Added shielding will not affect image channel rejection.
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The Si4734 uses an image-rejection mixer architecture to
avoid images. This architecture depends on precise matching
between two signal paths within the chip to cancel the image
(rather than filters used in traditional radios). It is
probable that the difference between your two radios is just
natural manufacturing tolerances. Both are within spec, but
one of the radios has extra-good internal matching. The
larger antenna in the Pl-390 will also make images more
readily audible above the noise floor.
--- In ultralightdx@..., Richard Berler <lrdheat@...> wrote:
Is it possible for a novice to properly shield the circuitry? If so, would the sensitivity be degraded? Glad to see a explanation for what I observed...We have locals on 1550 and 1370...I was pretty sure that I was hearing images only at +90 KHz from locals...just checked to see if there was a mix of images on 1460 (+90 KHz from 1370, -90 KHz from 1550), but like you, only observed the one that was +90 KHz (the one from 1370, none heard from the 1550KHz local). I was conditioned to look for images 900 or 910 KHz BELOW strong stations (which is what you encountered with the GRUNDIG 350 with it's 455 KHz IF)!
--- On Tue, 8/31/10, Rick Robinson <w4dst@...> wrote:
From: Rick Robinson <w4dst@...>
Subject: [ultralightdx] Re:PL-390 External LW MW Antenna Mod
Date: Tuesday, August 31, 2010, 2:59 PM
Â An oddity, however, that is not present on the 310 is images of daytime locals 90 KHz above the local's frequency (for example, I hear a clear mirror image of 1490 on 1580). Strange also is that I do not hear mirror images of locals located below about 1300 KHz on the dial. Anyone else note this quirk?
I noticed an image at 1540kHz on my PL-380 when I was within 120' of my local 1450kHz transmitting tower.Â Image frequencies are at (the transmitter frequency +/- 2 X IF frequency) which in the case of the Si4734 chip is 45kHz. Â This gave me the image at 1450 + 90.Â Nothing was noted at 1360kHz, which I had expected to also find. Â What is odd is that the images are present on your PL-390 and not on your PL-310.Â I wonder if this is due to some shielding present in the '310 that is absent in the '390.Â I had a Grundig S-350 that picked up an image of my local 1450kHz station on 540kHz over 6 miles away.Â I've never encountered this on any other radio I've owned, ULR or otherwise.