Multi-band low frequency impedance tuner
View Patent ↗A multi-band, electro-mechanical programmable impedance tuner for the frequency range between 10 and 200 MHz uses cascades of three or more continuously variable mechanical capacitors interconnected with sets of low loss flexible or semi-rigid cables; for each frequency band a different set of cables and capacitors are used. The cables and/or variable capacitors inside each tuning block are switchable manually or remotely. Multi-section variable capacitors are also used. Instantaneous impedance tuning is effectuated by changing the state 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 at all frequency bands.
1. A method for broadening the operating frequency range of low frequency impedance tuners employs re-configurable tuning sections comprising manually or remotely in-situ switchable capacitors and interconnecting cable sections;
said tuners comprising a cascade of at least three tuning sections,
each tuning section comprising
a) shunt variable capacitors of which one terminal is floating and the other grounded,
and
b) a cable module comprising at least two parallel sections of coaxial cable connected between two remotely controlled RF SPDT switches;
whereby said cable modules inter-connect the floating terminal of each said capacitor starting at the tuner input port and terminating at the floating terminal of the last capacitor.
2. A method as in claim 1 , whereby said in-situ switching of said cable sections is effectuated manually.
3. A method for determining the optimum length of interconnecting cables and capacitance values in tuners as in claim 1 for maximum coverage of the Smith chart area over a wide frequency range comprises the following steps:
a) measure s-parameters of each capacitor independently as a function of the values of the capacitance between a minimum and a maximum value,
said capacitor forming a two-port of which the input and output ports are between the floating and grounded terminals of the shunt capacitor;
b) cascade the s-parameters of said capacitors in computer memory using microwave network analysis program and lengths of RF cable between said capacitors,
whereby said program allows varying numerically the electrical length of said interconnecting cables between said capacitors, for all possible combinations of capacitance values, and display the expected impedance tuning pattern on the Smith chart over the targeted frequency range;
c) select the optimum combination of said cable lengths for maximum reflection factor and frequency range.
4. A method as in claim 1 , whereby said tuning sections comprise capacitors configured parallel to each-other and in-situ exchangeable using remotely controlled SPDT RF switches.
5. A method as in claim 4 , whereby said capacitors can be manually switched.
6. A method as in claim 4 , whereby said capacitors are common axis two-segment rotary capacitors.
7. A calibration method for electro-mechanical impedance tuner,
whereby said tuner is operationally connected to a pre-calibrated vector network analyzer using RF cables and,
whereby said tuner comprises an input port and an output port and a cascade of three or more shunt capacitance blocks;
said capacitance blocks comprising variable capacitors of which one terminal is grounded and the other floating, and
said capacitance blocks being inter-connected using exchangeable lengths of low loss coaxial cable, exchanged by switching RF SPDT switches,
whereby said cable sections inter-connect the floating terminals of the cascade of said capacitors starting at the tuner input port and terminating at the output port,
said capacitances being mechanically set to various positions using stepper motors,
the total assembly being digitally remotely controlled by a computer and control electronics, which runs software allowing to control the stepper motors, and communicate with said network analyzer in order to collect s-parameter data of said tuner at the various capacitor positions.