MENU

Measuring E band microwave connections

Measuring E band microwave connections

Technology News |
By eeNews Europe



The high frequencies are a challenge for T&M equipment; not only when developing transmit and receive modules, but also when measuring transmission systems.

E band: extra bandwidth for more data

Over 30 years ago at the World Radiocommunication Conference WARC-79 in Geneva [1], the International Telecommunication Union (ITU) passed the decision to dedicate the E band frequencies from 71 GHz to 76 GHz and from 81 GHz to 86 GHz for transmission applications. It took more than 20 years before commercial interest in these applications emerged and led the US Federal Communications Commission (FCC) and European authorities to issue licenses for these bands and to specify the technical requirements for their use. The reason why interest finally evolved was that, by then, it had become possible to commercially manufacture components for this frequency range. At the same time the demand for increasing transmission rates made it necessary to use new frequency bands. Transmission links with data rates of several Gbit/s are no problem in the E band. The two frequency bands, each with a continuous range of 5 GHz, make transmission bandwidths of several 100 MHz possible. Combined with a simple modulation method such as BPSK, high data rates can be achieved. Consequently, it is possible to implement simple and reliable transmit and receive modules for these millimeter-wave connections. It goes without saying that more complex types of modulation may be used as this technology evolves. The achievable range in these frequency bands is only insignificantly shorter than for example in the 38 GHz band. This was proven with open field tests done in normal weather conditions with an attenuation of 0.5 dB/km [2].

The high frequencies pose new T&M challenges. Although licensing protects against interference from other microwave sources, the power and spectrum of the transmitters must be measured to ensure disturbance-free coexistence of licensed communications. The requirements for transmitters in this frequency range, especially for the radiated power (EIRP) spectrum density mask, are described in ETSI TS 102 524 V1.1 [3].

Spectrum measurements in the E band – harmonic mixers are essential

Spectrum analyzers are the most suitable instruments for these sophisticated measurements. However, commercially available spectrum analyzers have a continuous frequency range of up to 67 GHz only. To carry out spectrum measurements in the E band, they must be used together with external harmonic mixers. The mixers multiply the spectrum analyzer’s local oscillator output signal and use a suitable harmonic to downconvert the millimeter-wave signal to be measured to the analyzer’s intermediate frequency. However, the large number of harmonics created in the mixer and the input signal’s harmonics produce a multitude of signals in the spectrum. The image frequency is not suppressed because there is no preselection.

This will not create any problems as long as only CW signals are present at the mixer’s input. With this type of signal, the spectrum analyzer can tell the difference between real signals and unwanted mixing products and their image-frequency signals that are present at the mixer output. To enable this distinction, the analyzer conducts a reference measurement prior to the actual measurement. During the reference measurement, the local oscillator frequency is increased to a value that is twice the intermediate frequency. Only signals that are detected in the reference measurement and in the actual measurement are real signals and are displayed in the spectrum.

If modulated signals are present at the mixer input, the task is more complicated. The real signal and the signal received on the analyzer’s image frequency may overlap each other, especially in the case of very wideband signals, so that it is no longer possible to tell them apart.

Click image to enlarge. Figure 1: Measuring a 500 MHz bandwidth E band input signal with the R&S FSQ signal and spectrum analyzer. The blue curve shows the results of the actual measurement, the black curve represents the reference measurement. It is apparent that the image signal, which has a frequency positioned above that of the input signal, can still be subtracted (orange curve). This would not be possible with 1 GHz bandwidth input signals.

Figure 1 shows a spectrum measurement performed with a high-end signal and spectrum analyzer, which no longer belongs to the newest generation and has an intermediate frequency of 404 MHz. The frequency difference between the input signal and the image-frequency signal is 808 MHz. With this 500 MHz bandwidth input signal, it is just still possible to test if it complies with the EIRP spectrum density mask according to ETSI TS 102 524 V1.1, by subtracting the reference measurement spectrum from that of the actual measurement. If the input signal had a bandwidth of 1 GHz, that would no longer be possible because the input signal and image-frequency signal would superimpose on each other. The influence of the image-frequency signal would strongly distort time domain analysis of the signal (I/Q data), where correction using a reference measurement is not possible.

