The K3 has very high dynamic range, requiring a high-level local oscillator
(the synth). One way to minimize spurs ("birdies") in a superhet design of this
type is to include very narrow-band filtering, which we used throughout the
radio. The ham-band filters are all just a bit wider than the ham bands
themselves, there are multiple I.F. rejection traps, and there's a very narrow
triple-tuned filter on the noise blanker.
The other way to minimize spurs is to enclose every subassembly in its own
shield and connect them when coax cables. While we use shielding where
absolutely necessary, we tried to avoid extensive shielding because it would
have dramatically increased the size, weight, and cost of the K3. Instead, we
used careful PCB layout, just the right number of coax cables, and balanced
low-pass filtering in the first mixer that rolls off the VHF and UHF synth
harmonics. The combination of these techniques results in very few spurs within
ham bands.
Those spurs which persist are generally very low level, inaudible with any kind
of antenna connected. For any "fast-tuning" (VFO harmonic-related) spurs that
remain above the noise floor, we provided the CONFIG:SIG RMV menu entry -- a
variation on spur-mapping that can be quite effective.
It's not surprising that certain very low-level spurs would change in level
with repositioning of the coax cables, since they typically arise from UHF
harmonics of the VFO.
73,
Wayne
N6KR
On May 4, 2015, at 2:17 PM, Richard Ferch <ve3...@storm.ca> wrote:
> My experience has been that the tuning noise is strongly affected by the
> positioning of the TMP cables, regardless of whether you have the old KSYN3's
> or the new KSYN3A's.
>
> I had heard some slight tuning noise on higher bands with the old KSYN3's,
> but only with no antenna connected, which is not a realistic test. After I
> put the new KSYN3A's in, at first I had much higher tuning noise (a buzz or
> rapid clicks when turning the tuning knob), but I opened up the rig and
> repositioned the TMP cables and since then the tuning noise has completely
> disappeared, even with no antenna connected.
>
> I do not have the VFO tuning noise reduction mod installed.
>
> As for the rapidly tuning birdies (the ones that can be eliminated with the
> CONFIG:SIG RMV option), I have not done a careful check. However, while they
> may appear at different places than they did before (perhaps that might be
> related to a reference oscillator adjustment), they do not seem to me to be
> any worse than they were with the old synth boards.
>
> 73,
> Rich VE3KI
> K3 #1595
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