9.318 Level-Control

The Level-Control logical device provides a gain setting linearizer for TWT amplifiers as well as a level control loop mode. The figure below explains how the Level-Control device works. The RF devices controlled by the Level-Control are shown in cadet blue, the level setting appears in red and the level measurement is marked blue.

When the operator sets an EIRP value, the Level-Control device first computes the amplifier output power which is required for this EIRP. The conversion is done according to a user supplied calibration table, it takes the actual signal frequency into account as well as the signal path.

The requested amplifier output power then is translated into the requested drive level. For TWT amplifiers the Pin/Pout compression curve is used for this. The Level-Control device is able to measure this characteristic by itself, however, the measurement points of this curve may be entered manually, too.

Before setting the drive value at the device used for this (usually a modem, upconverter or the gain setting parameter of the amplifier itself), the Level-Control adds a parameter called "gain drift" to the setting.

After the signal passed the amplifier, the Level-Control reads the measured output power. With amplifiers providing a sufficiently precise output power reading, this value can be used, with other amplifiers a separate power meter will be required. The measured output power gets translated to EIRP by means of the same calibration table mentioned above.

In close loop mode, the Level-Control device takes the difference between the requested output power and the measured power on a regular basis, sets the gain drift parameter to this value and adjusts the drive level accordingly. The closed loop level adjustment, even if enabled by the operator, automatically is suppressed in one of the following cases:

• The requested power is below a user specified threshold, the meter readings are expected to be noisy or even invalid in this range.
• The requested power is above a user specified threshold, near the amplifier saturation the closed loop adjustment could overdrive the amplifier.
• The measured power differs from the requested value less than a user specified threshold. This prevents the loop from oscillating with 0.1 dB around the programmed value.
• The measured power differs from the requested value more than 3 dB. It is likely that something is going completely wrong in this case (output power is completely missing, e.g.). It is not desirable, that the drive level rises to the maximum in such a case. Instead, the Level-Control shows a fault and freezes the drive level.

Device window pages

The following table shows which device window pages are available with this individual device type. Tool-bar functions not mentioned here are described at the general description of device windows .

• --- This page shows the operational parameters, meter readings and status displays.
• --- This page shows the parameter required for the calibration of the amplifier's Pin/Pout characteristic.
• --- The faults page.
  • High Drive Limit : The output power reaches the high limit
  • Low Drive Limit : The output power reaches the low limit
  • Gain Deviation : Warns that there is not the expected output power, maybe because of a fault in the transmit chain.
• --- The device info page.
• --- The maintenance page. The configuration parameters described below are set at this page.

Configuration parameters

At the maintenance page of the device window there are a couple of configuration parameters which must be set to interconnect the Level-Control device to the devices it monitors and controls.

Calibration parameters

At the "Calibration" page of the device window there are a number of parameters which mark out the basic performance of the device. They are explained below.

• parameter --- description
• gain drift --- The gain drift is a corrective offset which the Level-Control device adds to each drive setting in order to compensate the long term gain drift with respect to the Pin/Pout calibration. In closed loop mode the gain drift is calculated and updated from the target/actual comparison. You may overwrite this value by entering another value, this value will work as a new starting point for the closed loop control.
• calibration state --- During the Pin/Pout calibration process this field shows what's going on. You start the calibration process by entering the text "start calibration" here (mall letters in lower case, no additional spaces).
• min output power --- Enter here the lowest output power reading which is not falsified by noise. Do not set this value too low, the deviance produced by the level control due to noisy power readings mostly causes more problems than a nonlinearity of the drive setting attenuator.Closed loop mode is suppressed for power settings below this limit. During the Pin/Pout calibration power readings below this limit are discarded.
• max output power --- Here the true maximum output power of the amplifier must be entered (more precisely, the power reading achieved at the maximum amplifier output). The Level-Control device may modify this value if it detects a lower saturation point during the Pin/Pout calibration procedure.
• delay --- This parameter determines the delay between a drive setting and the next measurement sample taken for the target/actual comparison. The delay also applies for the Pin/Pout calibration.
• hysteresis --- In closed loop mode, the Level-Control device does not send a new drive value if the target/actual values differ by this value or less. In calibration mode the hysteresis value is used for the saturation point detection.
• compression curve --- The Pin/Pout transfer characteristic as a list of measurement samples. The format is "Pout 1 ,Pin 1 ,Pout 2 ,Pin 2 , ... Pout n ,Pin n ". The samples need not to be equally spaced, however the Pout values must be in ascending order. In most cases you will not enter this series of measurement manually. The Level-Control device is capable to perform this measurement by itself.

Leveling And Pin/Pout Calibration

To make the Level-Control device work properly, the level ranges must be adjusted and the Pin/Pout calibration curve must contain values which exactly represent the amplifier's characteristic. When defining the levels / level ranges the following should be considered.

• All devices in the transmit chain should be operated at their nominal input level if possible, the maximum input level must not be exceeded.
• Usually the invariant gain values are set in a way, that the amplifier may be overdriven by 2 .. 3 dB with the maximum drive setting. This ensures that the amplifier reaches it's maximum power, even if the overall gain of the transmit chain degrades.

Below the steps necessary to adjust the Level-Control device for a certain setup are described.

