Nice sleuthing work, Scott. I noticed another issue with the KXV3. One
of the first things I did with my finished K3 kit was to look at the IF
output prior to connecting it to my panadapter project. I was mainly
looking to check the overall stage gain from antenna input to IF output.
What I saw on my spectrum analyzer was a lot of LO leakage from the K3.
I don't see this anywhere else, but there are LO signals, and harmonics
of them, at a level of about -50 to -60 dBm in the IF output. There are
distortion products of passband signals as well. This has caused me to
rethink the input filter specs on my panadapter. I don't know if this is
typical or a problem with my K3. You may want to check it with your SA
as well. I have pictures if you want to exchange them.
73,
Larry N8LP
Date: Sat, 2 Feb 2008 18:22:57 -0800
From: "Scott Prather" <[EMAIL PROTECTED]>
Subject: [Elecraft] K3/100 RF Feedback/Spurious Oscillation Verified
Via RX Input
To: <[email protected]>
Message-ID: <[EMAIL PROTECTED]>
Content-Type: text/plain; charset="us-ascii"
When I read about this report from Stewart last night (as well as the
responses indicating that other had experienced a similar effect) I decided
to set up the bench test to see if I could emulate the problem. I've since
discovered that these reports are valid, the K3 PA will generate a severe
spectral "comb" if there's insufficient isolation between the RX and TX
antennas when the KXV3 RX input is used.
To execute this test, I ran the K3 into a dummy load through an RF coupler
that displays 26 dB of isolation. The output of this coupler was split (3 dB
splitting loss) between a Tektronix 496 spectrum analyzer and a Tektronix
2701 50-ohm step attenuator. The output of the 2701 was fed into the K3's RX
input. Initially, the attenuator was set to 0 dB.
The K3 was keyed at full output (100W or +50 dBm), which results in a +21
dBm signal back into the K3's RX input. Under this condition, if the RX port
is enabled, the K3 will generate a spectral comb with the most severe spurs
only 10 to 20 dB down from the carrier. Turning the RX input off clears the
problem. I was able to create this comb spectrum on 160, 80, and 40 meters,
with 40 displaying the highest-power spurs.
Lowering the input power to the K3 via the 2701 attenuator indicates that
the "sweet spot" for generating this spurious oscillation occurs when
there's an input signal of about +20 dB. This would correspond to a minimum
of 30 dB of TX antenna to RX antenna isolation when operating at 100W output
power or 40 dB of isolation at 1 kW. I have spectrum analyzer photos of the
spurious signals generated by the K3 under these conditions if anyone is
interested.
I ran the same test under the same conditions with my K2/100, and this
problem does not occur.
The bottom line here is that a +20 dBm signal (100 mw) will create severe
spurs on the K3 but not the K2 when operating in the 40M band and below.
Above 40M I was unable to duplicate the problem, perhaps a higher power to
the RX input is required.
It's important to note that anyone who has +20 dBm or more coming back on
the receive antenna is asking for trouble in the first place, and additional
isolation and/or a shorting relay during TX operation is probably a good
idea.
>From the reports, it sounds like there have been a few folks who had rather
high power coming back in through the RX input for a while and didn't notice
it until this secondary symptom was noted in the K3. I haven't looked at the
RX input circuit in the K2 to see how it differs from the K3 so that I can
find the root-cause of this problem.
Scott
N7NB
K2 #1285
K3 #219
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