Page 1 of 6 To keep the hardware of the utracer as simple as possible, the complete operation of the utracer is performed under software control. The program which controls the utracer is called the Graphical User Interface (GUI). The GUI is a standalone program that can be downloaded for free. It communicates with the utracer via a serial data link. If your computer is somewhat older it probably has a good old serial COM port. In that case you won t have to do anything extra. In case you own a more modern computer without a serial port, you will need a USB to Serial Adapter. When you buy one, be careful! There are a number of chip-sets that USB-Serial adapter manufacturers use. Make sure that you buy one which uses an FTDI chip set; that one will certainly work flawlessly. I had a bad experience with an adapter which used the Prolific chip-set. Under Windows 7 it simply wouldn t work. Browsing the internet it appeared to be a general problem for this chipset in combination with Windows 7, so fortunately nothing related to the GUI. Figure 1 Main form of the utracer 3.5 GUI. The GUI was written in Visual Basic 6. Installing it should be very straightforward, and on the computers I have tested it on usually takes less than a minute. Before you start building the utracer it is a good idea to install the GUI on your computer first and check if it is operating properly. The download page gives exact details, and even a video, of how the GUI can be installed and tested. On this page the main features of the GUI will be discussed. Before (or instead of) reading this page, you might also want to have a look at a set of videos which show you around the GUI. The videos were recorded using the version 3.4 GUI but, apart from a few details, most of the main features are identical to version 3.5. Measurement Setup Graphical Output Transconductance Communication Click on one of the images for a guided tour through the new GUI! Figure 1 shows the main form (a Visual Basic term) of the GUI. There are three areas on this form. The upper-left area is for the measurement set-up. Things like the type of measurement, the voltage ranges and details about range selection, averaging and current
Page 2 of 6 compliance can be set here. The whole right part of the form is for the graphical output of the measurement data. The small section at the bottom-left is reserved for all the other stuff such as: storing the measurement data / plot, modifying the calibration values and debugging communications. In the remainder of this page the different sections will be discussed. Measurement Setup The set-up of a new measurement is very simple and intuitive. The best thing is to go through the measurement set-up section from top to bottom. Most settings have been pre-programmed or are automatic so that usually very little adjustments are necessary. The most important thing to specify is the type of measurement. This is done by selecting one of the measurement types from the drop-down menu on the top of the form (Fig. 2). At this moment in total 6 measurement types have been defined. The most important ones are the anode (and screen) current as a function of grid bias I(Vgs) step Va, Vs, Vf constant and the currents as a function of the anode voltage I(Va) step Vgs, Vs, Vf constant. The next thing to set are the measurement voltages. Selection of a new measurement type will automatically copy a set of default values into the voltage selection text boxes. In most cases these default values will already produce a nice set of curves, but of course the values may be changed as required. The following minimum and maximum values need to be observed: the anode and screen voltages have to be within 2 to 300 V, the grid bias between -50 and 0V, while the filament voltage must be between 0 and the power supply voltage (19.5 V). In most cases this is enough to set-up the GUI and can a measurement be started. This is done by pressing the Heater On! button. The heater will now be slowly switched on with increments of 10% of the nominal heater voltage a second. When the filament is already hot, or when you don t care about a slow-startup of the heater, the same button (which now displays Heating ) can be pressed again so that the full nominal heater voltage is applied to the filament. It is up to the user of the GUI to decide when the tube has stabilized enough to start a measurement. Pressing the same button again (which now displays the text Do Measurement ) will start the measurement. Both the measurement, as well as the heating of the filament, can be interrupted at any time by pressing the abort button. Since the scales of the axis in the output section are set by default to Automatic Scaling, the measurement will always result in a nice graph. Figure 2 Measurement Setup section of the GUI. In the drop-down boxes captioned Range:, the measurement range can be selected [MORE]. Normally, it is best just to leave the ranges set to Automatic. There are however two situations in which you might want to set the range manually. The first situation occurs when you want to make a very nice and smooth graph of a set of curves, and there is a change of measurement range in the middle of a curve. Sometimes this can result in a small bump or dip as a result of small gain differences, although the effect is