Stephan Grossklass wrote in
:

Conan Ford schrieb:

I have a Sangean ATS-803a and a Degen DE-1103. On both, I get images
in the LW band of MW band stations. They are at the frequency of the
medium wave station divided by 10, i.e. 1010 khz shows up at 101.
Why do they show up at 1/10 the frequency? These are both dual
conversion radios.

That is not (1010/10) kHz = 101 kHz, but (1010 - 2x 455) kHz = 100
kHz. Non-ideal 2nd IF image rejection isn't uncommon particularly
among smaller and/or less expensive rigs with high 1st IFs (Sony's
ICF-SW7600[|G|GR] models are also affected). The crystal filters used
there should be selective enough by themselves, but apparently leakage
around the filter (better receivers don't have rather large ground
planes around the 1st IF filters for no reason - RF tends to go its
own ways as you get to higher freqs) and possibly also mixer related
issues limit ultimate rejection. Single conversion sets with frontend
tracking (varicap tuned in PLL based sets, with conventional tuning
capacitors otherwise) may actually be in advantage on low frequency
ranges like MW and LW, that's why high performance AMBCB sets have
never needed to use dual conversion. (In fact, one more mixer may also
mean more noise.) PLL based dual conversion designs are usually
wideband receivers without a lot of front-end filtering. (Bandspread
dual conversion analogs are a different matter, these can easily use
band filters for SW. That's why they get along with much lower 1st
IFs.) I guess including additional front-end tracking on LW/MW only
would cost even more than getting 1st IF filtering right.
Better-quality portables use switched front-end filters (the Satellit
700 even had a tracking function, not always that well aligned BTW),
but these require care in application as well, given switching diodes
used for selecting a filter electronically can introduce intermod
themselves! (One of the Kenwood R-2000 mods comprised the replacement
of the stock switching diodes with, I think, PIN types. Actually a
number of older rigs will benefit from such a measure, including the
JRC NRD-515.)

Stephan

Interesting, I'd seen a lot of references to LW station really being MW
stations on this group, but not a detailed explanation like this. I had
always thought that dual conversion took care of this. I did always
wonder on my (even lower end) dual conversion DE-1102 why the MW
broadcast station bled into SW bands.

For the DE-1103, I know there is some filtering switching in for SW
frequencies, in particular a lowpass filter for 1700 khz or so, and a
highpass filter for 30000 khz. This gets rid of the MW band bleeding
into the SW bands, which you see on other radios like the DE-1102 (dual
conversion) and the PL-550 (single conversion). I suppose the ATS-803A
must have some similar scheme as well. I imagine that there wasn't as
much care or concern about the LW band when these radios were designed.