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All-in-one Phase Noise Tester and Spectrum Analyzer

June 1, 2006
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The characteristics of transmit and receive modules for radio transmissions are determined primarily by the phase noise of the oscillators in use. Therefore, the measurement of characteristics, particularly phase noise, is just as essential for effective communications and broadcast systems as it is for special high tech applications such as radar. For simple, commercial applications, a spectrum analyzer is usually sufficient but if the requirements for dynamic range, accuracy and flexibility are greater then a phase noise measurement using the phase-locked loop (PLL) method is preferred. The new R&S FSUP Signal Source Analyzer unifies both options in one device. It offers the user a high end spectrum analyzer (R&S FSU) up to a maximum frequency of 50 GHz, combined with a phase noise tester based on the PLL method.


The PLL Method

The PLL method normally requires very complex set ups and the effort to calibrate the measurement is greater than for measurements using a spectrum analyzer. However, this new instrument simplifies the process by making phase noise measurements possible at the push of a button. It also offers the flexibility needed to adapt the test setup to special situations, and external and internal references can be used. The user decides which source regulates phase quadrature on the comparator. However, the most frequently used method — an internal phase comparator and internal reference — is predefined as the default value. Other test modes can also be selected from a clear menu.

If a more complex test setup with an external reference is needed, there is graphical help. A wiring schematic displayed on the screen shows the user how to connect the various modules, while LEDs on the front panel show which outputs and inputs should be connected.

The premeasurement covers all important oscillator parameters such as power and tuning slope and the instrument then automatically selects the optimum parameters for the test. Depending on the input frequency, the device uses internal frequency multipliers to operate the internal reference within the optimum range. However, the user also has the option of changing the defined parameters.

The offset range is easily set, as are the other measurement parameters such as bandwidth, filter type and number of averages. The menu set up is similar to that of R&S FS-K40 Application Firmware and makes operation very simple for the user, particularly when changing between different test modes. Predefined settings additionally simplify handling when a rapid or a very stable measurement is required.

After the phase noise measurement starts, the display shows ‘Locked’ or ‘Unlocked’ to indicate whether the PLL is locked and a successful measurement can begin. The loop bandwidth can be adapted as needed, and the voltage on the phase detector can be displayed during the measurement. An efficient algorithm then allows the user to select one of the following options: display during the measurement of all spurious emissions caused, for example, by AC frequency interference or the phase detector frequency or suppress all or specific, clearly defined interference. Integral parameters such as residual FM/PM or RMS jitter are also displayed. The calculation applies to the entire test range and the user can define the integration limits.

Figure 1 shows a typical phase noise measurement using the phase detector method, with the signal frequency, level and residual noise being displayed. Unwanted spurious emissions are detected automatically and can be suppressed. It is also possible to obtain a listing of these spurious emissions (shown top right) and the exact frequency.

To ensure uncorrupted measurements on oscillators, the phase noise of the internal reference must be negligible when compared with the oscillator. The internal source of the R&S FSUP possesses good phase noise values for this application. At an input frequency of 640 MHz and a frequency offset of 10 kHz, the phase noise value is < –136 dBc (1 Hz); at a frequency offset of 10 MHz it is < –165 dBc (1 Hz). The phase noise of the internal reference source for various input frequencies is shown in Figure 2.

Cross-correlation

With the R&S FSUP-B60 option, the signal source analyzer is equipped with two parallel receive paths. This symmetrical structure allows a cross-correlation between the two paths, allowing the uncorrelated inherent noise of the two reference sources to be eliminated. Complex parts are eliminated by averaging so that only the correlated signal of the device under test (DUT) remains. As a result, sensitivity is no longer limited by the phase noise of the internal references. The degree of improvement depends on the number of averages and can be up to 20 dB. Figure 3 shows the principle of cross-correlation to increase sensitivity during phase noise measurements.

Figure 4 shows the measurement on an oscillator with a subsequent filter, which illustrates the improvement in sensitivity achieved by cross-correlation. The yellow trace shows a measurement without cross-correlation, while cross-correlation was used with the two other measurements. It is clear how an improvement in sensitivity can be achieved, depending on the number of averages.

Spectrum Analyzer Functions

The new signal source analyzer is linked to a high end spectrum analyzer, which enables the measurement of the phase noise directly in the spectrum. This method determines the spectral power density in the sidebands. However, the disadvantages are obvious — it requires more time, sensitivity is lower because the carrier is not suppressed, and thus the dynamic range is severely limited. In addition, cross-correlation is not possible and there is no way to distinguish between amplitude noise and phase noise. The calculation or suppression of spurious emissions is also more complicated for the user. Conversely, the obvious advantage of measurements using the spectrum analyzer lies in the fact that significantly higher frequency offsets can be measured. As a result, the spectrum analyzer is clearly a necessary addition during phase noise measurements.

Even when measuring harmonics or interference, the instrument offers interesting options over and beyond the normal functions of a spectrum analyzer, such as the spurious emissions measurement function. It is possible to define a list of various sweep ranges with specific parameters in which the analyzer automatically searches for interference and spurious emissions. Up to 100,000 measurement points can be evaluated and the results listed in a table.

The measurement of the adjacent channel power is also an important function during characterization of signal sources. The instrument has simple measurement functions that allow rapid determination of power in the adjacent channels. Users can select from predefined default settings, or they can also separately define the channel widths and spacing. Also offered is a large dynamic range.

Finally, the device, like an FM/PM/AM demodulator, records the oscillator signal over time. Broadband resolution of transient effects or switching resulting from high frequency sources is thus possible.

Oscillator Characterization

In order to record characteristics and to allow measurement of the phase noise using the PLL method, the user must set the supply and tuning voltage of the oscillator exactly. To this end, the R&S FSUP is equipped with two independent, very low noise DC ports and the supply voltage and tuning voltage can be individually defined for each port via a clear menu. The values will change based on the settings needed for the measurement application but the maximum and minimum values will not be exceeded. It is also possible to define the sequence in which the various voltages are applied at the start of the measurement. For special applications, a negative supply is additionally available.

To characterize a voltage-controlled oscillator the following typical measurements are applied. At a constant supply voltage, it is possible to change the tuning voltage (tuning characteristic) or the supply voltage while maintaining a constant tuning voltage (DC dependencies). A combination of the two variants (pushing) is also available. In addition, characteristic parameters can be measured not only for the fundamental, but also for the harmonics and the tuning voltage or the frequency can be selected to allow scaling of the X-axis. For these measurements the user can define the number of measurement points to optimize resolution for the application.

Conclusion

With the functionality of a phase noise tester and a spectrum analyzer combined into a single device and with a maximum input frequency of 50 GHz, the R&S FSUP is the optimum instrument for development and production. With this device, investments for signal source analysis drop significantly, test setups are easier and flexibility increases. All functions can also be remotely controlled via LAN or GPIB, so that the device can be easily integrated into production lines.

Rohde & Schwarz, Europe: Munich, Germany,
Tel: +49 1805 12 4242,
e-mail: customersupport@rohde-schwarz.com
USA & Canada:
Tel: +1 888 837-8772,
e-mail: customer.support@rsa.rohde-schwarz.com
Asia:
Tel: +65 65130 488,
e-mail: customersupport.asia@rohde-schwarz.com

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