The PLL tab allows convenient setup of a two independent phase-locked
loop for high-speed tracking of frequency modulated signals. This tab is
only available when the HF2-PLL Dual Phase-locked Loop option is
installed on the HF2 Instrument (see Information section in the Device
Demodulators that are used by an active PLL are set to read-only values
on the Lock-in tab.
The PLL tab offers a convenient way to set up a phase-locked loop. In
this way the frequency of an external signal can be mapped onto one of
the instrument’s internal oscillators. An advisor functionality based on
mathematical models helps the user finding and optimizing the PID
parameters and quickly optimizing the servo bandwidth for the
application. Whenever the tab is closed or an additional one of the same
type is needed, clicking the following icon will open a new instance of
Table 1: App icon and short description
Features all control, analysis, and simulation capabilities of the phase-locked loops.
The PLL tab (see Figure 1) is
divided into two side-tabs corresponding to the two PLL units. It
contains a settings section on the left and an advisor section with
graphical display on the right.
In a typical work flow to set up a PLL one would first define the center
frequency, frequency range, and the phase setpoint in the left section.
If the frequency is not know beforehand, it can often be measured using
the Sweeper or Spectrum tool. Then one would set a target bandwidth in
the Advisor sub-tab and subsequently click on the
button, and then enable the PLL. If the Error field now displays very
small values, the phase lock has been successful. One can now iterate
the process and e.g. play with the target bandwidth in the PLL Advisor
to calculate a new set of feedback parameters. Displaying the oscillator
frequency in the Plotter along with a Histogram and Math function (e.g.
standard deviation) can help to characterize residual phase deviations
and further improve lock performance by manual tweaking.
The frequency range in the PLL Settings section should exceed the target
bandwidth by at least a factor of 5 to 10.
PLL 1 uses demodulator 7 as phase detector, and PLL 2 uses demodulator
8. The Input selection determines which signal is connected to the
corresponding demodulator. This setting is the same as the Input Signal
setting in the Lock-in tab.
Center frequency of the PLL oscillator. The PLL frequency shift is relative to this center frequency.
Auto Center Frequency
ON / OFF
The PLL Center Frequency is determined automatically. In this mode, the instrument sweeps the operating range until it finds a suitable frequency. Note: Auto Center Frequency works only for open loop systems. Closed loop systems require manual mode.
Set the frequency range of the PLL controller output relative to the center frequency
Set the harmonic used in the phase detector. A setting of 2 means the PLL generates a sub-harmonic of the external reference.
Filter time constant of the demodulator used as the phase detector.
Auto TC Enable
ON / OFF
When On, the PLL is running at full bandwidth. Use manual mode (off) for low-noise performance.
Filter BW (Hz)
Filter bandwidth of the demodulator used as the phase detector.
Filter order of the demodulator used as the phase detector.
Phase set point in degrees (i.e. PID setpoint). Controls the phase difference between the input signal and the generated signal.
Automated adjustment of PID coefficients
ON / OFF
If turned on together with Auto TC Enable and Auto Center Frequency, the PLL is in ExtRef mode
PID proportional gain P
PID integral gain I
PID derivative gain D
Current sampling rate of the PLL control loop.
Note: The numerical precision of the controller is influenced by the loop filter sampling rate. If the target bandwidth is below 1 kHz is starts to make sense to adjust this rate to a value of about 100 to 500 times the target bandwidth. If the rate is set too high for low-bandwidth applications, integration inaccuracies can lead to nonlinear behavior.
Current phase error of the PLL (Set Point - PID Input).
PLL lock LED
Indicates when the PLL is locked.
The PLL error is sampled at 5 Sa/s and its RMS value is calculated. If the result is smaller than 5 degrees the loop is considered locked.
Freq Shift (Hz)
Current frequency shift of the PLL (Oscillator Freq - Center Freq).
Copy the current PLL settings to the PLL Advisor.
Table 3: PLL tab: Advisor sub-tab
Calculate PID coefficients based on application mode and given settings.
Only PID coefficients specified with the advise mode are optimized. The Advise mode can be used incremental, means current coefficients are used as starting point for the optimization unless other model parameters are changed in-between.
The percentage of design algorithm already done when the Advisor is in progress.
