IQ Monitor System

• Source code
• RF oource overview
• Control system overview

The Four Quadrant IQ Phase Detector provides in-phase and out-of-phase signals of the cavity field with respect to the RF Frequency source signal. These signals are not only used for phase-locking the RF Frequency source to the RFQ resonance but are a valuable diagnostic. The Blinky-Lite^TM control platform supports vector devices so this diagnostic can be viewed remotely as shown in Figure 1. The Red Pitaya Stemlab 125-14 platform can be used as an inexpensive headless vector data acquisition system (i.e. oscilloscope) instead of using an expensive oscilloscope with display.

To use the Red Pitaya Stemlab 125-14 as a headless vector data acquisition system, the SCPI server on the Red Pitaya Stemlab 125-14 must be enabled. A Python script that continuously acquires vector data from the Red Pitaya was written. Since the Red Pitaya Stemlab 125-14 is a Linux computer, a Blinky-Lite^TM tray can be directly installed on the Red Pitaya Stemlab 125-14. The tray code is written in the Node-RED programming environment as shown in Figure 2. The tray flow is a modified version of the standard Blinky-Lite^TM tray for serial communications. The modified tray flow calls the Python script using an Node-RED Exec node. The script continuously pipes the binary vector data to the stdout which the Exec node pickups and is decoded in the subsequent function node “Read Traces”. The data is then packaged in vector format and sent to the Mirrotron Application Server. There is a Node-RED Dashboard node down stream of the “Read “Traces” function node that can be used to plot the vectors locally for debugging purposes.

The IQ monitor requires a number of parameters to be set for correct operation.

• The loopDelay determines the interval that the Python Script returns vectors.
• The monitor is triggered using the DIO0_P pin on the Red Pitaya. The source of the trigger comes from Channel 1 of the Timing system.
• The preTrigger defines how much data to acquire before the trigger start.
• The decimation power determines the sample rate of the acquistion.
  • A decimation power of 8 gives a sample rate of (125 / 2⁸) MS/sec = 488kSamples/sec
• The recordLengthPower determines number of points in the record.
  • A record length of 10 gives 1024 points.

The IQ monitor provides a sampled output taken from the middle of the RF pulse of the magnitude and phase. The sampled output is averaged over 10% of the RF pulse.

Figure 1. IQ Monitor Display

Figure 2. IQ Monitor System Blinky-Lite^TMTray Node-RED flow

Figure 3. IQ Monitor parameters