The circuit consists of three parts. The frequency generator is provided by the LTC1799 integrated circuit with a frequency range from 100 KHz to 30 MHz: this SMD component represents the only problem in assembly, otherwise it is normal administration. The second part is made up of the measurement bridge and is centered around the transformer made up of a 0.8 mm three-wire winding on an FT37-43 toroid. The third part is composed of the classic frequency meter with the PIC 16F84: A small modification was made with the addition of the LM339, a comparator with the aim of seeing the equilibrium condition of the bridge on the display: this modification obviously also entailed a modification of the soft in the PIC (to whom it may be interested I can provide all the necessary information)

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Second version: as mentioned, the aim is to display the impedance, SWR and reactance in addition to the frequency. Hardware modifications on the old circuit: the PCB was cut to make room for the self-made double potentiometer (see photo), it was not possible to find a double potentiometer with a value of 220 Ohm and therefore it was folded using a 220 Ohm one coupled with a 100 Ohm one. For the reactor it was possible to use the section of the 100 pF variable capacitor which remained unused in the previous version. The SWR is measured directly on the instrument. The 1KOhm output resistance for the frequency counter is increased to 10KOhm. Obviously the digital part is completely redone: the display becomes a 16 x 2, the PIC used is a 16F876-20 to have sufficient memory for the program, furthermore the two ADCs are necessary to create the Voltmeter. Brief description of the circuits: frequency meter input is shown the circuit of the previous version is on par with the NE592 while for the measurement of the capacitance of the capacitor varying from 100 pF the LM311 is used which in our case generates a frequency from approximately 470 KHz to 490 KHz. Use the same frequency meter software with a conversion table to read the values. Here I open a parenthesis: I originally used a switching circuit that alternately powered the two circuits (frequency meter and capacitance meter) unfortunately a solution abandoned because the two circuits influenced each other so I fell back on the 4066: another problem not having the 74HC4066 I tried some CD4066 to measure 30 MHz (some stop at 26/27 MHz). Then we have the LM358 for the input to the two voltmeters which measure the voltage drop on the double 100 OHm potentiometer and the other directly measures the voltage on the instrument. Furthermore, the first prototype was made on a millefora base (see photo) but then the printed circuit was also created. Naturally available to provide the software for the PIC, printed circuit design, further information . Below: detail of the double potentiometer.