What you are seeing are 3rd order intermodulation products that are being
generated in your 1st mixer. If you will notice, the frequencies that you
are hearing the stations on are mathematically related to the station minus
900kHz.
The way that you can eliminate them is to use a low-pass filter that cuts
off at 500kHz at the antenna input of your receiver. Some of today's
receivers use a 1MHz low-pass filter as the first preselector range. Not a
good idea, since in the Chicago area, you can get 3rd order mixing products
from WGN 720 mixing with WBBM 780, with a resultant on 60kHz.
Another example would be WGN 720 mixing with WIND 560. The resultant product
would be 720 - 560, or 220kHz. Please note that the 3rd order IMD products
are not only difference frequencies, but you can also have the sum of the
two frequencies showing up as an upper sideband in addition to the lower
sideband that manifests itself in the LW band.
That other fellow's post about the (2 X 450kHz) image was a good suggestion,
but in this case, you are encountering 3rd order mixing products.
Two solutions...............use band limit filters ahead of the receiver's
front end, or use a mixer that requires a much higher LO injection level. A
diode ring mixer or some kind of FET switching mixer will fill the bill
here. The key is having a high enough LO injection level in this case, so
that the LO is actually turning on and switching the devices in the mixer
instead of the signal that appears at the RF port of the mixer.
This is one of the things that you pay for when you purchase a more
expensive communications receiver. In the old days, this problem wasn't
really an issue, since receivers of that time either used a tracking front
end that had a multiple gang tuning capacitor that would tune the input
circuits as you varied the frequency...........or, they had a separate
preselector control that you would peak to the desired frequency.
I hope this helps.

Pete

"gccradioscience" wrote in message
...

If you live in a city area or some rural area with a real strong AM
station its likely it will be found on the longwave bands from 150 to
519 kHz (1050 kHz to 1419 kHz) the 900 kHz carrier image. Please
understand that this station is not a real longwave radio station.
Its a image from the broadcast station when the station is strong
enough to bleed on other longwave frequencies. The radios that I
have is the DX-380, DX-390, DX-392 and the yes the new Grundig G5.
It has do something with the dual conversion circuitry . I am
hoping that manufactuers, hobbyists, and technicians will use the 900
kHz formula to see other image stations to show that these AM stations
should not be on the LW bands.

Desired AM Station (kHz) - 900 kHz = Image Station

The images I am getting here locally in Virginia Beach, VA on LW is


150 kHz = 1050 kHz WVXX -AM
210 kHz = 1110 kHz WYRM-AM
330 kHz = 1230 kHz WJOI -AM
410 kHz = 1310 kHz WGH-AM
450 kHz = 1350 kHz WGPL -AM
500 kHz = 1400 kHz WPCE-AM

AM Band Images that are annoying

550 kHz = 1450 kHz
650 kHz = 1550 kHz

Adam E.