usually very small (the PGA113 s are really very good). In that case a fixed gain can give a better result. The other situation occurs when there is no screen current to be measured (as in the case of a diode or triode), or when the screen current is not of interest. Tip! It should be realized that even when the screen current is not plotted, the utracer still records both the anode as well as the screen current. This implies that when the ranging is set to Automatic for the screen current, the utracer wants to measure it as accurately as possible. For a low current (open terminal) this means maximum gain, but also maximum number of averages and hence, since the anode and screen currents are measured simultaneously, a longer measurement time. By simply setting the screen current range to 200 ma, the shortest measurement time is obtained. If you are in a hurry, and don t care too much about a bit of noise in the Graph, but just want to get a quick impression of the quality of a tube, the averaging can be switched of by selecting the proper option in the Average: drop down box. Alternatively a higher number of averaging can be specified to get an even better quality graph [MORE]. Finally there is the menu where the user can select the current compliance. As mentioned before, there are two over-current protection mechanisms in the utracer 3 [MORE]. In the first place there is a very fast hardware protection which simply limits the maximum current to 250 ma. Then there is a second, programmable protection mechanism the current compliance - which, since it is software controlled, is a bit slower (typically 10-20 us). This second mechanism protects the high voltage switch against excessive dissipation during a short-circuit.
Page 3 of 6 The user can set the current compliance to a number of values between 0 and 200 ma. The compliance applies to the anode current as well as the screen current. It is also possible to switch the compliance protection off! I would not recommend using this setting unless you are absolutely sure that the tube is ok, because a flash-over in that case will surely destroy the high-voltage output transistor. The measurement range is extended to 250 ma when the compliance is switched off. Finally there is a last box where a delay can be specified which is inserted in between the application of a new bias condition and the measurement pulse. This delay is used e.g. when the anode and screen currents as a function of the heater bias are measured. The delay is also very useful when magic eyes are tested. In this case the anode of the magic eye is directly connected to the high voltage reservoir capacitor (see examples). Graphical Output Section In the current version of the GUI, the graphical output section has been thoroughly revised. Also the structure of the program behind it has been completely renewed and especially restructured. Many new features have been included such as a second independent Y-axis, extraction / plotting of the transconductance and output resistance, and the implementation of a smart cursor. Figure 3 Graphical output section of the GUI. Just beneath the plot are the controls that determine what is plotted and how it is plotted. The controls have been organized into three rows. The top row for the X-Axis, the second row for the first (left) Y1-axis and the bottom row for the second (right) Y2-axis. The X-axis variable is always identical to the running variable specified in the Measurement Setup section. For both Y-axes the user has the possibility to choose from 7 variables (Fig. 3). The first two are the anode and the screen currents. The next three parameters are related to the plotting of the transconductance and I will come back to those later in this section. Finally the last two variables retrieve the stored data. Just like in the version 2 GUI, the anode and screen currents can be stored for comparison with a new measurement by pressing the Store command button. This feature can be used to compare or match tubes. There are a number of options to scale the axis, and the options differ slightly for the different axis. By default the scaling of the X-axis is set to Track. This means that the minimum and maximum values along the X-axis are copied from the start and stop values from the running variable in the measurement setup section. A small thing needs explaining here. Because of the way how the utracer is constructed, the minimum anode and screen voltages cannot be lower than 2 V. Having 2 V as a lower X-axis value looks a bit silly, that is why the software, when it detects that a minimum X-axis value lower than 2 V has been entered, automatically sets it to 0 V. Alternatively the user can set the minimum and maximum values of the X-axis manually. Both the Y-axes are by default set to Automatic. A special algorithm ensures that there are always sensible and round numbers along the axes and that the number of marker ticks is adjusted accordingly. Next to the possibility to set the axes manually, there is also a Track option. When this option is selected, the maximum axes values are copied from the selected range in the measurement setup section. The second Y-axis has a fourth option Copy. In this case the minimum and maximum Y2-Axis values are always copied from the Y1-Axis, even if the scaling for