Thanks Lexa and Joe, I see what you are saying about the PIN diode switch.
Looking at the schematic, the T/R switch for the LPA uses the MA4P which is a
true high power pin diode, but the KPA3 actually uses the S1M which is a
rectifier diode. It makes me wonder why different type were used in the two
designs. I assume this is correct since my K3 is one of the early ones (S/N
732). I t looks like both can handle 1A forward current. So... it looks like
my options are to take apart my old faithful K3 (again) and change the resistor
R58 as you suggested, or try to fabricate a high power bandpass or high pass
filter, or just ignore it and stay away from 1840. I guess I'll have to
seriously consider the first option.
Russ, N3CO
On Tuesday, November 11, 2014 9:38 AM, "Joe Subich, W4TV" <li...@subich.com>
wrote:
> 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.kobra...@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 <eru...@att.net>
> Komu: Elecraft@mailman.qth.net <Elecraft@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" <li...@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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