Instrument Function > RF Block > RF Frequency > The Configurable Main PLL Bandwidth
The Configurable Main PLL Bandwidth

The latest generation of the R&S SMA provides an extended SynEx RF assembly which improves the phase noise performance. It is achieved by reducing the PLL bandwidth of the main synthesizer (see "Main Pll Bandwidth") for a certain frequency range and offset.

  • To change the PLL bandwidth requires that your device is equipped with the enhanced phase noise and FM/ɸM modulator (R&S SMA-B22) and the extended synthesis assembly (SynEx) with part number 1413.1800.02. This assembly is installed in all instruments with serial number 112000 or higher.
    If your instrument is equipped with the previous version of the SynEx assembly, the normal bandwidth is provided and th "Main Pll Bandwidth" parameter is suppressed in the settings dialog.
  • The range of the adjustable PLL bandwidth is limited to 3 GHz. For frequencies higher than 3 GHz, the instrument uses the standard bandwidth automatically. The "Main Pll Bandwidth" selection switches to "Normal" mode.
  • The narrow bandwidth increases the frequency settling time to approximately 1 ms.
    Therefore, the narrow PLL bandwidth mode and the list mode exclude each other. To generate a signal in list mode, the standard PLL bandwidth setting is required.
SSB Phase Noise, comparison of the main synthesizer PLL bandwidth "Narrow" and "Normal"

The phase noise performance for frequency offsets higher than 100 kHz is improved in the narrow PLL bandwidth mode (blue measurement curve). In the offset range 10 kHz to 100 kHz, the pedestal phase noise deteriorates.

Interactions of the Frequency and Phase Modulation Modes and the Configurable Main PLL Bandwidth

If you generate a frequency- or phase-modulated RF signal, you can also set the narrow PLL bandwidth, provided you perform the modulation in "Low Noise" mode. Consequently, some operation modes exclude each other.

The table shows the operating modes which can be activated simultaneously (+) or which deactivate each other (-):

Main PLL

Bandwidth

FM Mode

"Normal"

FM Mode

"Low Noise"

ɸM Mode

"Low Noise"

ɸM Mode

"High Bandwidth"

ɸM Mode

"High Deviation"

Normal

+

+

+

+

+

Narrow

-

+

+

-

-

Behavior of the instrument when you change the frequency or modulation modes

When you enable a mode, while another excluding mode is already active, the R&S SMA automatically adjusts the parameters of this previously set mode. Consequently, the instrument always performs the signal generation with the current settings and adjusts the foregoing settings accordingly, if required.

A message in the info line indicates when there is a conflict between the modulation mode and the PLL bandwidth setting.

 


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