AM receiver filter characteristics
 

For a direct conversion AM receiver for the commercial broadcast band
(550-1710kHz), could someone suggest appropriate low pass filter
specifications? I'm looking at a standard (cheap) superhet receiver with a
two stage transformer coupled IF amplifier/filter, and then end up with a
bandwidth of +/3kHz (around 455kHz). Hence, this would be equivalent to a
4th order filter with a corner frequency of 3kHz -- correct? I was thinking
I'd need somewhat more aggressive (but wider) filtering than this (more like
an 8th order filter with a corner at 5kHz).

If a commercial AM station is transmitting at, say, 550kHz, is it a safe bet
that the FCC wouldn't have stuck another reasonably powerful station at
560kHz, the next 'channel', in the same geographic location? That they'd at
least wait until 570kHz to do so? Historically I would think they'd have
had to do so in order to allow these simple IF transformer coupled
amps/filters to perform satisfactorily.

Thanks,
---Joel Kolstad

"Joel Kolstad" wrote in message
...

If a commercial AM station is transmitting at, say, 550kHz, is it a safe
bet
that the FCC wouldn't have stuck another reasonably powerful station at
560kHz, the next 'channel', in the same geographic location? That they'd
at
least wait until 570kHz to do so? Historically I would think they'd have
had to do so in order to allow these simple IF transformer coupled
amps/filters to perform satisfactorily.

You won't find stations closer than 40 (sometimes 30) kHz "in the same
town". Rimshot stations are often closer spaced. However at night you have
long distance ionospheric propagation. At night, most stations are only
protected to the contour where their signal and the signal 10 kHz away are
EQUAL. What signal level this is differs by class of station and also
stations can accept worse. IIRC, some may not be protected beyond their 25
mV/m contour at night. 2.5 or 5 mV/m is more common. Pretty much all bets
are off for stations on the graveyard channels of 1230, 1240, 1340, 1400,
1450 and 1490.

The FCC never allocated on the basis of the worst possible receiver. Super
el cheapo radios may well have trouble within the "interference-free"
contour.

"Joel Kolstad" wrote in message
...

If a commercial AM station is transmitting at, say, 550kHz, is it a safe
bet
that the FCC wouldn't have stuck another reasonably powerful station at
560kHz, the next 'channel', in the same geographic location? That they'd
at
least wait until 570kHz to do so? Historically I would think they'd have
had to do so in order to allow these simple IF transformer coupled
amps/filters to perform satisfactorily.

You won't find stations closer than 40 (sometimes 30) kHz "in the same
town". Rimshot stations are often closer spaced. However at night you have
long distance ionospheric propagation. At night, most stations are only
protected to the contour where their signal and the signal 10 kHz away are
EQUAL. What signal level this is differs by class of station and also
stations can accept worse. IIRC, some may not be protected beyond their 25
mV/m contour at night. 2.5 or 5 mV/m is more common. Pretty much all bets
are off for stations on the graveyard channels of 1230, 1240, 1340, 1400,
1450 and 1490.

The FCC never allocated on the basis of the worst possible receiver. Super
el cheapo radios may well have trouble within the "interference-free"
contour.