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From: whinery@hale.ifa.hawaii.edu (Alan Whinery)
Newsgroups: sci.electronics
Subject: Re: How to read the value of small ceramic capacitors, and Why use an IF?
Message-ID: <11909@uhccux.uhcc.Hawaii.Edu>
Date: 12 Mar 91 19:08:15 GMT
References: <1991Mar11.181039.22769@uncecs.edu>
Sender: news@uhccux.uhcc.Hawaii.Edu
Organization: Institute For Astronomy, Hawaii
Lines: 31

In article <1991Mar11.181039.22769@uncecs.edu> crisp@uncecs.edu (Russ Crisp) writes:
>

>Secondly, why is the rf in a radio receiver mixed with a 
>local oscillator to provide an IF?  Why not go straight
>through with the carrier frequency?  What advantage does
>this IF signal provide?

If you wanted to have much tuning range in a radio with no IF, you'd
be faced with the formidable task of tuning a chain of filters, which
are necessary to achieve selectivity, simultaneously. If it doesn't sound
formidable then consider that the tuning element would have to be linear
across a wide range of frequency, regardless of temperature changes, 
parasistic capacitance and inductance, etc. 

If you "beat" the incoming RF against a Local Oscilllator frequency, 
thus producing the IF, all you have to tune is the LO. The RF amps 
ahead of the mixer (beater, whatever) are wide band, the IF filters
are all tuned to 1 frequency (usually 455 khz or 10.7 Mhz) and they
stay that way. It also makes the detection easier, because the detector 
in an FM receiver, say, can be built to operate at 10.7 Mhz instead
of at 88-108 Mhz, which would complicate the design considerably, not
to mention making it more expensive.

Alan
whinery@hale.ifa.hawaii.edu

--
Regards,
Alan Whinery
whinery@hale.ifa.hawaii.edu