K3 IF Design

[VK5ABQ]

When considering the differences between the K3 and IC-7800/7700/756PRO3 or FTDX-9000/2000, apart from the K3's lack of band-scope, numerous large knobs and colour screens, the other thing that stands out is the selection of the 1st IF frequency.

Most other radios use a VHF 1st IF (typically about 70MHz) whereas the K3 has been designed to use 8MHz. Icom and others point out that a VHF IF allows their radios to obtain superior image rejection. This is done at the expense of non-ideal filter bandwidths though, as it's difficult to design narrow filters at around 70MHz. I think the K3 manages to provide 1st IF filter bandwidths of just 400Hz doesn't it?

Does anyone have a useful link to the definition of image rejection?

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
South Australia.

[Bill W4ZV]

Shane White wrote

Most other radios use a VHF 1st IF (typically about 70MHz) whereas the K3 has been designed to use 8MHz. Icom and others point out that a VHF IF allows their radios to obtain superior image rejection. This is done at the expense of non-ideal filter bandwidths though, as it's difficult to design narrow filters at around 70MHz. I think the K3 manages to provide 1st IF filter bandwidths of just 400Hz doesn't it?

G'day Shane.  Dave G4AON answered your questions about image rejection but I'll add a comment about the 1st IF filters.  The K3 actually has a 200 Hz filter option.  The narrowest 1st filters available from Icom (7800, 7700, Inrad mod for 756Pro family) or Yaesu (9000, 2000, Inrad mod for 1000 family) are ~5 kHz.  Both Yaesu and Icom "advertise" 3 kHz filters but they are actually ~5 kHz (i.e. they lie).  Ten-Tec's Orion II (which has a very similar front-end to the K3) offers an optional 300 Hz filter and Inrad offers a 250 Hz.  Your comments about the difficulty of manufacturing narrow BW filters at VHF are correct.

73,  Bill  W4ZV

[VK5ABQ]

Thanks Bill.
It's amazing to learn where Icom and Yaesu place their priorities. I suppose they have chosen a different compromise between style and substance than Elecraft. Icom and Yaesu's brochures and radios are adorable though.

Shane.