Signal source analyser

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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. The measurement of characteristics, in particular phase noise, is therefore just as essential for effective communications and broadcast systems as it is for special high-tech applications such as radar.

In the case of simple, commercial applications, a spectrum analyser is often sufficient. However, if the requirements for dynamic range, accuracy, and flexibility are higher, then phase noise measurement using the phase-locked loop (PLL) method is preferred. The Signal Source Analyser R&S FSUP unifies both options in one device: It offers the user a high-end spectrum analyser (R&S FSU) up to a maximum frequency of 50 GHz, combined with a phase noise tester based on the PLL method.

Measurement of phase noise using the PLL method

Normally, the PLL method requires very complex setups. In addition, the effort to calibrate the measurement is greater than for measurements using a spectrum analyser. The R&S FSUP greatly simplifies the entire process for the user and makes phase noise measurements possible at the push of a button. It also offers the flexibility needed to adapt the test setup to special situations. External and internal references can be used. The user decides which source will be used to regulate phase quadrature on the comparator. However, the most frequently used method - measurement using an internal phase comparator and internal reference - is predefined as the default value. The R&S FSUP also offers other test modes that can be selected from a clear-cut menu.

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

The premeasurement covers all important oscillator parameters such as power and tuning slope. The R&S FSUP 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 averagings. The menu setup is similar to that of Application Firmware R&S FS-K40 and makes operation - particularly when changing between different test modes - very simple for the user. Predefined settings additionally simplify handling when a rapid measurement or a very stable measurement is needed.

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 all spurious emissions that are caused, for example, by AC frequency interference or the phase detector frequency. Alternatively, 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; the integration limits can be defined by the user.

To ensure uncorrupted measurements on oscillators, the phase noise of the internal reference must be negligible as compared to the oscillator. The internal source of the R&S FSUP possesses excellent 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 (1Hz), and at a frequency offset of 10 MHz it is <-165 dBc (1Hz).

Cross-correlation for reducing phase noise

With the R&S FSUP-B60 option, the signal source analyser 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 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 averagings and can be up to 20 dB.

Required functions for the spectrum analyser

The Signal Source analyser R&S FSUP is linked to a high-end spectrum analyser. This makes it possible to measure the phase noise directly in the spectrum. This method determines the spectral power density in the sidebands. 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 is also more complicated for the user. The obvious advantage of measurements using the spectrum analyser lies, however, in the fact that significantly higher frequency offsets can be measured. As a result, the spectrum analyser 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 analyser, such as the spurious emissions measurement function. It is possible to define a list of various sweep ranges with specific parameters in which the analyser automatically searches for interference and spurious. 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 R&S FSUP 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. The large dynamic range offered by the R&S FSUP also sets the standard for signal source analysis.

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.

Complete characterization of oscillators

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. Supply voltage and tuning voltage can be individually defined for each port via a clear-cut menu. The values will change based on the settings needed for the measurement application, but the maximum and minimum values will not be exceeded or fallen below. 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 similarly to change 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. 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 his application.

With the functionality of a phase noise tester, a transients recorder, and a spectrum analyser combined into a single device 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.