From: rbono@necis.UUCP ( NM1D)
Newsgroups: rec.models.rc
Subject: Re: AM vs FM AGAIN? - Have Mercy
Date: 14 Sep 92 18:36:37 GMT
Organization: NEC Technologies, Inc., Boxborough, MA

In article <BuKoo1.Ixs@usenet.ucs.indiana.edu>, ntaib@silver.ucs.indiana.edu (Iskandar Taib) wrote:
> In article <1992Sep5.042114.3993@nntpd.lkg.dec.com> fisher@kay.enet.dec.com (kay::fisher) writes:
>>Someone else writes:
> 
>>>   Dual conversion on an AM?   I thought that dual conversion was impossible/
>>>   useless on an AM receiver because of the method of detecting an AM signal.
>>>   Besides,   I've yet to see an AM receiver that had 'Dual Conversion' on it
> 
>>Well - I didn't want to answer till I was sure.
>>Check out Model Aviation October 1992 issue page 160.
>>The add for Kraft Midwest Inc. shows a dual conversion AM receiver.
> 
> There isn't anything magical about dual conversion + AM. Dual conversion
> simply means that the receiver uses two intermediate frequencies instead
> of one. Single conversion receivers use 455 KHz for the IF, so the osci-
> llator crystal on the receiver (which is mixed with the incoming RF) is
> 455 KHz above or below the frequency to be detected. A filter then admits 
> 455 KHz to the rest of the receiver. Problem is with the megahertz wide band
> we operate in these days the frequencies at the top and bottom of the band
> may interfere with the ones in the middle. 
> 
> Double conversion receivers get around this by using a 10 MHz or so IF,
> with a 455 KHz second stage. Such receivers are also supposedly better
> at discriminating between adjacent signals. 
> 
> Now: Whats to prevent someone on a totally different band (i.e. 10 MHz
> away) from interfering with you?
> 
  Nothing... except good front end design.  In a good design, there will
be some sort of band pass filter placed before the first IF stage.  There is
a lot of controversy over the placement of this first filter.  It can be
either before or after the first RF amplifier.  Before the first RF amp will
provide better isolation and probably better overload characteristics but
can degrade the noise figure (ability to receive WEAK signals).  After
the first RF amp will (should) not affect the noise figure (good weak signal
reception) but can allow the first RF amp to be overloaded by out of band
signals (which can cause internal-to-the-receiver self-generated intermodulation
interference).  Note that 'good design' is a relative term.  The design engineer
has to make choices to match the specifications (and price) that the receiver
is supposed to perform to.

 The front end filter is ideally supposed to attenuate (decrease) the level of
out-of-band signals (this helps to remove them from causing an image problem)
without attenuating the in-band (desired) signals.

 Of course, with all designs there are comprimises so there are no 'perfect'
or ideal receivers.  With a strong enough out-of-band signal, there is a
possibility for out-of-band image interference.  The idea here is that there
will (hopefully) be less of a chance of a STRONG out-of-band signal than
there is of a STRONG in-band signal.  Therefore, the dual (or triple)
conversion designs usually will perform better than a single conversion
design in this regard.

 Remember that dual conversion does not mean that there will not be image
problems with in-band signals.  If the in-band signals were STRONG enough, then
there could be some interference to the 455 KHz IF in the receiver.

 Again (as stated), single-conversion or dual-conversion, has NOTHING to do
with the modulation method used (AM or FM).

-- 
 /**************************************************************************\
 * Rich Bono (NM1D)              IMRICH              rbono@necis.ma.nec.com * 
 * (508) 635-6300                                       nm1d@nm1d.nh.usa.na * 
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