--- Kevin Purcell <kevinpurcell@pobox.com> wrote:
> Yes, it's all in the loop design.
>
> The Elecraft K2 is rather smart about this in moving in big
> steps and
> them tweaking the PLL reference (using a DDS) to move the
> PLL between the big steps.
>
> The RX320 (and I presume the Pegasus and Jupiter?) already
> uses a
> "big (2.5kHz) step in the PLL" approach which should help
> them
> tighten up the loop and then fine tunes by moving the
> passband around in the DSP.
Yep, they all do the 2.5 kHz step for the first LO, then fine
tune in the DSP. If I'm not mistaken the RX-330/331/340 use a
1 kHz step. Don't know what thier PLL is.
> I was looking at the Sherwood Receiver Test table today
>
> http://sherweng.com/table.html
>
> and noticed that the TT-340 (another similar design, I
> think) is
> right near the bottom unlike pretty much all the other TT
> rigs. It
> has a terrible close-in IMD (46dB SFDR) with 5kHz
> separation. With a
> wide (100kHz - outside the roofing filter) spacing the IMD
> is very
> good (93dB SFDR). The LO phase noise is not that good
> either.
If Sherwood ever tests the Pegasus/Jupiter/516 I suspect that
they would all land not very far from the RX-340 in his
table. The ARRL close spaced IMDDR numbers already suggest
this (look specifically at the 516 review).
My own experiences with the Pegasus in heavy signal
conditions show spurious/image responses every 12 kHz up and
down from the very strong signal. I've attributed this to the
last analog IF being at 12 kHz and the roofing filter being
~15 kHz wide. Or at least if that is not the cause, that
12/15 kHz relationship likely agravates the underlying
limitation. And if the infamous "IF gain pot fully clockwise"
tweak has been done then this behavior is noticeably worse. I
posted to this group some of my findings about this topic a
few years ago.
Duane
N9DG
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