Shane
In the case of 14 MHz and the K3, the first IF is 8.215 MHz. An
oscillator at 22.215 MHz mixes with 14 MHz to give the difference
frequency of 8.215 MHz, however there is another difference of 14 MHz
minus 8.215 MHz which is 5.785 MHz which is the image frequency. The
ratio in dB between the wanted frequency and the image gives the image
rejection. There are other images involving the 2nd IF of 15 KHz too.
With the K3, the main images are greater than 70 dB below the wanted
signal. My own measurements give figures of 70 dB on 28.1 MHz (worst
case) to better than 110 dB on 1.8 MHz and 50 MHz.
The big advantage of a low 1st IF is the ability to use narrow "roofing"
filters to reject nearby interference. The disadvantage is potential for
poorer image rejection.
Image rejection is only one aspect of a transceiver design, of model A
was better in all respects than model B (and was cheaper) everyone would
buy model A, but life isn't so simple!
73 Dave, G4AON
http://www.astromag.co.uk/k3/
----------------------------------------
Does the K3 manage to obtain image rejection comparable to Icom's and
Yaesu's flagships? If so then it seems to me that Icom and Yaesu seem to be
relying on a proven RF design and instead are concentrating their current
efforts on ergonomics and presentation. There's nothing wrong with that
though.
Thanks,
Shane
VK5ABQ
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