems tend to have two frequency regimes; a low band for signal processing and the millimeter-wave band for transmission from an antenna. For the purposes of testing, is it necessary to purchase a VNA which covers both the low and the high bands contiguously? Such instruments are available, but they are extremely expensive. A much more economical approach is to purchase a VNA which covers the range from less than 1 MHz up to 9 GHz and a frequency extender module which operates only in the millimeter-wave band of interest. 

There are two advantages. First, the cost will be a fraction of the cost of a wide-band VNA which might cover the entire range. Secondly, the frequency extender “head” can be placed physically close to the Device Under Test. This minimizes signal loss and improves the dynamic range of the measurement. For automatic testing of integrated circuits, the extender can be bolted to the test fixture, with the VNA itself mounted a few feet away in the processing hardware of the test station and only low frequency cables connected between them. 

In another application, the extender heads might be mounted directly to the arms of a probe station with only a few inches between the millimeter-wave connection and the probe itself. Again, this greatly improves the dynamic range of measurement by keeping the distances short and the losses down. 

“Banded” VNA solutions simply make the best sense for most millimeter-wave applications. The cost is lower and the measurement itself might be better. 

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