super heterodyne receiver

[pankaj jha]

hello everyone!!!!!!!!
Super heterodyne receiver is a dual conversion architecture, in which, at the first state RF is down-converted to IF and then, in second stage it is from IF to baseband signal.

Can anyone tell me why is it necessary to downconcert the first to IF , and then to baseband.

why do we have to convert the Rf signal to baseband finally?

 

[enjunear]

Typically you convert to an IF such that you can use very sharp, single-frequency tuned, filters to knock down interfering signals that are nearby your carrier, in frequency. That way, the receiver can demodulate just the signal on the RF carrier you choose, and not be drowned out by signal from a nearby carrier. This is called selectivity... how well can your receiver "hear" the desired signal, and reject the undesired signal. If you went from RF down to baseband, directly, you'd need absurdly steep filters that would be impossible to create.

The baseband signal is the "information" you want to send/receive. The RF is just a carrier... the way that carrier is modulated contains the information. Like a terrestrial radio station. The carrier frequency may be 88.1 MHz, but the music is Frequency Modulated (hence, why it's called FM) onto it. If you looked at 88.1 MHz on a spectrum analyzer, you'd see that it has a bandwidth of a few tens of kilohertz... which is the bandwidth of the audio being sent out. You mix the whole signal from RF to IF (the modulation remains on the IF signal), you run it through a steep filter (like a quartz crystal filter) to attenuate all the other signals coming in from the antenna, then mix the IF down to 0 Hz, and what you are left with is just the modulation signal (the audio).

Example:
RF = 88.1 MHz
LO1 = 58.1 MHz (first downmix)
LO2 = 30 MHz (second downmix)
IF = 30 MHz

RF - LO1 = IF (88.1 MHz - 58.1 MHz = 30 MHz)
IF - LO2 = Baseband (30 MHz - 30 MHz = 0 Hz)

 

[kspalla]

Selectivity under LO frequency stability (PPM) would require IF conversion,
if your LO is highly stable or in other words if LO drift is not sensitive based on the application then you can go for direct conversion.