[Elecraft] KXV3: (1) RX ANT signal level during TX; (2) spectra at buffered IF output

[Stewart Baker]

Wayne,
Thank you for your very comprehensive mail, it certainly has put 
my mind at rest re potential issues involving separate receive 
antennas into the K3.

73
Stewart G3RXQ
On Sun, 3 Feb 2008 14:42:19 -0800, wayne burdick wrote:
> Hi all,
>
> I'll address both of these issues.
>
> 1. RX ANT input signal handling during TX
>
> The KXV3's ANT IN jack includes a carrier-operated relay circuit 
(COR).
> If the COR kicks in too soon, they relay will switch off/on 
during
> keying. So its threshold is set high. It will still serve its 
primary
> function, which is to protect the transceiver when unsafe signal 
levels
> are present, such as when the RX antenna is in the near field of 
a KW
> transmit signal, or when a transmitter is accidentally connected 
to
> this port.
>
> This leaves a gray zone where the COR doesn't kick in, but the 
transmit
> signal may still be very high. High-power contest stations 
sometimes
> use additional external T/R switching if they can't avoid the 
use of
> closely-coupled receive and transmit antennas.
>
> We recently to a close look at this gray zone, and found that we 
could
> make an "RX ANT jack on steroids" by adding two more diode 
isolation
> sections. The resulting parts count is considerably higher, but 
we feel
> that the added protection during transmit would be worthwhile. 
So this
> change will be included in production units (once we use up our 
present
> supply of assembled KXV3 modules).
>
> Meanwhile, if your antenna configuration and power level warrant 
this
> higher degree of isolation, and you already have a KXV3, you can 
modify
> it. We're designing a small add-on board to make this easier. It 
will
> be supplied free of charge, on request.
>
> This unit has not yet been assigned an Elecraft part number, and 
isn't
> available yet, so PLEASE don't request it until we announce the
> details.
>
> 2. Buffered I.F. output spectra
>
> The pick-off point for the K3's buffered I.F. output -- right at 
the
> output of the 1st mixer's post-amp stage -- was selected to give 
the
> widest possible bandwidth. Thus there is no filtering of any 
kind from
> here to the BNC jack on the KXV3. This creates the opportunity 
to do
> *very* wide spectral sweeps, if required -- limited only by the
> bandwidth of the K3's band-pass filters.
>
> As some (armed with spectrum analyzers) have noted, this output 
signal
> includes the usual spectra one would expect from a high-level 
mixer.
> (And it *is* a very high-level mixer.)
>
> A mixer's output includes a broad range of products relating to 
the sum
> and difference of its inputs (RF and LO). It also includes 
products
> derived from harmonics of the sum and difference, etc. 
Collectively,
> these will appear as a "comb" pattern on an analyzer. This is 
why the
> mixer must be followed by band-pass filtering. (In transmit mode 
this
> function is performed by the K3's narrow ham-band band-pass 
filters; in
> receive mode, by the I.F. crystal filter and DSP filtering.)
>
> But the buffered I.F. output is at a different point in the 
signal
> chain. So any filtering needed for a panadapter must be included 
either
> in the panadapter itself, or in series with it. Some panadapters 
will
> require no filtering at all, while some will need various 
degress of
> L-C filtering (the L-C filter used on our I.F. noise blanker is 
a good
> starting point).
>
> If you were only interested in a narrow band sweep, you could 
use a
> ceramic or crystal filter. Our FM crystal filter, at about 13 
kHz wide,
> would be a good choice here.
>
> Excellent shielding should be used, and possibly additional 
buffering,
> to prevent re-radiation of 8.215 MHz back into the K3.
>
> A related topic is the exact frequency of the buffered I.F. 
output. It
> is nominally 8.215 MHz with our 8-pole filters, and typically 
0.8 to
> 0.9 kHz lower with our 5-pole filters. In addition, the portion 
of this
> passband actually present at the audio output is determined by 
the DSP
> controls (SHIFT, WIDTH, etc.). We can work with panadapter 
suppliers to
> provide this data via the RS232 port, and of course any future 
Elecraft
> panadapter would also have this information available.
>
> I'll be adding this subject to the Owner's Manual. If you're
> interfacing a panadapter to the K3 and have any further 
questions, feel
> free to e-mail me directly.
>
> 73,
> Wayne
> N6KR
>
>
> ---
>
> http://www.elecraft.com
>
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