that axis is set to Automatic. All the other plot settings and options are pretty straightforward; the only item left to discuss is a check box captioned Voltage Correction. As a result of the voltage drop over the current sense resistor and the discharging of the buffer capacitors, the actual anode and screen voltages on the tube will be somewhat lower than the voltage to which the anode and screen buffer capacitors have been charged. Fortunately these (most of the times very small) voltage drops are known and can be corrected for. This is why sometimes in e.g. an output characteristic the plotted anode voltage at high current levels can be a bit lower than the set anode voltage. With the check box in this section the user has the possibility to switch the anode voltage correction off. For the convenience of the user the possibility to plot a load line or a maximum dissipation hyperbole have been added. They can be added to the plot by entering a non-zero value into one of the respective boxes. The lines can be removed by setting the value to zero. At the bottom of the section there is also the possibility to enter a plot title. The title can be placed in the graph, or slightly above it. There is also a Save Plot option which directly stores the graph with the file name title.bmp. Measuring the transconductance and output resistance The transconductance of a tube is the variation in anode current as a result of a small variation in gate voltage at a certain bias point, which is nothing more than a combination of DC grid and anode voltages. In other words, it is the slope of the anode current versus grid voltage at a specified bias point. In mathematical terms: the transconductance is the first derivative of the anode current with respect to the grid voltage.
Page 4 of 6 This simplest way to measure the transconductance is to draw a line through two consecutive data points on the Ia(Vgs) curve. The transconductance is then nothing else than slope of this line. However, differentiating a measured curve this way usually results in a very noise transconductance curve as a result of (small) fluctuations in the measured data. In the version 3 utracer I have therefore used a slightly different approach. After measurement of a set of Ia(Vgs) curves, first a polynomial is fit through the measurement points. This polynomial smoothes out the fluctuations in the measured data. After the fit the polynomial is analytically differentiated resulting in a perfect noise free transconductance curve [MORE]. Figure 4 Left: Ia versus Vg set of curves for an EL84; Right: extracted transconductance (solid lines) and anode current (dashed lines) Figure 4 shows an example of a transconductance extraction from a set of Ia(Vgs) curves. The original set of curves is shown on the left. The first step to extract the transconductance is to determine the degree of the polynomial to be fitted. The degree of the polynomial can be set to any value between 1 and 25! I usually do this by trial and error. I usually plot the data on the Y1-axis and then switch off the line so that only the markers are shown. On the Y2-axis the fitted anode current is plotted with the markers switched off. In this way it is easy to compare the fit to the measured data. A 5th order polynomial usually gives a good result. The order of the polynomial should not be chosen too high because that will result in oscillations in the transconductance curve. When the fit is ok, the derivative of the current to the voltage (dia/dv) can be plotted. An alternative way to find the optimum polynomial degree is to directly plot the transconductance, and then to vary the polynomial degree. At first increasing the order of the polynomial should result in a significant change in the transconductance curves. At a certain point the curves will not change anymore when the order is increased. However, when the order is increased again beyond a certain point oscillations in the transconductance curves will appear. The most optimal point is somewhere in the flat part where the transconductance is relatively independent of the degree of the polynomial. The right graph in Fig. 4 shows the extracted transconductance (solid lines) and the original anode current as a function of grid bias. The marker shows the transconductance at the bias point recommended in my Muiderkring tube handbook (see inset) Vg = -7 V and Va = Vs = 250 V. Both the transconductance as well as the anode current are exactly the same as the values mentioned in the handbook, respectively 11.5 ma/v and 48 ma, which shows what an accurate and useful instrument the utracer is! Exactly the same procedure is followed to extract the output conductance of a tube. In that case the anode current is measured as a function of anode voltage. Since it is more common to give the output resistance it is also possible to plot the reciprocal of the conductance: dv/dia. Miscellaneous The miscellaneous form houses a variety of items, mostly related to I/O. The Debug button opens a form that can be used to set the proper COM port and to monitor the data transfer between the utracer and the GUI. The Cal. Button opens a form that can be used to calibrate the utracer. The Save Data and Save Plot buttons finally give the possibility to store the measurement data in data or graphic format.
