Virtual Thru-Reflect-Line (TRL) By John E. Penn Introduction In measuring circuits at microwave frequencies, it is essential to have a known reference plane, particularly when measuring transistors whose characteristics are affected simultaneously by input and output impedances. Various calibration techniques are used to measure devices at microwave frequencies. One of those techniques using standards of a thru line, reflect typically an open circuit, and a second line, or multiple lines, each longer than the thru line. Measurements can be taken over a frequency range correlated to the Various calibration difference in electrical length between the thru and the line standard, or techniques are used to standards.these TRL standards allow direct de-embedding of measurements up to the edge of a standard transition which is half of the thru measure devices at microwave frequencies. connection (See Reference 1). For probe measurements of transistors, a typical launch structure smoothly transitions the coplanar ground-signal-ground (GSG) probes to microstrip connections at the transistor. Many probe calibration methods yield a measurement reference plane at the point of contact of the GSG probes. The TRL method allows for moving the reference plane to the edge of the gate and drain connections of standard transistor layout, as shown in figure 1. 22 High Frequency Electronics Another advantage of including TRL calibration structures with an IC fabrication is that some process variation can be compensated for when deembedding the common half thru launch. If the line Figure 1 Reference Planes of Measured HEMTs. impedance of the transition changes due to changes in the substrate thickness, or, over or underetching of line widths on a PC board, these variations will be de-embedded leaving the reference plane at the transistor edges as desired. In this example, on chip TRL calibration structures were not available, so an EM simulation of the physical layout of the half thru launch is used to de-embed the transistor measurements. If variation in substrate thickness did occur, and was known, it could be included in this virtual TRL calibration de-embedding method. Electromagnetic Simulation of GSG to Microstrip Launch A lack of a TRL calibration kit does not negate the use of a physical structure to de-embed the common GSG to microstrip transition. In this case, the nominal physical description of the transition is known and can be simulated with an Electromagnetic (EM) Simulator such as Sonnet EM, Keysight s Momentum, or AWR s Axiem simulator. Simulations of a similar GSG launch yield reasonably close agreement among these three EM simulators, though only the Axiem EM structure is shown here (figure 2). A key requirement of the 2.5D EM simulator is the ability to solve a three connection GSG coplanar port defined internal to the pads where the probe pins make contact. Each port has the signal designated as 1, or 2, while the differential (ground) contacts
PRODUCTS TO SOLUTIONS RF Products are -1 and -2. While figure 2 shows the physical layout of an entire thru standard, a separate EM simulation was performed on half of this structure to de-embed the common coplanar to microstrip launch. This half thru can then be easily de-embedded from s-parameter measurements of the desired transistors, or other circuits. De-embedding load pull measurements uses the same concept but requires additional effort, as described later. Figure 3 shows the schematic in Microwave Office (MWO) for subtracting the Axiem EM simulated GSG launch to de-embed the measured HEMT s-parameter data up to the gate and drain connections. A Smith Chart plot of the deembedded measured data (up to 67 GHz) compared to vendor supplied data (up Ducommun has more than 45 years of experience with the design, testing and manufacturing of standard and custom millimeter wave amplifiers. High Power, Single DC power supply/ internal sequential biasing -1-2 32 to 36 GHz Power Amplifier