.. _calibration: Calibration *************** .. currentmodule:: skrf.calibration.calibration .. contents:: Intro --------------- This tutorial illustrates how to use **skrf** to calibrate data taken from a VNA. The explanation of calibration theory and calibration kit design is beyond the scope of this tutorial. Instead, this tutorial describes how to calibrate a device under test (DUT), assuming you have measured an acceptable set of standards, and have a coresponding set ideal responses. **skrf**'s default calibration algorithms are generic in that they will work with any set of standards. If you supply more calibration standards than is needed, skrf will implement a simple least-squares solution. Creating a Calibration ---------------------------- Calibrations are performed through a :class:`Calibration` class. Creating a :class:`Calibration` object requires at least two pieces of information: * a list of measured :class:`~skrf.network.Network`'s * a list of ideal :class:`~skrf.network.Network`'s The :class:`~skrf.network.Network` elements in each list must all be similar (same #ports, frequency info, etc) and must be aligned to each other, meaning the first element of ideals list must correspond to the first element of measured list. Optionally, other information can be provided when relevent such as, * calibration algorithm * enforce eciprocity of embedding networks * etc When this information is not provided skrf will determine it through inspection, or use a default value. Saving and Recalling a Calibration ----------------------------------- Like other **skrf** objects, :class:`Calibration`'s can be written-to and read-from disk. Writing can be accomplished by using :func:`Calibration.write`, or :func:`rf.write() `, and reading is done with :func:`rf.read() `. One-Port -------------- This example is written to be instructive, not concise. :: import skrf as rf ## created necessary data for Calibration class # a list of Network types, holding 'ideal' responses my_ideals = [\ rf.Network('ideal/short.s1p'), rf.Network('ideal/open.s1p'), rf.Network('ideal/load.s1p'), ] # a list of Network types, holding 'measured' responses my_measured = [\ rf.Network('measured/short.s1p'), rf.Network('measured/open.s1p'), rf.Network('measured/load.s1p'), ] ## create a Calibration instance cal = rf.Calibration(\ ideals = my_ideals, measured = my_measured, ) ## run, and apply calibration to a DUT # run calibration algorithm cal.run() # apply it to a dut dut = rf.Network('my_dut.s1p') dut_caled = cal.apply_cal(dut) # plot results dut_caled.plot_s_db() # save results dut_caled.write_touchstone() Concise One-port ----------------- This example is the same as the first except more concise. :: import skrf as rf my_ideals = rf.load_all_touchstones_in_dir('ideals/') my_measured = rf.load_all_touchstones_in_dir('measured/') ## create a Calibration instance cal = rf.Calibration(\ ideals = [my_ideals[k] for k in ['short','open','load']], measured = [my_measured[k] for k in ['short','open','load']], ) ## what you do with 'cal' may may be similar to above example Two-port --------- Two-port calibration is more involved than one-port. skrf supports two-port calibration using a 8-term error model based on the algorithm described in [#]_, by R.A. Speciale. Like the one-port algorithm, the two-port calibration can handle any number of standards, providing that some fundamental constraints are met. In short, you need three two-port standards; one must be transmissive, and one must provide a known impedance and be reflective. One draw-back of using the 8-term error model formulation (which is the same formulation used in TRL) is that switch-terms may need to be measured in order to achieve a high quality calibration (this was pointed out to me by Dylan Williams). Switch-terms ++++++++++++++++++++++++ Originally described by Roger Marks [#]_ , switch-terms account for the fact that the error networks change slightly depending on which port is being excited. This is due to the internal switch within the VNA. Switch terms can be measured with a custom measurement configuration on the VNA itself. **skrf** has support for switch terms for the :class:`~skrf.vi.vna.HP8510C` class, which you can use or extend to different VNA. Without switch-term measurements, your calibration quality will vary depending on properties of you VNA. Example ------------------- Two-port calibration is accomplished in an identical way to one-port, except all the standards are two-port networks. This is even true of reflective standards (S21=S12=0). So if you measure reflective standards you must measure two of them simultaneously, and store information in a two-port. For example, connect a short to port-1 and a load to port-2, and save a two-port measurement as 'short,load.s2p' or similar:: import skrf as rf ## created necessary data for Calibration class # a list of Network types, holding 'ideal' responses my_ideals = [\ rf.Network('ideal/thru.s2p'), rf.Network('ideal/line.s2p'), rf.Network('ideal/short, short.s2p'), ] # a list of Network types, holding 'measured' responses my_measured = [\ rf.Network('measured/thru.s2p'), rf.Network('measured/line.s2p'), rf.Network('measured/short, short.s2p'), ] ## create a Calibration instance cal = rf.Calibration(\ ideals = my_ideals, measured = my_measured, ) ## run, and apply calibration to a DUT # run calibration algorithm cal.run() # apply it to a dut dut = rf.Network('my_dut.s2p') dut_caled = cal.apply_cal(dut) # plot results dut_caled.plot_s_db() # save results dut_caled.write_touchstone() Using one-port ideals in two-port Calibration ++++++++++++++++++++++++++++++++++++++++++++++ Commonly, you have data for ideal data for reflective standards in the form of one-port touchstone files (ie s1p). To use this with skrf's two-port calibration method you need to create a two-port network that is a composite of the two networks. There is a function in the WorkingBand Class which will do this for you, called two_port_reflect.:: short = rf.Network('ideals/short.s1p') load = rf.Network('ideals/load.s1p') short_load = rf.two_port_reflect(short, load) .. rubric:: Bibliography .. [#] Speciale, R.A.; , "A Generalization of the TSD Network-Analyzer Calibration Procedure, Covering n-Port Scattering-Parameter Measurements, Affected by Leakage Errors," Microwave Theory and Techniques, IEEE Transactions on , vol.25, no.12, pp. 1100- 1115, Dec 1977. URL: http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=1129282&isnumber=25047 .. [#] Marks, Roger B.; , "Formulations of the Basic Vector Network Analyzer Error Model including Switch-Terms," ARFTG Conference Digest-Fall, 50th , vol.32, no., pp.115-126, Dec. 1997. URL: http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4119948&isnumber=4119931