The rx340 is bad on the close in dynamic range because the DSP in the RX340
only has a 40 db range. So any signal in the roofing filter passband (+- 10
khz) can only have 40 db of AGC applied to it. The hardware provides the
other 80 db of AGC. So any difference of more than 40 DB in the roofing
filter passband forces the hardware to use some AGC which causes desense.
In actual practice I do not find this much of a problem. I also added a 4
khz roofing filter at the second 455 khz IF that greatly improves this
problem to my rx340.
Carl Moreschi N4PY
121 Little Bell Drive
Bell Mountain
Hays, NC 28635
----- Original Message -----
From: "Duane - N9DG" <n9dg@yahoo.com>
To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
Sent: Wednesday, October 03, 2007 10:48 PM
Subject: Re: [TenTec] dual receive
>
> --- 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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