Clearly you're much more familiar with it than I, and I'll accept
your, thourough, description. My use of the term "dirty" was only ment
to imply "it contains many things, some you want, some you filter
out".

Either way, I think it "clicked" in the OP's mind, so it's all good.
:o)

-Denny


> I don't know how dirty, it contains the fundamental (the receive
carier),
> the reference (local oscillator), the sum and product of the two
mixed
> signals. Once you grasp that concept, you understand how a radio
receiver
> works. Delving deeper off topic, since you want the modulated
information
> (in the case of an AM broadcast receiver), the reference oscillator
(or
> local oscillator) generates a frequency 455 khz above or below the
desired
> listening signal. The "intermediate frequency" amplifier passes only
the
> sum or product and attenuates the local oscillator and receive
frequency.
> This acts just like a hi/low pass filter more commonly known as a
band
> pass filter. It also increases your selectivity by the sharpness, or
> stages of IF amplification. The signal is then demodulated by means
of a
> diode to remove the IF frequency and what is left behind is your AM
audio
> information that was broadcast....
> There.. now you know how a superhetrodyne AM receiver works.
> Rick C.
> Chief Engineer - WTRM-FM
>
>
> Denny Esterline wrote:
>
> > When you mix the freqs you get a very dirty signal that contains
the
> > sum freq (extremely high) and the difference freq (low, usually
audio)
> > Considering the enormous difference between these two components
they
> > are very easy to separate with lowpass filters.
> >
> > (do I here a "click" yet?) :-)
> >
> > -Denny
> >
> > ----- Original Message -----
> > From: "Tim Hart" <Tim.Hart@HAWORTH.COM>
> > To: <PICLIST@MITVMA.MIT.EDU>
> > Sent: Wednesday, November 12, 2003 11:21 AM
> > Subject: Re: [OT]: Simple questions...
> >
> > The heterodyne just sounds counterintuitive....
> > Ultra High + Local High = Low?
> >
> > AM is a good example....I've not actually built an AM radio...but
I
> > guess it does the same thing.  Turns 600 AM into something I can
hear.
> > One person out on the web converted an AM radio into a
heterodyne....
> >
> > I'll keep reading....it will "click" sooner or later....
> > Once again Thanks for the help!!!
> > Tim
> >
> > >>> rixy@VVALLEY.COM 11/11/03 04:22PM >>>
> > Just looked at the bat detector schematic. It's just lie I
explained.
> > The LM567 is the oscillator. Q1 and Q2 make up a preamp and high
pass
> > filter. Both feed the fet mixer Q3 via R7/C2 and R11/C5. The
resultat
> > "tone" or ticking is amplified by U1. Simple.
> > Rick
> >
> > Tim Hart wrote:
> >
> > > I found some "Bat Detector" circuits that seem to be exactly
what
> > I'm wanting to do.  I'm still lost as to why a local signal mixed
with
> > an ultrahigh signal makes something you can hear?
> > >
> > > They are used an LMC567 "Phase Locked Loop IC" to make the local
> > oscillations.  Which is Greek to me...  I've used the LM386 before
so
> > I'm cool with that...but the rest still doesn't make much sense.
> > >
> > > Maybe you can make sense of it...
> > > http://www.njsas.org/projects/bat_detector/populel_sch.html
> > >
> > > I appreciate the help!!!
> > > Tim
> >
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