AHP-344353-1 Gain: 3 db (Min) Gain Flatness: +/- db (Max) P-1D db: 34 dbm (Typ), 33 dbm (Min) 1 2-1 -2 32 to 36 GHz Power Amplifier ALN-331443-1 Gain: 3 db (Min) Gain Flatness: +/-1. db acoss the band Noise Figure : db (typ) For additional information, contact our sales team at +1 (31) 513-7256 rfsales@ducommun.com 24 High Frequency Electronics CONTACT US Figure 2 57um Thru Line 2D and 3D view of Axiem EM Simulation Structure. to 5 GHz) at the HEMT connections shows good comparison (figure 4). The DC biases were similar, but different, and while the HEMT device was from the same wafer lot, it was likely a different device too. Figure 5 shows the magnitude of s11, S22, and S21 of the two sets of measurements as a rectangular plot. Load Pull Measurements Load pull measurements were performed on HEMTs at various frequencies. These also required a shift in the reference plane using a similar method to deembed the electrical length of the GSG launches.microwave Office was used to plot the load pull contours. There did not appear to be a convenient mechanism
PORT P=1 Z=5 Ohm NEG2 ID=N1 NET="GSG_CAPs_Ax" (-) 26 High Frequency Electronics Measured S2P 1 2 SUBCKT ID=S1 NEG2 ID=N2 NET="GSG_CAPs_Ax" (-) PORT P=2 Z=5 Ohm Figure 3 Schematic for De-embedding GSG Launch from Measured HEMTs. Figure 4 Measured and De-embedded 8x5um HEMT (-67GHz) vs. Vendor (-5GHz). Figure 5 Measured and De-embedded 8x5um HEMT (-67GHz) vs. Vendor (-5GHz). to shift the contours in a method similar to the one used for the s-parameter measurements. After trying tedious methods to de-embed the measured impedances and transform a few load pull files, a simple perl script was created to easily process the load pull files. The script reads in the measured impedances (Gamma_dut) and rotates them by approximately twice the electrical length, as determined by the EM simulation of the GSG launch. Conversions are made between the real and imaginary reflection coefficient for each load pull data point, converts to magnitude and angle, rotates to de-embed the GSG launch, then converts back to a final real and imaginary reflection coefficient. A sample plot of load pull contours measured at the GSG probe pads at 16 GHz is shown in figure 6, and the reference plane shift corresponding to the drain connection of the HEMT is shown in figure 7. The de-embedded plot shows a smooth virtually identical set of contours with the reference plane shifted to the drain connection of the HEMT. Using an EM simulation of the GSG launch to deembed, or shift the reference plane, of s-parameter data appears to be a useful substitute when an actual TRL calibration kit is not available. This technique works well for measured load pull data as well. One could easily extend this idea to de-embed a network, eg matching network, using the full 2 port s-parameters. This example using a nearly lossless GSG launch results in a simple rotation of the reference plane. Load Pull Reference Plane; Perl Script to De-embed GSG Launch from Maury Load Pull Measurements A simple Perl script was created to read in a Maury load pull, or source pull, file and translate the impedances to the device gate or drain connection. Matlab27 was used to run the perl script to process these load pull files. It is very important to maintain the exact formatting of the file, in order for it to plot successfully in MWO. Each load pull reflection coefficient is converted from real and imaginary to a magnitude and angle, then rotated by the twice the angle of the GSG launch ($angle), then adjusted in magnitude to compensate for the virtually lossless GSG launch (1/$magn), then converted back to real and imaginary for the de-embedded impedance, and finally, written to the output. This file could be re-written to take the magnitude and angle of the GSG launch as an input parameter, but for this example it is included in the script and needs to be modified at each frequency (eg 8, 16, 24, GHz), as well as adjusted for the appropriate GSG launch. Following is
