|Radio and Electronics (DED Philippinen, 66 p.)|
|6. RECEPTION OF RADIOSIGNALS (AM - TYPE)|
The problem arising with RF-amplifiers in the TRF was: the RF-amplifier did not only amplify the desired signal of selected radiostation, but also to each additional signal, passing the tuned circuit.
So, by inserting the RF-amplifier the SENSITIVITY of the radio was improved (it could respond to weaker incoming signals too) but at the same time the SELECTIVITY (the ability to filter out a single radiostation only) was decreased, because now it could happen, that the speaker gave the sound of more than one radio-station at the same time.
The question was now: How is it possible to amplify (to add energy) the signal of the desired radiostation exclusively?
The answer was found in the beginning of the 40ties of this century. It was in physical sense the effect of SUPERHETERODYINING.
The function is in short like shown in fig. 42. The signal coming from the tuned circuit - from now on the RADIOSIGNAL (a) is mixed in a MIXER-STAGE with a RF-frequency signal supplied from the OSCILLATOR - from now on called the OSCILLATORSIGNAL (b) resulting at the end to a frequency f3 called the INTERMEDIATEFREQUENCY SIGNAL (c)
The oscillatorsignal is a signal with a constant frequency and a constant amplitude. You might assume that the oscillatorfrequency is desired to be exactly equal to the radio frequency in order to add energy to the incoming radiosignal.
For two reasons this is not true:
1. it would be very difficult to make sure that the oscillatorfrequency is exactly equal and in phase with the radiofrequency (if not phase, it would diminish the radiosignal).
2. It has a very big advantage to mix with a frequency distant from the radio frequency but to keep the distance constant. This advantage will be cleared during the next chapter.