[Elecraft] K3 IF Design

[ab2tc]

Hi,

Somehow the math here is wrong. With a 1st IF of 8.215MHz and the local
oscillator at 22.215MHz in order to receive 14.000MHz, the image frequency
is 8.215+22.215 = 30.430MHz. Either way, it 's not hard to achieve excellent
image rejection across the HF band with this IF frequency. Icomwood would do
better arguing that the IF rejection will be poor when receiving close to
the IF frequency, but with a trap tuned the IF, the K3 does OK even though
there is a hole in coverage close to the IF frequency. Another disadvantage
of the low IF is that the synthesizer must cover a much larger range
relatively speaking, but it appears that the K3 synthesizer performs
admirably in spite of this challenge. The advantage of being able to offer
excellent filters at the 1st IF is undisputable.

Knut - AB2TC


Shane White wrote:
> 
> Thanks Dave. I'll read your review.
> Having been out of HAM radio for over a decade, I'm just trying to
> understand where each company involved in selling radios is heading and
> what their priorities are.
> 
> 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/
> 
> 
> 

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