These are frequency meters with the possibility of adding or subtracting any value to be set manually. Therefore it represents ideal for reading the various oscillators internal to the receivers where the medium frequency is to be added/subtracted depending on the case. Article taken from the website www.ik3oil.it. Naturally the version that reaches 1.3 GHz also has the prescaler and the JP jumper inserted to compensate the reading (% 64). Two buttons are used to set the desired value of the medium frequency: IF SET sets the value to add or subtract: IF+SET IF-SET adds or subtracts: PRESCALER value to divide for the use of the prescaler. In the FreqIF.zip file there are in addition to the Hex file for programming the PIC, there is also a PDF file for further information on the implementation.