Re: K3/100 RF Feedback/Spurious Oscillation Verified

[N8LP]

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" <[hidden email]>
> Subject: [Elecraft] K3/100 RF Feedback/Spurious Oscillation Verified
> Via RX Input
> To: <[hidden email]>
> Message-ID: <031401c8660b$b1454740$9c01a8c0@spvaio>
> 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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