Measurement accuracy up to 50 GHz and higher

Rohde & Schwarz (Australia) Pty Ltd
Saturday, 11 December, 2004

The growing demand for radiocommunications calls for higher frequencies since all lower-range frequencies have already been allocated worldwide.

Frequency bands above 40 GHz have traditionally been reserved mainly for military applications due to the highly sophisticated technology involved.

Advances in technology will, however, result in the commercial use of the higher microwave bands.

For example, a frequency range from 10 to 66 GHz is planned as part of network standardisation for point-to-point connections.

The commercial use of higher microwave ranges and the mass production of corresponding components present new challenges for T&M equipment.

These challenges have been taken into account by Rohde & Schwarz in the development of its Microwave Spectrum Analyser R&S FSU50 (Figure 1).

The analyser is largely based on the lower-frequency models of the R&S FSU family. It offers the same top-quality characteristics as well as the same operation and measurement functions.

In addition to highly sensitive spectrum analysis with a wide dynamic range, the analyser features top-quality characteristics with regard to measurement accuracy.

Dynamic range is one of the most important characteristics of a spectrum analyser and is influenced by several parameters such as sensitivity, compression point (1 dB compression) and phase noise. The FSU50 provides good performance with any of these.

To achieve high sensitivity, the analyser uses fundamental mixing in the entire frequency range from 20 Hz to 50 GHz.

This means that the mixer uses the fundamental of the local oscillator to mix the input signal to the first IF, which yields low conversion loss and thus very high sensitivity.

By comparison, many other microwave analysers use a harmonic mixing method starting at a specific input frequency.

This method significantly reduces sensitivity, which means that you cannot measure low level signals.

The high sensitivity of the FSU50 also offers an advantage in speed. For achieving a specific degree of sensitivity, its low inherent noise allows the use of a filter with a high resolution bandwidth.

Since measurement speed depends on the square of the resolution bandwidth, doubling the bandwidth yields a fourfold increase in measurement speed.

When very small signals are measured, the low inherent noise of the FSU50 produces a large S/N ratio and thus results in good measurement accuracy (Figure 2).

But the instrument also measures accurately because it is influenced by tracking preselection (YIG filter). For many analysers, compliance with specifications regarding level measurement accuracy depends on the use of preselector peaking.

In this technique, preselection is tuned to the highest level reading by means of an applied input signal. In contrast, this instrument uses a frequency response correction mechanism and internal automatic adjustment to correctly set the preselection filter.

This yields good stability and repeatability of results even without the use of peaking.

You can therefore achieve considerable savings in time, especially when you use the analyser in production.

Typical application examples of the analyser are measurements on microwave components and systems used in radio relay and radar links.

It provides a number of built-in measurement routines that enable you to perform complex measurements such as C/N or determine the intermodulation point by pressing a single key (Figure 3).

In this case, the analyser is not limited to frequencies under 50 GHz. Its frequency range can be extended beyond 50 GHz by means of an optional external mixer. The company offers harmonic mixers FS-Z60/-Z75/-Z90/-Z110, which cover all frequency bands from 40 to 110 GHz (Figure 4).

However, operation is not limited to these external mixers. The analyser can also be operated with any conventional mixer you may already have.

It supports both three-port and two-port mixers and the diplexer required for operation has already been integrated.

Since all important parameters are displayed in setup summary, you can quickly configure external mixing (Figure 5).

Preconfigured settings are available for microwave bands up to 330 GHz, but you can also generate and store the specific settings you need.

The harmonic that is used is especially important for measurements at very high frequencies since it influences the sensitivity and the phase noise of the measurement.

Its internal local oscillator covers the frequency range from 7 to 15 GHz. Thus, the analyser uses a harmonic with a far lower order than that used by conventional microwave spectrum analysers.

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