[Elecraft] K3 6/10M pre-amp

[Joe Subich, W4TV]

Ed,

 >> Bypassing the ATU did not change the level so I guess the ATU was
 >> already adjusted to 50-ohms.

I have not checked the KX3 schematic but based on the K2 and K3
"bypassing" the ATU only adjusts it to a stored condition that the CPU
'thinks' represents 50 Ohms.  I've seen several of the other rigs be
very far off when the ATU is bypassed and known 50 Ohm load attached.

I would always "tune" into a known 50 Ohm load before switching the
ATU to bypass - or in the case of the K3 use the RX IN - before making
any measurements if the radio under test has an ATU installed.

73,

    ... Joe, W4TV


On 7/10/2013 1:32 PM, Edward R Cole wrote:
>
>> Date: Wed, 10 Jul 2013 00:12:45 -0800
>> To: Wayne Burdick <n6kr at elecraft.com>
>> From: Edward R Cole <kl7uw at acsalaska.net>
>> Subject: Re: K3 6/10M pre-amp
>>
>> Wayne,
>>
>> I re-tested on 50.120 and was able to reduce the -107 dBm signal from
>> the XG3 down to -135 dBm at 2.8-KHz bw.  Correcting for bw=500-KHz
>> this results in -142.5 dBm.  This is barely detectable so lets say
>> -140 dBm for copyable CW.  Adjusting the step attenuator further did
>> not change the signal so suspect -135 dBm is the lowest level I can
>> generate with the current setup.  -135 dBm is equivalent to NF=4 dB.
>> That is surprisingly low for the BDR performance which usually
>> requires a trade-off with MDS.
>>
>> Bypassing the ATU did not change the level so I guess the ATU was
>> already adjusted to 50-ohms.
>>
>> I did adjust the pitch up to about 700-Hz which made the signal easier
>> to hear.  The other day it was probably about 400-Hz.
>> I did not verify the 0-dBm output of the XG3 but would not expect
>> there to be significant level above zero.
>>
>> The K3+PR6 or other Gasfet preamp will exceed the KX3 so still the
>> preferred setup for 6m-eme will be the K3.  Still -140 dB is no small
>> achievement for the KX3.  My summer project includes getting the
>> 6-element 6m yagi installed and 800-1000w 6m sspa QRV for eme/ms.
>>
>> thanks for a really sweet little radio.
>>
>> Ed - KL7UW
>>
>>
>>> > At 08:31 AM 7/9/2013, you wrote:
>>> >> Typical KX3 MDS with preamp on is about -138 to -140 dBm in a
>>> 500-Hz bandwidth, on all bands.
>>> >>
>>> >> If a KX3 is measuring -134 dBm MDS in this bandwidth, chances are
>>> there's a measurement error, or the ATU has been left in-line at an
>>> impedance other than that of the source RF generator, etc.
>>> >>
>>> >> 73,
>>> >> Wayne
>>> >> N6KR
>>
>> 73, Ed - KL7UW
>> http://www.kl7uw.com
>> dubususa at gmail.com
>> "Kits made by KL7UW"
>
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