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.
But there must be some other changes. I Jupiter schematic would be
rather helpful!
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.
This is going to wrap badly ... but the tabs did line up.
MDS AGC Th AGC Cmp BDR Sens LO Spcing Front End FiltUlt
SFDR-W Spcing
SFDR-N Spcing
dBm uV dB dB uV dBc kHz Selectivity dB
dB kHz dB kHz
Ten-Tec 340
-123 0.5 3 109 0.4 113 10 B 0.5 Octave 70
93 100 46 5
Ten-Tec 340 preamp on and measured with SSB filter
-133ab 0.13 0.14
On Oct 3, 2007, at 11:32 AM, Duane - N9DG wrote:
> The same MC145140 PLL chip is used in the 320 as the Pegasus
> (Jupiter too I presume). I think a very similar if not the
> same PLL is used in the Elecraft K2. It's phase noise
> behavior is largely determined by how it is configured. The
> Jupiter was also notably better for phase noise than the
> Pegasus per the ARRL tests. But I've also noticed that my
> early S/N Pegasus does not have the tuning "tick" every 2.5
> kHz like the newer S/N's ones do, or that has been reported
> by Jupiter users. My speculation is that the PLL
> configuration was tweaked slightly to improve phase noise
> performance but in so doing introduced the "tick".
--
Kevin Purcell
kevinpurcell@pobox.com
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