- - - - p27 p26 p25 p24 p23 - p29 p28 LP_4x5_15_16G_Pout_all p1: Gt_dB = 6 db p2: Gt_dB = 6.4 db p3: Gt_dB = 6.8 db p4: Gt_dB = 7.2 db p5: Gt_dB = 7.6 db p6: Gt_dB = 8 db p7: Gt_dB = 8.4 db p8: Gt_dB = 8.8 db p9: Gt_dB = 9.2 db p1: Gt_dB = 9.6 db p11: Gt_dB = 1 db p12: Gt_dB = 1 db p13: Gt_dB = 1 db p14: Gt_dB = 11.2 db p15: Gt_dB = 11.6 db p16: Gt_dB = 12 db p17: Gt_dB = 12.4 db p18: Gt_dB = 12.8 db p19: Gt_dB = 13.2 db p2: Gt_dB = 13.6 db p21: Gt_dB = 14 db p22: Gt_dB = 14.4 db p23: Pout_dBm = 16 dbm p24: Pout_dBm = 16.5 dbm p25: Pout_dBm = 17 dbm p26: Pout_dBm = 17.5 dbm p27: Pout_dBm = 18 dbm p28: Pout_dBm = 18.5 dbm p29: Pout_dBm = 19 dbm p3: Pout_dBm = 19.5 dbm p31: Pout_dBm = 2 dbm p32: Pout_dBm = 2.5 dbm p33: Pout_dBm = 21 dbm p34: Pout_dBm = 21.5 dbm p35: Pout_dBm = 22 dbm p36: Pout_dBm = 22.5 dbm p37: Pout_dBm = 23 dbm p38: Pout_dBm = 23.5 dbm p39: Pout_dBm = 24 dbm p4: Pout_dBm = 24.5 dbm p41: Pout_dBm = 25 dbm p42: Pout_dBm = 25.5 dbm p43: Pout_dBm = 26 dbm p44: Pout_dBm = 26.5 dbm p45: Pout_dBm = 27 dbm p46: Pout_dBm = 27.5 dbm p47: Pout_dBm = 28 dbm p48: Pout_dBm = 28.5 dbm p49: Pout_dBm = 29 dbm p5: Pout_dBm = 29.5 dbm p51: Pout_dBm = 29.763 dbm p52: Gt_dB = 14.763 db -1. 1. 1. - p46 p38 p39 p4 p1 p9 p41 p11 p12 p42 p13 p43 p14 p44 p15 p45 p16 p17 p18 p47 p19 p48 p2 p37 p49 p21 p35 p36 p6p7 p8 p5 p22 p4p5 p52 p51 29.763 Swp Max Mag 3 95 Ang 75.58 Deg - Pcomp_PORT_2_1_M_DB Pout Pcomp_PORT_2_1_M_DB Max PoutMax Converged Points LPCM(2,6,,3,1,5,) PAE LPCMMAX(3,1,5,) PAEMax - p34 p2 - p33 p1 p32 p31 p3 Sw p Min Figure 6 Load Pull Contours 4x5 HEMT at 16GHz (Power-blue, Efficiency-red). - - - LP_4x5_15_16G_Pout_all_DeembedPL - p23 p24 - p27 p26 p25 p29 p28 p1: Gt_dB = 6 db p2: Gt_dB = 6.4 db p3: Gt_dB = 6.8 db p4: Gt_dB = 7.2 db p5: Gt_dB = 7.6 db p6: Gt_dB = 8 db p7: Gt_dB = 8.4 db p8: Gt_dB = 8.8 db p9: Gt_dB = 9.2 db p1: Gt_dB = 9.6 db p11: Gt_dB = 1 db p12: Gt_dB = 1 db p13: Gt_dB = 1 db p14: Gt_dB = 11.2 db p15: Gt_dB = 11.6 db p16: Gt_dB = 12 db p17: Gt_dB = 12.4 db p18: Gt_dB = 12.8 db p19: Gt_dB = 13.2 db p2: Gt_dB = 13.6 db p21: Gt_dB = 14 db p22: Gt_dB = 14.4 db p23: Pout_dBm = 16 dbm p24: Pout_dBm = 16.5 dbm p25: Pout_dBm = 17 dbm p26: Pout_dBm = 17.5 dbm p27: Pout_dBm = 18 dbm p28: Pout_dBm = 18.5 dbm p29: Pout_dBm = 19 dbm p3: Pout_dBm = 19.5 dbm p31: Pout_dBm = 2 dbm p32: Pout_dBm = 2.5 dbm p33: Pout_dBm = 21 dbm p34: Pout_dBm = 21.5 dbm p35: Pout_dBm = 22 dbm p36: Pout_dBm = 22.5 dbm p37: Pout_dBm = 23 dbm p38: Pout_dBm = 23.5 dbm p39: Pout_dBm = 24 dbm p4: Pout_dBm = 24.5 dbm p41: Pout_dBm = 25 dbm p42: Pout_dBm = 25.5 dbm p43: Pout_dBm = 26 dbm p44: Pout_dBm = 26.5 dbm p45: Pout_dBm = 27 dbm p46: Pout_dBm = 27.5 dbm p47: Pout_dBm = 28 dbm p48: Pout_dBm = 28.5 dbm p49: Pout_dBm = 29 dbm p5: Pout_dBm = 29.5 dbm p51: Pout_dBm = 29.746 dbm p52: Gt_dB = 14.746 db -1. 1. 1. Swp Max p35 p4 p36 p5p6 p37 p7 p38 p8 p39 p4 p1 p9 95 p41 p11 p42 p12 29.746 p13 p43 p14 p44 p15 p45 p16 p17 p46 p18 p47 Mag 2 p19 p48 p2 p22 p5 p49 p21 Ang 47.12 Deg p52 p51 - - Pcomp_PORT_2_1_M_DB Pout Pcomp_PORT_2_1_M_DB Max PoutMax Converged Points LPCM(2,6,,3,1,5,) PAE LPCMMAX(3,1,5,) PAEMax - - p33 p1 p32 p31 p3 p34 p2 Swp Min Figure 7 Load Pull DeEmbed 4x5 HEMT at 16 GHz (Power-blue, Efficiency-red). The MX6L Wideband Integrated-LO Mixer A standalone RF mixer with programmable internal local oscillator perfect for downconverting. 1-6 MHz RF Range LF - 6 MHz IF Range Input P1dB > +12dBm Standalone Signal Generators to 22GHz Ultra-Wideband Output Adjustable Level Control Micro 2.75 Enclosure USB Remote Control Only $149 Only $599 USB & Panel Controls Low conversion loss Auto 1MHz Reference 12GHz Programmable RF Step Attenuator - 63dB in.5db Steps Low Insertion Loss USB Powered & Controlled 1MHz to 12GHz Coverage Only $799 ds instruments WWW.DSINSTRUMENTS.COM an example of a typical Maury load pull file referenced to the probe pads, followed by its output from this perl script to create the de-embedded load pull file referenced to the drain of the HEMT. Each line that starts with Gamma_ dut is the impedance of a given tuner measurement as a real and imaginary reflection coefficient. That impedance is modified to reflect the shift in the reference plane, otherwise the rest of the file is identical. Perl Script: #!