Some questions about transceiver
I am a sophomore student at a university and we have a term project for the course: "Analog Electronics". We will be doing a transceiver at public band level (28.-29.7). We have learned some information about components of transceiver, but I could not understand something for receiver circuitry. First of all
We have two IF Amplifiers. Why we use those amplifiers? The second question is that do we need last Low-Pass filter? The last question is (look for pic2) after second IF Amplifier, we have left part of copy of signal left. Is it gone by envelope detector or Low Pass Filter?
Thank you
Two IF (Intermediate Frequency) amplifiers. In order to reduce the possibility of another tranmitter from interfering with your reception, most receivers will use a 10.7 MHz first IF stage. This mixes with the incoming signal, to produce a useable signal.
The second stage uses a frequency of 455 KHz. That way only audio levels will be allowed to pass. Each time a mixing occurs, there is some signal degradation, so an amplifier is placed after the mixer stage to enhance the signal. Similarly, the Audio Amplifier is used to boost the level to make it audible.
Actually the posted receiver is a superheterodine using only one conversion, so there cannot be two IF's (10.7MHz AND 455kHz).
The reason of two (or multiple) IF stages is to get the receive system budget for gain and sensitivity.
The reason of the LPF after the detector is to eliminate any high frequencies greater than audio (voice in this case) frequencies.
Answer to the last question: by the envelope detector.