"Chuck Harris" wrote in message
...
John Woodgate wrote:
I read in alt.binaries.schematics.electronic that Albert

wrote (in ) 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