Spectrum analysis: able to handle wideband-modulated signals

Modern signal and spectrum analyzers such as the R&S FSW equipped optional LO/IF connectors for external mixers have a major advantage compared to conventional instruments. With an intermediate frequency of 1.3 GHz, the analyzers have an image-free frequency range of 2.6 GHz. This makes it easy to measure the EIRP spectrum density mask of wideband-modulated signals, even if their bandwidth reaches into the GHz range. Together with the harmonic mixers from Rohde & Schwarz, e.g. the R&S FS-Z90 (60 GHz to 90 GHz), the achievable dynamic range is truly unique. The mixer has a typical conversion loss of 23 dB at 80 GHz, resulting in a displayed average noise level (DANL) of approximately –150 dBm/Hz for the complete test setup, including the R&S FSW. A 1 dB compression point of nominally –3 dBm results in a dynamic range sufficient for measuring the spectrum mask. The ETSI technical specification defines a value of 50 dB. The R&S FS-Z90 harmonic mixer is also equipped with an isolator at the input, which makes a VSWR of typically 1.4:1 possible. Power measurement errors due to reflections at the input resulting from mismatch are typically reduced by a factor of 5 compared to mixers without isolators.

Click image to enlarge. Figure 2: Measurement of the same signal as in Figure 1 with the R&S FSW signal and spectrum analyzer. The input and image-frequency signal are 2.6 GHz apart. Measuring the spectrum mask or analyzing the modulation quality of significantly wider signals is possible without any difficulty.

Figure 2 shows the measurement of the same signal of an E band microwave connection as Figure 1. The 500 MHz bandwidth input signal and the image signal are 2.6 GHz apart, and it is possible to measure if the spectrum is within the prescribed mask (red line). The required dynamic range of at least 50 dB is also easily achieved with this setup.

The R&S FSW can measure the spectrum as well as the modulation quality. Its optional high analysis bandwidth of up to 320 MHz makes it possible to capture wideband signals, demodulate them with the vector signal analysis option and to analyze the modulation quality.

Click image to enlarge. Figure 3: Modulation analysis of a 300 MHz bandwidth QPSK signal. Besides graphic displays, e.g. the constellation diagram or input signal versus time, values displayed in tabular form offer information about the modulation quality at a glance.

Figure 3 shows the analysis of a 300 MHz bandwidth QPSK signal. The error vector magnitude (EVM) as a measure of the modulation quality, as well as the frequency error, the symbol rate error and many more parameters can be measured. The R&S FSW signal and spectrum analyzer displays the results in tables or graphs. For example, the phase and amplitude are displayed in a constellation diagram, which delivers a visual impression of the modulation quality.

Summary

E band microwave connections are becoming more and more popular due to the constantly growing demand for higher data volumes to be transmitted. This range offers the highest achievable data rates of all available wireless transmission technologies. A spectrum analyzer with an external harmonic mixer is required to measure the spectrum. The R&S FSW signal and spectrum analyzer’s high intermediate frequency offers a wide image-free range. The low conversion loss of the Rohde & Schwarz harmonic mixers provides a high dynamic range, and the good matching results in high power measurement accuracy. This makes the R&S FSW together with the R&S FS-Z90 harmonic mixer a peerless solution for spectrum measurements in the E band.

References

[1] Radiofrequency Use and Management, Impacts from the World Administrative Radio Conference of 1979, WARC-79, chapter 4, overview, actions and impacts, page 77.
[2] ITU-R P.676-6, Attenuation by atmospheric gases, 2005.
[3] ETSI TS 102 524 V1.1. Technical Specification: Fixed radio systems; point-to-point equipment; radio equipment and antennas for use in point-to-point millimeter-wave applications in the fixed services (mmw FS) frequency bands 71 GHz to 76 GHz and 81 GHz to 86 GHz.

If you enjoyed this article, you will like the following ones: don't miss them by subscribing to :    eeNews on Google News

Share:

Linked Articles
10s