Target BW (Hz)
Target bandwidth for the PLL closed loop feedback system which is used for the advising of the PID parameters. This bandwidth defines the trade-off between PLL speed and phase noise.
Select the PID coefficients that are optimized. The other PID coefficients remain unchanged but are used during optimization. This enables holding selected coefficients at a fixed value while optimizing the rest.
The advise time will increase significantly with the number of parameters to be optimized.
Only optimize the proportional gain.
Only optimize the integral gain.
Only optimize the proportional and the integral gain.
Optimize the proportional, integral, and derivative gains.
The model to use for the parameter calculation.
1 to 1023
Multiplier of the for the reference frequency of the modelled demodulator.
Defines the low-pass filter time constant of the selected demodulator input.
ON / OFF
Adjusts the demodulator bandwidth to fit best to the specified target bandwidth of the full system. If disabled, a demodulator bandwidth too close to the target bandwidth may cause overshoot and instability.
In special cases the demodulator bandwidth can also be selected smaller than the target bandwidth.
Defines the low-pass filter characteristic of the selected demodulator input.
Selects the filter roll off between 6 dB/oct and 48 dB/oct of the modelled demodulator.
1st order filter 6 dB/oct
2nd order filter 12 dB/oct
3rd order filter 18 dB/oct
4th order filter 24 dB/oct
5th order filter 30 dB/oct
6th order filter 36 dB/oct
7th order filter 42 dB/oct
8th order filter 48 dB/oct
Proportional gain P coefficient used for calculation of the response of the PID model. The parameter can be optimized with PID advise or changed manually. The parameter only gets active on the PID after pressing the button To PLL.
Integral gain I coefficient used for calculation of the response of the PID model. The parameter can be optimized with PID advise or changed manually. The parameter only gets active on the PID after pressing the button To PLL.
Derivative gain D coefficient used for calculation of the response of the PID model. The parameter can be optimized with PID advise or changed manually. The parameter only gets active on the PID after pressing the button To PLL.
RT load dependent
PID sampling rate used for simulation.
The advisor will update the rate to match with the specified target bandwidth. A sampling rate close to the target bandwidth and excessive higher bandwidth will results in a simulation mismatch.
Simulated bandwidth of the full close loop PLL with the current PID settings. This value should be larger than the target bandwidth.
Target BW LED
Green indicates that the target bandwidth can be achieved. For very high PLL bandwidth the target bandwidth might be only achieved using marginal stable PID settings.
Simulated phase margin of the PID with the current settings. The phase margin should be greater than 45 deg for stable conditions. An Infinite value is shown if no unity gain crossing is available to determine a phase margin.
Green indicates that the phase margin is fulfilled and the PID system should be stable.
Copy the PLL Advisor settings to the PLL.
Table 4: PLL tab: Display sub-tab
ON / OFF
Enables manual selection of display and advice properties. If disabled the display and advise settings are automatically with optimized default values.
Select the display mode used for rendering the system frequency or time response.
Display the Bode magnitude plot.
Display the Bode phase plot.
Display the step response plot.
Start frequency for Bode plot display. For disabled advanced mode the start value is automatically derived from the system properties and the input field is read-only.
Stop frequency for Bode plot display. For disabled advanced mode the stop value is automatically derived from the system properties and the input field is read-only.
Start time for step response display. For disabled advanced mode the start value is zero and the field is read-only.
Stop time for step response display. For disabled advanced mode the stop value is automatically derived from the system properties and the input field is read-only.
Start point for the plant response simulation for open or closed loops. In closed loop configuration all elements from output to input will be included as feedback elements.
Start point is at the demodulator input.
Start point is at the setpoint in front of the PID.
Start point is at PID output.
Start point is at the instrument output.
Start point is at the DUT output and instrument input.
End point for the plant response simulation for open or closed loops. In closed loop configuration all elements from output to input will be included as feedback elements.
End point is at PID output.
End point is at the instrument output.
End point is at the DUT output and instrument input.
End point is at the demodulator input.
End point is in front of the PID error calculation (Setpoint-System Output).
ON / OFF
Switches the display of the system response between closed or open loop.
ON / OFF
Switch between display of gain parameters (I, D) and time constants (Ti, Td) for the integral and differential parts. The following relations hold: I=P/Ti, D=P*Td.