[Elecraft] K3 IF Design

[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
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