Image frequency math on Tecsun PL-880
VK-SWL6002 > 25-10-2018, 05:54 PM
Hello.. this is my first post so if I make any errors of etiquette please let me know.
I am trying to work out the math of inter-modulation frequencies using my Tecsun PL-880 as an example. I have been SWL on my new Tecsun PL-880 (loving it) using a 17.1m random wire antenna with 9:1 unun and getting plenty of signal. Even setting the SW antenna gain to 'local', I seem to still be getting quite strong frequency images of the local AM radio stations.
I have been doing a lot of research, spreadsheeting and math, but can't quite work out the exact math of why the image frequencies are where they are.
I know the PL-880 uses an IF of 45 kHz and 10.7 MHz, but can't quite get the math to replicate why for example, I am receiving images of 720am (station 6WF here in Perth, Australia) on 6525,7675,8155,12870,15590,16070.
One clue is the frequency difference between all images is one third of the frequency difference between the original stations. For example, the difference between original station 720 6WF and 6RN 810 kHz is 90kHz, but the difference between images of those stations (eg 15560 for 810 and 15590 for 720 ) is one third of that .. ie 30kHz. This one third spread is apparent right across the images, ie the 495kHz MW band spread from say 585 6NR to 1080 6IX is three times of that in the image (say 15470 (6IX image) - 15635 (6NR image) = 165kHz or 1/3 of 495kHz!
Another clue. The spread between 720 6WF and one of its images at 16070, is 15530, half of which is 7675 which is also an image of 6WF
Another clue. The mid point between 720 6WF and one if its images at 15590, is 8155, which is also an image (15590 - 720)/2 = 7435, and 7435 + 720 is 8155 (mid way between 720 and 15590).
If any expert with a little time on their hands can decyper the math (ie various integer multiples of IF +/- RF) I would be very interested to know! I have spreadsheeted out all the images and still can't quite get integer multiples happening.
I hope someone can work it out, or point me to the math of an answer somewhere. Thanks for your time!
CP Perth, Australia