Page 5 of 6 Figure 5 Miscellaneous form. The Save Plot form is the simplest. There are two possibilities: the user can enter a file name via a dialog window, or the plot is simply stored as [title].bmp, with [title] the plot title entered in the graphical output section. Due to limitations of Visual Basic, it is only possible to store a plot as a.bmp file. The Save Data form is a bit more complicated. First of all there is the possibility to store the complete Measurement Matrix as it is used internally in the GUI [MORE]. Although this file contains all the measured data, it needs quite a bit of manipulation to prepare it so that the data can be read into e.g. excel. Especially for that purpose it is possible to store the data in two other formats. First the variable to be stored is selected. When the option Save as Block is chosen, the data is stored as a block in which the first column is the running variable while each next column contains the measured data for each consecutive stepping value. When the option Save as List is chosen, there are only two columns, the first one being the running variable again, and the second one the measurement data. In this case the data for the different stepping values is appended to the file. For both options it is possible to produce a data file with or without text. Figure 6 The calibration form The calibration form is used to compensate for component tolerances. By adjusting the slide bars the exact values for currents and voltages can be set [MORE]. The default value for each slide bar is 1.0 (center position). The maximum range over which can be compensated is 0.9 to 1.1, so +/- 10%. VaGain and VsGain adjust the anode and screen voltages. IaGain and IsGain adjust the measured currents, and with Vsuppl and Vgrid the exact supply and grid voltages can be set. Vsat adjusts the voltage drop over the high voltage switch during the measurement pulse. When the Save to Calibration File button is pressed the calibration data is saved to the file utracer_3p5.cal. At the same time and in the same file, the COM port number selected in the Communication section is stored. The Communications Form The last part left to explain is the Communications Form (Fig. 7). With a drop down menu it is possible to select another COM port than the default port 1 if needed. Furthermore there is a button to close the current COM port or to send an escape character to the utracer. An escape character always forces the utracer to return to its reset state.
Page 6 of 6 Figure 7 Communication debug form. The top of the form contains detailed information about the communication between the GUI and the utracer. This window was used extensively during the debugging phase of the program development, and I still use it regularly to check what is going on in detail. The form has three areas, the top part displays data that is send to the utracer by the GUI [MORE]. In the four columns we find the set-points of the voltages which together determine the bias point of the tube. The top row gives the decimal value. The row below that gives an integer representation of the set-point which has been translated to a value which makes sense for the 10-bit on-chip AD/PWM converters. The row below that just gives the hex representation of the integer value. The last row gives the values for the gain, averaging and compliance settings which are send to the utracer in the command string. These last values are already coded so that the utracer can easily interpret them. The four setpoints, headed by a command byte are combined to a command string which is shown in the middle section of the form [MORE]. Normally the utracer echoes every character sent to it. This process is monitored in the line below the command string which shows the echoed characters. In the bottom section of the form, the data is displayed which is sent back by the utracer in the result string. The result string is composed of three parts. It is headed by a status word which is displayed separately. A value of 10 indicates a successful measurement, while 11 indicates the occurrence of a compliance error. The bulk of the data in the result string is the readout of the 8 AD channels. The hex readout and the integer representations are shown in the top two lines. In the row beneath that the decimal values are shown whereby already the conversion from AD readout to real voltages and currents has been done. In the boxes at the bottom the actual gain and averaging values which were used for that particular measurement point are shown. They may vary from point to point when the automatic ranging and averaging options have been chosen. It is instructive to see them change during a measurement. previous page next page