IQ&A Module

• SPEAR LLRF Controls

Module Description:

Each PEP RF station contains 2 (2-Cavity Station) or 3 (4-Cavity Station) IQ&A modules. Each module measures 8 RF signals. The module performs two main tasks:

1. The IQ portion (short for in-phase and quadrature) provides very accurate measurement of the real (in-phase) and imaginary (quadrature) component of the RF vector.  Digital down conversion is used to measure both I and Q, the values for each channel are shown on the lower right of the IQ&A module EPICS panel.  The amplitude and phase of the vector are calculated as SQRT(I^2 + Q^2) and ATAN2(I,Q) respectively.  These calculations are performed in the EPICS database.  Slow feedback loops such at the cavity tuners, drive power and station gap voltage set point use IQ data and operate at about a 1 Hz update rate.  The bandwidth of each IQ channel is programmable via a file written to the decimating digital filters (DDF)  for that channel.  Currently all channels are programmed for 5 Hz bandwidth with a 50 Hz update rate, except for the klystron output channel which is set up for 10 kHz bandwidth and 23 kHz update rate needed for the ripple loop.

2. The Amplitude portion of the module measures the envelope of each RF vector with  8 diode detector circuits.  The output of each detector drives a programmable comparator "trip" circuit.  Any RF signal  whose amplitude exceeds the comparator set point will trigger a fault signal on the VXI crate back plane trigger line representing a station RF fault.  The fault causes the klystron drive to be removed quickly and HVPS turned off.

The IQA module also performs these additional tasks:

1. The IQ&A module in VXI slot 7 (left most slot is slot 0) has the additional task of measuring the klystron output and sending the information to a DSP in the RF Processing (RFP) module. This information is used to form a feedback loop (the ripple loop) across the  RF modulator (in RFP module), drive amplifier and the klystron.  As the klystron high voltage is changed the phase length of the tube also changes.  The ripple loop keeps the phase length of the path constant.  It was originally intended for the ripple loop to also cancel klystron modulations caused by power line harmonics appearing on the klystron high voltage but this task is now handled by the integral compensation option of the direct RF feedback loop.  The klystron output is assigned to channel 1 of the IQ&A in slot 7.  The klystron output power data is sent to the RFP digitally over a dedicated serial link which operates at 23 kHz.  There are two versions of IQ&A modules.  The IQ&A in slot 7 MUST be a version II to properly send data to the DSP.  IQ&A modules in slot 9 or slot 11 can be either version I or version II.

2. The output of each diode detector may be directed to the lemo connector located on the IQ&A module front panel for viewing with a scope.  The user must select which channel gets connected by  typing in the channel number (1-8) in the "Signal Selection" box on the IQ&A module EPICS panel.  The amplitude of the selected channel is also digitized and displayed  in volts on the EPICS panel.  Note that if the station is in FM_Processing mode the channel number is constantly being switched by EPICS to measure the amplitude of each channel (the FM signals exceed the 5 Hz bandwidth of the IQ measurements).  The circuit is designed to display the true RF voltage when the signal is viewed with high impedance (NOT 50 ohms!).

3. The output of both the IQ portion and one envelope detector is constantly being digitized and stored in a long (512K) history buffer.  When a RF fault occurs, these buffers are frozen and can be read out into files for later viewing (see the Fault File Control and Analysis Panel for more detail). Since the IQ channels are low bandwidth, they are probably not very interesting to look at.  The amplitude channel, however, is wide band and can provide a good transient history of the RF fault (provided the "Signal Selection" was preset to the channel  we wanted to review).

Module Inputs and Outputs:

Inputs:

• LO Input - The top SMA connector is the input for the 471.2 MHz local oscillator.  The LO is generated by the clock module (slot 2).  The nominal LO power is +13dBm.
• RF inputs (1-8) - There are 8 SMA connectors for the 476 MHz RF signals which the IQ&A module will measure and interlock.  The maximum RF power is 1 watt (+30 dBm).

