Low frequency electro-mechanical impedance tuner
A segmented, electro-mechanical, remotely controlled programmable impedance tuner for the frequency range between 10 and 200 MHz uses a cascade of three continuously variable mechanical capacitors interconnected by a set of two low loss flexible or semi-rigid cables; the electrical length of the interconnecting cables between the capacitors determines the frequency at which tuning coverage of the entire area of the Smith chart is optimum; for maximum impedance coverage the length is to be chosen such as to generate a transmission phase shift of 60 degrees at the center frequency between each capacitor stage. Remote tuning is possible by changing the value of the capacitors using electrical stepper motors. The tuner is calibrated using a vector network analyzer and the data are saved in the memory of the control computer, which then allows tuning to any user defined impedance within the tuning range. Reflection factor values between 0 and higher than 0.9 can be obtained using this tuner.
1. An electro-mechanical impedance tuner comprising an input port and an output port and a cascade of three or more capacitance blocks being interconnected using exchangeable lengths of low loss coaxial cable, said capacitances being mechanically adjustable using stepper motors, the total assembly being digitally remotely controlled by a computer, which runs software allowing to synthesize arbitrary, user defined, impedances, in an open loop configuration, based on previously generated tuner calibration data.
2. A tuner as in claim 1 , in which said capacitance blocks include one high tuning range rotary capacitor each, which is connected in parallel between a floating point and ground, said floating point serving as connection point between the central conductors of said lengths of coaxial cable between capacitance blocks, each capacitor being driven by an electrical stepper motor, allowing settings between a minimum and a maximum capacitance value in discrete steps, said motor being remotely driven by a digital driver circuit, which receives digital electric control signals from a system computer running appropriate control software.
3. A method for choosing the optimum length of said interconnecting cables between capacitance blocks, as in claim 1 , for either optimum coverage of the Smith chart area, or, for focusing the tunable points in other areas, by first measuring the scattering parameters (s-parameters) of each tuning block independently as a function of the values of said capacitance between a minimum and a maximum value and cascading them in the computer using a commercially available microwave circuit analysis program, which allows varying numerically the electrical length of said interconnecting cables between said capacitance blocks and displaying the expected impedance tuning pattern on the Smith chart.
4. A calibration procedure at a frequency F 0 , in which the tuner of claim 1 is connected to a pre-calibrated RF vector network analyzer (VNA) and four scattering (s-) parameters, S 11 , S 12 , S 21 and S 22 , are measured between the input and output port of said tuner; said calibration procedure consisting of the following steps: in step 1 the s-parameters of said tuner are measured with all capacitors set to their minimum values, the result is saved as a zero matrix {S 0 }; in step 2 s-parameters of said tuner are measured when varying the first capacitor in N equal steps between its minimum and maximum values and saved in a matrix {S 1 }; in step 3 s-parameters of said tuner are measured when varying the second capacitor in N equal steps between its minimum and maximum values, cascaded with the inverse matrix {S 0 } −1 and saved in a matrix {S 2 }; in step 4 s-parameters of said tuner are measured when varying the third capacitor in N equal steps between its minimum and maximum values, cascaded with the inverse matrix {S 0 } −1 and saved in a matrix {S 3 }; in step 5 s-parameter matrices {S 1 }, {S 2 } and {S 3 } are cascaded to generate the tuner calibration matrix containing N 3 number of s-parameters and saved in a calibration data file, at the frequency F 0 , for later use by the impedance synthesis software.