Set term push # save current terminal settings Plot 'MVAM125.dat' using 1:2 title 'Datasheet' with points 3, \Ĭ(x) title sprintf('Curve-fit c0 = %f, v0 = %f, m = %f', c0, v0, m) Set title "MVAM125 capacitance and curve fit" Set key title "c(v) = c0/((1 + v/v0) ^ m)\n" # title to key Here is the gnuplot script that performs the curve fitting for our varactor capacitance: c(x) = c0/((1+x/v0) ** m) # function to be fitĬ0 = 1000 v0 = 0.5 m = 0.5 # initial guess for c0, v0 and mįit c(x) 'MVAM125.dat' using 1:2 via c0, v0, m
GNUPLOT CURVE FITTING HOW TO
I won't go into the details on how to setup gnuplot to fit a function to a set of data points, as this is well explained in the relevant section of the gnuplot manual. There are several applications that can be used for curve fitting, a few specialized in this domain, other designed for general numerical analysis, like GNU Octave, but this time I have used the curve fitting capability of gnuplot. So this is the equation we will use for the data fitting.
GNUPLOT CURVE FITTING SIMULATOR
V data are available in tabular for we need to fit a suitable curve practically every circuit simulator has a model for the capacitance of a reverse biased junction which uses the standard equation C(v) = C0/((1+v/Vj)^m)
The end results of the data extraction process is a nice table with the coordinates of the selected points of the graph, expressed in voltage and capacitance: 1.15182409227 495.251088405 The following picture shows g3data at work on the MVAM125 capacitance curve shown previously: V data in a tabular form, instead of a graph, and then try to fit these data to an equation to model the capacitance variation with applied voltageĪ very nice (and free, GNU GPL) application that can be used for extracting data from graphs is g3data I have used it a few times in the past on graphs of different kind and quality and always obtained good results for the extracted data. V curve is essential, to be able to simulate the response of a circuit built using this component one needs to build a model for the circuit simulator. reverse voltage curve taken from the Motorola datasheet: Realizing a filter that can be tuned over the entire 3 MHz to 30 MHz spectrum with using varactors is almost impossible, since that means covering a frequency ratio of 10 and so a capacitance variation of 100 over that frequency range anyway to reduce the number of filters to be implemented it's better to select a varactor with a large capacitance variation with voltageĪ good candidate is, for example, the MVAM125 tuning diode, which was designed for the tuning of AM receivers and has a specified capacitance ratio of 15 when changing the voltage applied from 25 to 1 . voltage curveĪ while ago I started looking for a preselector circuit for an HF receiver I was interested in controlling the preselector tuning using varactor, hoping to reduce the circuits size (maybe at the expense of performances, but that's another story.).
0 kommentar(er)