• Detected RF Amplitude - Lemo connector is multiplexed to the 8 envelope detectors.  Multiplexer control is "Signal Selection" on the IQ&A module EPICS panel.  Connect to high impedance scope channel (NOT 50 ohms).

Module Setup:

• Trip Circuit Speed Switch Settings

  The speed of the trip circuit is set by 8 DIP switches (SW2) on the main circuit board.  All channels are set for slow response (3 us) except for channel 2 of the IQ&A module in slot 7, this channel is protecting the klystron from reflected power and must be set to respond fast.  The status of the SW2 setting is read by EPICS and displayed as entry "FEST" on the IQ&A module EPICS panel.  Set SW2 for:

  Slot 7: FEST = 0xFDFF
  Slot 9 or 11: FEST = 0xFFFF

• DDF Coefficient File Names

  The bandwidth of the IQ detector is determined by a coefficient file.  Presently all channels except for the klystron output channel are set for the lowest possible bandwidth (maximum filtering).  The klystron output channel (IQ&A module in slot 7, channel #1) is setup with sufficient bandwidth to support the ripple loop.  File details:

  /tbl/iqaDdf_50Hz.rpt: Bandwidth = 5 Hz, Output Rate = 50 Hz
  for all channels except klystron output

  /tbl/iqaDdf_23KHz.rpt: Bandwidth = 10 kHz, Output Rate = 23 kHz
  for the klystron output channel (IQ&A slot 7 channel #1)

• Amplitude History Buffer Settings

  AHSZ - Amplitude history buffer size.
  This field is set by the EPICS database.  The normal value is 512 (k).

  AHFS - Amplitude history file name.
  This field is set by the EPICS database.  Something like "/dat/FAULTIqadAmp_dd", where the "d" represent integers that identify the IQ&A module and the fault file number.

• I/Q Data History Buffer Settings

  IQHS - I/Q  history buffer size.
  This field is set by the EPICS database.  The normal value is 512 (k).

  AHFS - I/Q history file name. 
  This field is set by the EPICS database.  Something like "/dat/iqaIqHistory.dat"

• IF Mode Setting

  On the lower right of the IQ&A EPICS panel there is a button for selecting the mode of the IQ processing.  The IF mode should always be set to 8 chan(nel).  The 1 chan(nel) mode is for test purposes and not supported by the EPICS database.

• Interrupt Mask (IMSK)

  Determines what types of interrupts are passed to the VXI crate processor.  The proper value for this field is 0xFFFFFFF8 (248).

• Amplitude Threshold Set Points

  These setpoints are set during IQ&A module calibration. The voltage steeping for each channel is determined in the EPICS database (Power/Phases panel then on to the Power Trip Limit panel).  If the original steeping is unknown, the correct value can be restored by placing the cursor in the associated trip limit entry field on the Power Trip Limit panel and entering a carriage return.  This will force the EPICS database to recalculate the trip voltage and restore the correct value.

EPICS Panel Details:

• Software Reset

  Resets the module and reloads the DDF files and comparator trip points.  The module will become unsynchronized with the RF reference after a Software Reset.  It will be necessary to force a "resynch" on the clock module panel to restore the proper phase relationships.  This is most important if the IQ&A module is in slot 7 since the klystron phase is measured in channel 1 and is directly tied to the phase margins of the direct RF feedback loop.

• Threshold Fault Reset Button

  Clears the latched amplitude fault.  This button is pushed by a station Reset and should not need to be activated from the IQ&A panel (except during testing in the lab).

• Stop Trigger Reset Button

  Clears a latched stop trigger.  This trigger stops the internal ADC from writing the IQ and Amplitude information to the long memories, holding the data until it can be read out to a file.  This button is also pushed by a station Reset and should not need to be activated from the IQ&A panel (except during testing in the lab).

EPICS Database:

Contact:  Stephanie Allison
Last Modified:  Thursday, Jul 11, 2008