1. First of all, the message identifiers for all devices communicating with the Level-Control device must be set. See the paragraph "Configuration Parameters" above for details.
2. To see reasonable EIRP readings, a EIRP calibration table with at least rough values must be present. Edit the calibration table with a text editor program of your choice, the paragraph "EIRP Calibration Table" below explains the format of this file.
3. Now the invariant gain settings in the transmit chain must be set to values which meet the requirements mentioned above. If you set these values as configuration parameters at the maintenance page of this device, you can rely on the values to be set with each RF-ON you command.
4. Enter the min/max amplifier output power to the adequate fields at the "Calibration" page. The maximum value should be set to the true saturation output power, the minimum value to the lowest power reading you will rely on.
5. On the calibration page, set the parameter "delay" to 10 seconds, the "hysteresis" parameter to 0.2 dB.
6. Now make sure, that the amplifier is switched on and warmed up and the output is routed to a dummy load which is capable to admit the full amplifier power. Enter the text start calibration to the field "calibration state" and hit RETURN. The command must be entered exactly as shown, this is a kind of safeguard against an unintended start of the calibration procedure.

The Level-Control device now starts to measure the Pin/Pout characteristic of the amplifier. It sets the lowest possible drive value, waits 10 seconds (the value set with the "delay" parameter) an then takes a reading of the output power. If the power reading is below the minimum value, this measurement point is discarded. If considered to be OK, the measurement point is added to the power transfer characteristic. The Level-Control device increases the drive by 1 dB and starts the next measurement cycle. The measurement procedure stops if

• The maximum amplifier output power is reached.
• The maximum drive value is reached.
• The amplifier output power did not increase by at lest 0.2 dB (the hysteresis value) with respect to reading at the previous drive setting which was 1dB below. The Level-Control device assumes that the saturation point is just to be reached (This condition only applies for the upper half of the drive setting range. At the low edge noisy power readings unintentionally could trigger this.)

The ongoing measurement procedure can be watched with the messages displayed in the "calibration state" and "state" fields. At any time the measurement may be aborted by commanding a RF-OFF at the device's main settings page.

Unless aborted by the operator, the LevelControl device replaces the stored Pin/Pout curve by the recently measured one when it finishes. Additionally the "gain drift" value is cleared and the maximum amplifier output power value is adjusted to the saturation value if applicable.

EIRP Calibration Table

The Level-Control device requires an EIRP calibration table which defines the level difference between the measured output power reading and the EIRP over the frequency. This in most cases is the antenna gain reduced by the waveguide loss between the measurement point and the antenna.

The calibration table is a plain text file, called "XXXX.eirpCal" where XXXX is the name of the Level-Control device. The file must reside in the M&C software's home directory. Below an example for such a file is shown:

    #
    # /home/mnc/EIRP-1.eirpCal
    #
    #           TRANSMIT 1    TRANSMIT-2   
    #           X      Y      X      Y     
       14000.0  20.2   20.2   20.2   20.2   
       14100.0  20.27  20.27  20.27  20.27  
       14200.0  20.31  20.31  20.31  20.31  
       14300.0  20.44  20.44  20.44  20.44  
       14400.0  20.5   20.5   20.5   20.5   
       14500.0  20.5   20.5   20.5   20.5   

The file is formatted as a table of 5 columns and an arbitrary number of lines. Empty lines and lines starting with a "#" are ignored. Columns are separated by spaces.

Limit Checking

At the primary device page, two pairs of min / max threshold values may be defined. These values apply to the measured EIRP, they are checked while the Level-Set device is set RF-ON. If the measured EIRP exceeds the limit values a fault is generated. The fault gets cleared if the EIRP value returns inside the limit window of id the EIRP is switched off. If the Level-Set device shall not do any limit checking, set both threshold values to 0.0.

The minEirp/maxEirp settings are included into the 'gauge' parameter which controls the appearance of a Gauge screen element which follows this parameter. There is a second parameter of the same format called 'gauge2' which includes the minEirp2/maxEirp2 limits instead.

N:1 Redundant Configuration

For a Level-Control device operating a redundant transmit chain in a N:1 redundant configuration, the Pin/Pout calibration must be corrected depending on which chain actually is replaced.

For this case the Level-Control device provides a mechanism to monitor the actual "location" setting of the Protection-Switch-N-To-1 device managing the redundancy control an to apply individual offsets to the displayed EIRP and initial drive power values for each chain.

To use this feature, fill out the configuration settings redPosId, redEirpDeltas, and redDriveDeltas as described below:

• parameter --- description
• redPosId --- This parameter must be set to the parameter ID of the "location" of the redundancy control device. The Level-Control device is programmed to monitor a Protection-Switch-N-To-1 device, however other implementations of redundancy control devices may work as well if the publish the selected chain with a parameter containing something like "CHAIN-01" or "CHAIN-12".
• redEirpDeltas --- This parameter must be set to a comma separated list of deltas to be included into the EIRP calculation for each individual chain. The first value in the list applies to CHAIN 01, the last value applies to the highest numbered chain. If the location parameter contains a value which cannot be parsed as a chain number (e.g. "NONE") or if a chain number is addressed which is not contained in the list, a value of 0.0 is assumed.The delta value gets added to the amplifier output power (in addition to the regular calibration value) to get the displayed EIRP value. This means, if for a particular chain there is an additional attenuation in the signal path, this attenuation has to be entered as a negative value.
• redDriveDeltas --- This parameter must be set to a comma separated list of deltas to be included into the drive power calculation for each individual chain. The first value in the list applies to CHAIN-01, the last value applies to the highest numbered chain. If the location parameter contains a value which cannot be parsed as a chain number (e.g. "NONE") or if a chain number is addressed which is not contained in the list, a value of 0.0 is assumed.The delta value gets added to the drive / input power (in addition to the regular calibration value) when the Level-Control sets the power value. This means, if for a particular chain there is an additional attenuation in the signal path, this attenuation has to be entered here as a positive value.

Variables defined by this device driver