Let say your receiver is set to receive 72.555 mz in order to do this the 72.555 need to be changed to your I.F. (intermediate frequency) let say that your IF is set to 10.7mz this is called your band pass freq. because it will not allow any freq. above or below 10.7mz. To pass. So to receive 72.555 your mixing freq. needs to be 61.855mz (72.555 - 61.855 = 10.7mz) if you change your mixing freq. up by 1 mz to 62.855 you will now be receiving 73.555. The mixing freq. can be changed by changing a crystal or by using a programmable VFO (variable frequency oscillator) like tuning in a FM radio station. ( programmable VFO is a synthesized receiver) in a duel receiver this happened twice with two IF’s