[Elecraft] Fw: Interesting K3 QRM problem

[Joe Subich, W4TV]

> What helps - increasing current in diode switch on KPA board -
> change  of R25 to cca 65 Ohm and there are no IMD products as before.

This would tend to explain why Russ sees a more of a problem at low
power than at high power.  Current through the receive diodes in the
KPA3 is about three times as much as those on the RF board (13/200 vs.
7/320).

One would need to look carefully at the specifications of the PIN
diodes and the FETs being used for switching.  However, the resistors
to "tweak" would be R58 on the mainboard and R25 (as Lexa mentions)
on the KPA3.  The idea would be to keep the diodes turned on hard
enough to prevent them from turning off on RF peaks.

73,

    ... Joe, W4TV


On 2014-11-11 5:03 AM, alexandr.kobranov at seznam.cz wrote:
> Hi Russ,
> the same problem was here with BC station abt 5km away. Only with KPA in the
> chain.
> What helps - increasing current in diode switch on KPA board - change of R25
> to cca 65 Ohm and there are no IMD products as before.
> Hope this can help in any similar case in very special configuration of
> local BC station and non resonant ant used for higher bands.
> (I had IMD problems from 14MHz up, with with LW ant, BC station on 1062kHz)
>
> GL & 73!
> Lexa, ok1dst
>
>
> ---------- Původní zpráva ----------
> Od: Russ Tobolic <erusst at att.net>
> Komu: Elecraft at mailman.qth.net <Elecraft at mailman.qth.net>
> Datum: 10. 11. 2014 17:17:51
> Předmět: [Elecraft] Fw: Interesting K3 QRM problem
>
> "In this recent discussion here on K3 QRM from strong local broadcast
> stations, W4TV brought up the noise generated in the T/R switching diodes.
> In doing some experimentation with a bandpass flter in the loop between the
> RX antenna "out" and "in" on the KXV3A I have discovered something which has
> baffled me. I have a 2KW station on 1480 kc 1.5 miles north of me and a 20KW
> station on 1300 kc about 5 miles south of me. The 1300 station is
> directional north and I am right in the boresight. I am experiencing +30 to
> +40 dB over S9 intermod from these two stations on 1840 kc. An intermod
> calculator shows a 5th order intermod at 1840 from the two stations. I
> normally run QRP at 5W cw and what I have discovered is that when I crank up
> the power above 15W (activating the KPA3), the intermod drops to about +5 to
> +10 dB over S9. This behavior is very consistent when switching between low
> and high power.
>
> Can anyone explain this or is something broke in my K3?
>
> Thanks,
> Russ, N3CO
>
>
>
>
>
> On Thursday, October 23, 2014 4:52 PM, "Joe Subich, W4TV" <lists at subich.com>
> wrote:
>
>
>
>
>> Maybe, and if you're right, my suggestion may not help. But you
>> misunderstood my suggestion. There is an insert point between the
>> T/R switch and the RX input. That's the RX loop I was talking about
>> -- you insert the filter there.
>
> The insert point is *after* the PIN diode T/R switches for both the HPA
> (KPA3) and 10W LPA. Given Vic's description, it is likely the noise is
> being generated in one or more of the PIN diodes in the T/R switching
> and placing the highpass filter in the insert point will not resolve
> the noise. Vic, you can bypass (disable) the KPA3 to see if the noise
> generation is in the KPA3 T/R switch.
>
> 73,
>
> ... Joe, W4TV
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