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From: "Watson A.Name - "Watt Sun, the Dark Remover"" on 8 Apr 2005 21:38

"Chuck Harris" <cf-NO-SPAM-harris(a)erols.com> wrote in message
news:a7idndBatMl7AMvfRVn-sg(a)rcn.net...
> John Woodgate wrote:
> > I read in alt.binaries.schematics.electronic that Albert
<?@?.?.invalid>
> > wrote (in <1mn251h6aip5lcirupgoooumiaeh90krnr(a)4ax.com>) about 'MK484
> > single chip AM radio question', on Mon, 4 Apr 2005:
> >
> >> I hope to use the MK484 single chip AM radio at 20 kilohertz for a
> >> very low power receiver. The spec sheet says it only goes down to
150
> >> kilohertz however.
> >
> >
> > Is there any reason to suppose it contains a coupling capacitor or
> > something that would limit the low-frequency response? It seems
rather
> > unlikely. But the definitive test invokes a little-used technique in
> > these days of calculations and simulations. It's known as 'trying
it'. (;-)
> >
> > It isn't as if testing at 20 kHz is rocket science, after all.
>
> Since this is a superhet chip (IIRC), the problem is more likely one
of LO
> bleed through into the IF. The LO frequency would have to be on the
> high side of the receive frequency, so in this case, with a 455Khz IF,
> the LO would be at 475Khz. The LO signal bleeding through the IF
filter
> would probably be as strong, or stronger than the 20KHz signal you
were
> intending on receiving.

Two reasons for using an IF amp are to amplify a fixed freq even as the
receiver is tunes, and to amplify a lower freq so that the bandwidth is
narrower. But since neither of these is necessary, it makes less sense
to upconvert to 455kHz, or to use 455kHz as the IF.

So it seems more logical to reduce the IF amps to the receiving freq, by
padding the IF tuned circuit caps. There is one barrier though: you
can't pad a 455kHz ceramic filter.


> -Chuck


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