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

[wayne burdick]

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


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