RF Tube model

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RF Tube modeller is an aid to perform poweramplifier design in a more efficient way. Based on the by manufacturer supplied graphical data (constant current graphs) a number of key parameters can be calculated for different operating conditions. 

Overview

RF Tube Modeller provides the following features

  • Easy way of input the constant current graphs for anode and screen (G2) current as provided by the manufacturer
  • Graphs will show the constructed constant current data as you model them
  • Operating conditions can be set (a.o. idling current, anode working voltage and load line) and resulting operating parameters provided (a.o. all currents and poweroutput)
  • Graph of instantaneous anode and screen current for one cycle of grid-kathode RF voltage
  • Calculated will be 
    • DC and RF currents for anode and screen 
    • Power output, anode dissipated power and efficiency
    • RF load for optimum performance

Tube RF modeller was developed to provide a better insight in the behavior of RF tubes under operating conditions. When changing drive voltage (grid - kathode) immediate the effect can be seen on the resulting currents and the delivered power. Proposed used is during the design and analysis phase of poweramplifier circuits.

Main screen

Main screen will be displayed when RF Tube modeller is started and will reflect the last state as saved on previous exit.

rfTubeModel main 600 401

 

Menu items:

  • About/Help button : Provide information on the current version and contact details
  • File button: Opens another tube data file, Save as and exit function. Tube data files are saved as plain ascii XML files. 
  • Edit: Create/edit a tube data file

 

The main screen is divided into 3 distinct area's:

1. Constant current graph

In this graph the tube data as used by simulation is given

rfTubeMode graphs 534 378

Shown are

  • Every Ia (solid curves) and Ig2 (dotted curves) series
  • Iq, solid horizontal line
  • Load line, based on the values of UaMin, Ugk@UaMin, Ua. UaMin and Ugk@UaMin are normally set at the point where Ig2 current is starting to flow. 
  • Input range of Ugk peak-peak. (shaded area)

2.Resulting Anode and screen currents for one cycle of the RF input

In this tabbed form the resulting anode and screen currents are shown. The curves are calculated via interpolation of the provided constant current series. Normally anode current will be in amps (A) and screen current in milliamps (mA). Clearly the conduction angle (class A, AB, C, etc) can be seen as well as the peak value.

rftubemodeller ia 357 425
rftubemodeller ig2 357 425

3. Input parameters and calculated values

 Input parameters:

  • Iq or Ugk; Set the idling current Iq (Ugk will be calculated accordingly) or Grid-Kathode DC voltage Ugk (Iq will be calculated)
  • Ugk peak-peak; Peak-Peak value of the input sinewave signal
  • Auto Ugkpp; When checked, Ugkpp will be set to fully utilize the input swing as set by UaMin/Ugk@UaMin and Ua. Visually shown is a grayed area on the constant current chart
  • Ua; DC anode voltage
  • Ua min and Ugk@Ua min; Used to construct the load line. Ua min and corresponding Ugk will influence the resulting screen current under full load conditions

rftubemodeller parameters 600 86

based on the constant current graphs and input parameters the instantaneous anode and screen currents are calculated. To get best results as many as possible datapoints and current series should be provided (see edit Tube data). The application will use interpolation to create the anode and screen currents. These timedomain signals will be translated via Fast Fourier Transform to the frequency domain and the separate frequency components are calculated (DC and fundamental components are used at this moment). 

Output parameters calculated:

  • Ia/Ig2 peak; Peak values of anode and screen currents
  • Ia/Ig2 fundamental; Peak values of the currents at the fundamental frequency
  • Idc; DC component of the anode and screen current (the value shown on the meters...)
  • Pout; Resulting output power: ( Ia fund (pk) * Ua (peak) swing ) / 2   .Peak Ua swing is function of Ugk exitation and calculated according the load line and Ugkpp voltage.
  • Panode diss; Power dissipated by the tube anode (difference between input power (Idc*Ua) and output power
  • Eff (%); Efficiency achieved
  • Rload (fund); Required load impedance at fundamental frequency to get the tube working according the load line. This is the impedance you need to transform to the 50/75 Ohm output impedance by the anode matching network (PI or PI/L) 

Edit Tube data

This form will provide means to model the tube. For every Ia ad Iscreen current serie given by the manufacture datasheet an as much as possible equal graph should be constructed. This is the most tedious work...to get the best results as many as possible series and datapoints should be input. Setting the chart format equal to the chart axis as provided by the manufacturers drawing is probably the best way to get the fastest results.

rfTubeModel Model Ia 600 318
 

 Additional it is possible to import this data from graphics as supplied by the manufacture. Load the image (a.o. .jpeg, tif, png and .bmp formats are supported) which the curves given, calibrate the image by clicking some defined points. After calibration the tabular tube data (Ugk/Ua) will be automatically filled with the corresponding point on the image. A big time saver for import !

rfTubeModel loadImage 600 397
 

Deployment

To deploy RF Tube Modeller, please unzip  (see the  downloads area below) and store all files in the same directory. RF Tube modeller consists of 2 files; the actual executable (TubeModel.exe) and a DLL (libfftw3f-3.dll) which provides FFT algorithmes used for calculating the RF currents. (FFTW is an open source project). When initial started, data representing a 4cx1500 will be created automatically and stored as "noname.xml". 

Development

RF Tube modeller is developed using Microsoft Visual Studio 2008 Express as a Window Forms .NET (v4.0) application using c#. 
Computer requirements are moderate, a common personal computer with SVGA screen and running XP (SP3!) or Vista is sufficient, however it will also run on W7. please note that .Net 4.0 should be installed.

Copyright © 2011 Carel Mobach, PC5M

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