SmithScope: displaying r.f.impedance directly in a Smith Diagram

Joachim Köppen DF3GJ Kiel/Strasbourg/Illkirch Summer 2003

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The SmithScope

This shows the setup to measure the input impedance of a helical resonator bandfilter terminated with 50 Ohms at its output. The box to the left contains (among other things) the r.f. impedance bridge, the signal generator is to the right. The frequency is about 20 MHz.

Measuring the input impedance of a 21 MHz direct conversion receiver. The VSWR is better than 3:1, but that's the optimum.

An inside view. One notes the two sets of concentric circles of silvered copper wire which drive the LEDs.

Narrow band helical filters

The same as above, but for a filter designed for 45 MHz.

The impedance at the centre of the passband of the 45 MHz helical filter. Note that the transparency with the Smith diagram had been placed 180 degrees rotated from its proper position. One can also directly read off the VSWR of about 2:1, from the relative distance of the lit-up LEDs from the centre.

This is a further close up, at the frequency of the best match to 50 Ohms. Note that the green LED near the centre is lit in order to indicate that.

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