John Woodgate wrote:
I read in alt.binaries.schematics.electronic that Chuck Harris
wrote (in
) about 'MK484 single chip AM radio
question', on Fri, 8 Apr 2005:

Since this is a superhet chip (IIRC),


No, it's a TRF.

Ahhh! Well, that changes everything. I took a look for the spec
sheet, and found the sheet for the ZN414Z, which is apparently almost
identical to the MK484. There are two things that I think would limit
operation at lower frequencies:

1) the gain appears to be purposefully rolled off at about 40KHz. That
is to say, it approaches zero at that frequency.

2) the detected audio stage has bandwidth out to 20KHz.

Imagine what would happen if you had a signal with a 20KHz carrier
frequency. The carrier frequency would pass right through the detector
and into the audio channel. The AGC, which appears to be audio derived,
would get seriously upset, and if the 20KHz signal was modulated, you
create some very interesting hetrodynes.

That being the case, it appears that the manufacturer tried to keep the
operating frequency far enough away from the audio passband that a simple
filter would suffice to prevent interferrence.

It probably won't work well at all as a 20KHz receiver.

-Chuck