What you are running to is a characteristic, not a flaw of superheterodyne
receivers. This radio uses high-side local oscillator injection, which in
this case means that the LO is operating 455kHz above the received
frequency.
As an example, if your receiver is tuned to 1000kHz, that means that your LO
is operating at 1455kHz. Since your mixer has two sidebands at its I.F.
output, it can also respond to (1455 + 455kHz), or 1910kHz.
It is up the the tuned circuits ahead of the mixer to provide rejection of
that upper sideband. This is a big reason that double conversion circuits
are used.
If you have a 1st I.F. of 45MHz, for instance, your image response will be
90MHz away from your desired signal. A properly designed low-pass filter
ahead of the 1st mixer provides adequate rejection of this image frequency,
since the image response will be quite far down on the filter slope.
As Mr. Terrel suggested, a notch filter at the offending frequency would
help quite a bit.

Pete

"Michael A. Terrell" wrote in message
...
Alexander Kozik wrote:

Hi all,

Radio: Tecsun P230. Problem with AM reception (spurious signal?).
There is a strong AM station 50 kW with frequency 1530 kHz within 10
miles.
Tecsun P230 detects this station when it is tuned to 620 kHz.
No other radio detects this station at 620 kHz.
Actually, other radios detect weak distant stations at this frequency.
Obviously, there is something with heterodyne 1530 - 455*2 = 620

Could you explain what is wrong with radio or just point to the
useful web pages that describe the physical/electrical background
of this phenomena. Probably, I am not interested how to fix the problem,
I just want to understand why it happens.

Thanks a lot in advance,

-Alex


A tuned trap on the interfering frequency would help, or completely
eliminate the unwanted sign, if you are using an external antenna

--
Link to my "Computers for disabled Veterans" project website deleted
after threats were telephoned to my church.

Michael A. Terrell
Central Florida