/usr/bin/perl # $magn =.998 ; $angle = -13.9 ; while (<>) { # chop ($_) ; if (/Static /) { print $_ ; } elsif (/Gamma_dut: /) { ($temp, $real, $img) = split ; $mag1 = sqrt($real * $real + $img * $img) / $magn; $ang1 = atan2($img, $real) ; # convert to angle $ang1 = $ang1 + ( 3.141592654 * $angle / 9 ) ; $realn = $mag1 * cos($ang1) ; $imgn = $mag1 * sin($ang1) ; $realn = sprintf( %8.5f, $realn ) ; $imgn = sprintf( %8.5f, $imgn ) ; print $temp $realn $imgn \n ; } else { 28 High Frequency Electronics
} } print $_ ; Example Input Load Pull File Referenced to Probe Pads (4x5um HEMT at 8 GHz):! Load Pull Data file Frequency 8. GHz number of positions: 116 number of harmonics: 1 Static Gamma_dut: Source = (-.1732.3438)! Pin_avail_dBm Pout_dBm Gt_ db Vout_v Iout_mA Vin_v Iin_mA Eff_% Gamma_dut: 1916.35662 27.1785 17.1785 18.286 6.1883.56348.257 47.32898 Gamma_dut: 6232 9822 27.1179 17.1179 18.331 59.1119.56342 -.31 47.53132 Gamma_dut:.51335.3378 27.3412 17.3412 18.28 56.98186.56351.257 51.42559 Gamma_dut: 694.38839 27.3623 17.3623 18.286 58.4767.56351.4737 55828 Gamma_dut:.52322 936 27.4227 17.4227 18.318 55.66545.56344 -.3816 54.11629 Gamma_dut: 7475 6139 27.25421 17.25421 18.29 57.2819.56346 -.3816 52451 Gamma_dut: 1824 3978 Gamma_dut: 9273 4514 27.1785 17.1785 18.286 6.1883.56348.257 47.32898 Gamma_dut:.5255.1786 27.1179 17.1179 18.331 59.1119.56342 -.31 47.53132 Gamma_dut:.57792.19737 27.3412 17.3412 18.28 56.98186.56351.257 51.42559 Example Output Load Pull File De-embedded to HEMT Drain (4x5um HEMT at 8 GHz):! Load Pull Data file Frequency 8. GHz number of positions: 116 number of harmonics: 1 Static Gamma_dut: Source = (-.1732.3438)! Pin_avail_dBm Pout_dBm Gt_ db Vout_v Iout_mA Vin_v Iin_mA Eff_% Innovation in Millimeter Wave Solutions www.omlinc.com (48) 779-2698 29
Gamma_dut:.54879 6396 27.3623 17.3623 18.286 58.4767.56351.4737 55828 Gamma_dut: 643 7126 27.4227 17.4227 18.318 55.66545.56344 -.3816 54.11629 Gamma_dut:.57192.33343 27.25421 17.25421 18.29 57.2819.56346 -.3816 52451 Gamma_dut:.51187.3267 Summary: Measuring unmatched transistors requires precise calibration of the reference plane in order to use these measurements in designing circuits, especially at higher frequencies. A TRL calibration kit is a useful way to de-embed a standard launch structure. For this example of a HEMT in an MMIC process, a TRL calibration kit was not available. However, the physical layout of the common coplanar GSG to microstrip connection was utilized in a 2.5D EM simulation (Axiem). The use of the EM simulation as a virtual TRL calibration kit to translate the reference plane of the measured HEMTs to the standard gate and drain connections was quite successful, as shown in these measurements up to 67 GHz. A simple perl script was written to perform an adjustment of the reference plane to load pull measurements. An example of that perl script to adjust a load pull, or source pull, file at a single frequency is included. The results of the Axiem EM (AWR) simulation of the physical coplanar to microstrip launch were successfully used to perform the equivalent function of a TRL calibration kit, to de-embed up to the transistor connections. About the Author John E. Penn received a B.E.E. from the Georgia Institute of Technology in 198, an M.S. (EE) from Johns Hopkins University (JHU) in 1982, and a second M.S. (CS) from JHU in 1988. Since 1989, he has been a part-time professor at Johns Hopkins University where he teaches RF & Microwaves I & II, MMIC Design, and RFIC Design. Email: jpenn3@jhu.edu. Reference Agilent (Keysight) App Note 851-8A, Agilent Network Analysis Applying the 851 TRL for Non- Coaxial Measurements. 3 High Frequency Electronics