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
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